LibWasm: Start implementing WASI

This commit starts adding support for WASI, along with the framework to
implement all the functions (though only a couple are currently
implemented).

5 files changed, 1967 insertions, 1 deletions

diff --git a/Userland/Libraries/LibWasm/AbstractMachine/AbstractMachine.h b/Userland/Libraries/LibWasm/AbstractMachine/AbstractMachine.h
index 4c8ca6261f..dac570e96f 100644
--- a/Userland/Libraries/LibWasm/AbstractMachine/AbstractMachine.h
+++ b/Userland/Libraries/LibWasm/AbstractMachine/AbstractMachine.h
@@ -121,7 +121,7 @@ public:
     ALWAYS_INLINE Value& operator=(Value const& value) = default;
 
     template<typename T>
-    ALWAYS_INLINE Optional<T> to()
+    ALWAYS_INLINE Optional<T> to() const
     {
         Optional<T> result;
         m_value.visit(
diff --git a/Userland/Libraries/LibWasm/CMakeLists.txt b/Userland/Libraries/LibWasm/CMakeLists.txt
index 5b78f92da6..729156278c 100644
--- a/Userland/Libraries/LibWasm/CMakeLists.txt
+++ b/Userland/Libraries/LibWasm/CMakeLists.txt
@@ -5,6 +5,7 @@ set(SOURCES
     AbstractMachine/Validator.cpp
     Parser/Parser.cpp
     Printer/Printer.cpp
+    WASI/Wasi.cpp
 )
 
 serenity_lib(LibWasm wasm)
diff --git a/Userland/Libraries/LibWasm/Forward.h b/Userland/Libraries/LibWasm/Forward.h
index 4bb2c85f84..84006190ef 100644
--- a/Userland/Libraries/LibWasm/Forward.h
+++ b/Userland/Libraries/LibWasm/Forward.h
@@ -11,5 +11,10 @@ namespace Wasm {
 class AbstractMachine;
 class Validator;
 struct ValidationError;
+struct Interpreter;
+
+namespace Wasi {
+struct Implementation;
+}
 
 }
diff --git a/Userland/Libraries/LibWasm/WASI/Wasi.cpp b/Userland/Libraries/LibWasm/WASI/Wasi.cpp
new file mode 100644
index 0000000000..22cc5b369a
--- /dev/null
+++ b/Userland/Libraries/LibWasm/WASI/Wasi.cpp
@@ -0,0 +1,1017 @@
+/*
+ * Copyright (c) 2023, Ali Mohammad Pur <mpfard@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/ByteReader.h>
+#include <AK/Debug.h>
+#include <AK/FlyString.h>
+#include <AK/SourceLocation.h>
+#include <AK/Tuple.h>
+#include <LibCore/File.h>
+#include <LibWasm/AbstractMachine/Interpreter.h>
+#include <LibWasm/Printer/Printer.h>
+#include <LibWasm/Wasi.h>
+#include <dirent.h>
+#include <fcntl.h>
+#include <sys/stat.h>
+#include <time.h>
+#include <unistd.h>
+
+namespace Wasm::Wasi::ABI {
+
+template<typename T>
+Wasm::Value CompatibleValue<T>::to_wasm_value() const
+{
+    return Wasm::Value(value);
+}
+
+template<typename T>
+T deserialize(CompatibleValue<T> const& data)
+{
+    return deserialize<T>(Array { ReadonlyBytes { &data.value, sizeof(data.value) } });
+}
+
+template<typename T, size_t N>
+void serialize(T const& value, Array<Bytes, N> bytes)
+{
+    if constexpr (IsEnum<T>)
+        return serialize(to_underlying(value), move(bytes));
+    else if constexpr (IsIntegral<T>)
+        ReadonlyBytes { &value, sizeof(value) }.copy_to(bytes[0]);
+    else if constexpr (IsSpecializationOf<T, DistinctNumeric>)
+        return serialize(value.value(), move(bytes));
+    else
+        return value.serialize_into(move(bytes));
+}
+
+template<typename T, size_t N>
+T deserialize(Array<ReadonlyBytes, N> const& bytes)
+{
+    if constexpr (IsEnum<T>) {
+        return static_cast<T>(deserialize<UnderlyingType<T>>(bytes));
+    } else if constexpr (IsIntegral<T>) {
+        T value;
+        ByteReader::load(bytes[0].data(), value);
+        return value;
+    } else if constexpr (IsSpecializationOf<T, DistinctNumeric>) {
+        return deserialize<RemoveCVReference<decltype(T(0).value())>>(bytes);
+    } else {
+        return T::read_from(bytes);
+    }
+}
+
+template<typename T>
+CompatibleValue<T> to_compatible_value(Wasm::Value const& value)
+{
+    using Type = typename ToCompatibleValue<T>::Type;
+    // Note: the type can't be something else, we've already checked before through the function type's runtime checker.
+    auto converted_value = *value.template to<Type>();
+    return { .value = converted_value };
+}
+
+}
+
+namespace Wasm::Wasi {
+
+void ArgsSizes::serialize_into(Array<Bytes, 2> bytes) const
+{
+    ABI::serialize(count, Array { bytes[0] });
+    ABI::serialize(size, Array { bytes[1] });
+}
+
+void EnvironSizes::serialize_into(Array<Bytes, 2> bytes) const
+{
+    ABI::serialize(count, Array { bytes[0] });
+    ABI::serialize(size, Array { bytes[1] });
+}
+
+void SockRecvResult::serialize_into(Array<Bytes, 2> bytes) const
+{
+    ABI::serialize(size, Array { bytes[0] });
+    ABI::serialize(roflags, Array { bytes[1] });
+}
+
+void ROFlags::serialize_into(Array<Bytes, 1> bytes) const
+{
+    ABI::serialize(data, Array { bytes[0] });
+}
+
+template<typename T>
+void LittleEndian<T>::serialize_into(Array<Bytes, 1> bytes) const
+{
+    ABI::serialize(m_value, move(bytes));
+}
+
+template<typename T>
+LittleEndian<T> LittleEndian<T>::read_from(Array<ReadonlyBytes, 1> const& bytes)
+{
+    auto swapped = ABI::deserialize<T>(bytes);
+    return bit_cast<LittleEndian<T>>(swapped);
+}
+
+Rights Rights::read_from(Array<ReadonlyBytes, 1> const& bytes)
+{
+    Rights rights { .data = 0 };
+    bytes[0].copy_to(rights.data.bytes());
+    return rights;
+}
+
+void Rights::serialize_into(Array<Bytes, 1> bytes) const
+{
+    data.bytes().copy_to(bytes[0]);
+}
+
+void FDFlags::serialize_into(Array<Bytes, 1> bytes) const
+{
+    ReadonlyBytes { &data, sizeof(data) }.copy_to(bytes[0]);
+}
+
+FDFlags FDFlags::read_from(Array<ReadonlyBytes, 1> const& bytes)
+{
+    FDFlags flags { .data = 0 };
+    bytes[0].copy_to(flags.data.bytes());
+    return flags;
+}
+
+FSTFlags FSTFlags::read_from(Array<ReadonlyBytes, 1> const& bytes)
+{
+    FSTFlags flags { .data = 0 };
+    bytes[0].copy_to(flags.data.bytes());
+    return flags;
+}
+
+OFlags OFlags::read_from(Array<ReadonlyBytes, 1> const& bytes)
+{
+    OFlags flags { .data = 0 };
+    bytes[0].copy_to(flags.data.bytes());
+    return flags;
+}
+
+SDFlags SDFlags::read_from(Array<ReadonlyBytes, 1> const& bytes)
+{
+    SDFlags flags { .data = 0 };
+    bytes[0].copy_to(flags.data.bytes());
+    return flags;
+}
+
+void FDStat::serialize_into(Array<Bytes, 1> bytes) const
+{
+    auto data = bytes[0];
+    ABI::serialize(fs_filetype, Array { data.slice(offsetof(FDStat, fs_filetype), sizeof(fs_filetype)) });
+    ABI::serialize(fs_flags, Array { data.slice(offsetof(FDStat, fs_flags), sizeof(fs_flags)) });
+    ABI::serialize(fs_rights_base, Array { data.slice(offsetof(FDStat, fs_rights_base), sizeof(fs_rights_base)) });
+    ABI::serialize(fs_rights_inheriting, Array { data.slice(offsetof(FDStat, fs_rights_inheriting), sizeof(fs_rights_inheriting)) });
+}
+
+void PreStat::serialize_into(Array<Bytes, 1> bytes) const
+{
+    auto data = bytes[0];
+    ABI::serialize(tag, Array { data.slice(0, sizeof(tag)) });
+    if (tag == 0)
+        ABI::serialize(dir, Array { data.slice(offsetof(PreStat, dir), sizeof(dir)) });
+    else
+        VERIFY_NOT_REACHED();
+}
+
+void PreStatDir::serialize_into(Array<Bytes, 1> bytes) const
+{
+    ABI::serialize(pr_name_len, move(bytes));
+}
+
+void FileStat::serialize_into(Array<Bytes, 1> bytes) const
+{
+    auto data = bytes[0];
+    ABI::serialize(dev, Array { data.slice(0, sizeof(dev)) });
+    ABI::serialize(ino, Array { data.slice(offsetof(FileStat, ino), sizeof(ino)) });
+    ABI::serialize(filetype, Array { data.slice(offsetof(FileStat, filetype), sizeof(filetype)) });
+    ABI::serialize(nlink, Array { data.slice(offsetof(FileStat, nlink), sizeof(nlink)) });
+    ABI::serialize(size, Array { data.slice(offsetof(FileStat, size), sizeof(size)) });
+    ABI::serialize(atim, Array { data.slice(offsetof(FileStat, atim), sizeof(atim)) });
+    ABI::serialize(mtim, Array { data.slice(offsetof(FileStat, mtim), sizeof(mtim)) });
+    ABI::serialize(ctim, Array { data.slice(offsetof(FileStat, ctim), sizeof(ctim)) });
+}
+
+RIFlags RIFlags::read_from(Array<ReadonlyBytes, 1> const& bytes)
+{
+    RIFlags flags { .data = 0 };
+    bytes[0].copy_to(flags.data.bytes());
+    return flags;
+}
+
+LookupFlags LookupFlags::read_from(Array<ReadonlyBytes, 1> const& bytes)
+{
+    LookupFlags flags { .data = 0 };
+    bytes[0].copy_to(flags.data.bytes());
+    return flags;
+}
+
+CIOVec CIOVec::read_from(Array<ReadonlyBytes, 1> const& bytes)
+{
+    return CIOVec {
+        .buf = ABI::deserialize<decltype(buf)>(Array { bytes[0].slice(offsetof(CIOVec, buf), sizeof(buf)) }),
+        .buf_len = ABI::deserialize<decltype(buf_len)>(Array { bytes[0].slice(offsetof(CIOVec, buf_len), sizeof(buf_len)) }),
+    };
+}
+
+IOVec IOVec::read_from(Array<ReadonlyBytes, 1> const& bytes)
+{
+    return IOVec {
+        .buf = ABI::deserialize<decltype(buf)>(Array { bytes[0].slice(offsetof(IOVec, buf), sizeof(buf)) }),
+        .buf_len = ABI::deserialize<decltype(buf_len)>(Array { bytes[0].slice(offsetof(IOVec, buf_len), sizeof(buf_len)) }),
+    };
+}
+
+template<typename T>
+ErrorOr<Vector<T>> copy_typed_array(Configuration& configuration, Pointer<T> source, Size count)
+{
+    Vector<T> values;
+    TRY(values.try_ensure_capacity(count));
+    auto* memory = configuration.store().get(MemoryAddress { 0 });
+    if (!memory)
+        return Error::from_errno(ENOMEM);
+
+    UnderlyingPointerType address = source.value();
+    auto size = sizeof(T);
+    if (memory->size() < address || memory->size() <= address + (size * count)) {
+        return Error::from_errno(ENOBUFS);
+    }
+
+    for (Size i = 0; i < count; i += 1) {
+        values.unchecked_append(T::read_from(Array { ReadonlyBytes { memory->data().bytes().slice(address, size) } }));
+        address += size;
+    }
+
+    return values;
+}
+
+template<typename T>
+ErrorOr<void> copy_typed_value_to(Configuration& configuration, T const& value, Pointer<T> destination)
+{
+    auto* memory = configuration.store().get(MemoryAddress { 0 });
+    if (!memory)
+        return Error::from_errno(ENOMEM);
+
+    UnderlyingPointerType address = destination.value();
+    auto size = sizeof(T);
+    if (memory->size() < address || memory->size() <= address + size) {
+        return Error::from_errno(ENOBUFS);
+    }
+
+    ABI::serialize(value, Array { Bytes { memory->data().bytes().slice(address, size) } });
+    return {};
+}
+
+template<typename T>
+ErrorOr<Span<T>> slice_typed_memory(Configuration& configuration, Pointer<T> source, Size count)
+{
+    auto* memory = configuration.store().get(MemoryAddress { 0 });
+    if (!memory)
+        return Error::from_errno(ENOMEM);
+
+    auto address = source.value();
+    auto size = sizeof(T);
+    if (memory->size() < address || memory->size() <= address + (size * count))
+        return Error::from_errno(ENOBUFS);
+
+    auto untyped_slice = memory->data().bytes().slice(address, size * count);
+    return Span<T>(untyped_slice.data(), count);
+}
+
+template<typename T>
+ErrorOr<Span<T const>> slice_typed_memory(Configuration& configuration, ConstPointer<T> source, Size count)
+{
+    auto* memory = configuration.store().get(MemoryAddress { 0 });
+    if (!memory)
+        return Error::from_errno(ENOMEM);
+
+    auto address = source.value();
+    auto size = sizeof(T);
+    if (memory->size() < address || memory->size() <= address + (size * count))
+        return Error::from_errno(ENOBUFS);
+
+    auto untyped_slice = memory->data().bytes().slice(address, size * count);
+    return Span<T const>(untyped_slice.data(), count);
+}
+
+static ErrorOr<size_t> copy_string_including_terminating_null(Configuration& configuration, StringView string, Pointer<u8> target)
+{
+    auto slice = TRY(slice_typed_memory(configuration, target, string.bytes().size() + 1));
+    string.bytes().copy_to(slice);
+    slice[string.bytes().size()] = 0;
+    return slice.size();
+}
+
+static ErrorOr<size_t> copy_string_excluding_terminating_null(Configuration& configuration, StringView string, Pointer<u8> target, Size target_length)
+{
+    auto byte_count = min(string.bytes().size(), target_length);
+    auto slice = TRY(slice_typed_memory(configuration, target, byte_count));
+    string.bytes().copy_trimmed_to(slice);
+    return byte_count;
+}
+
+static Errno errno_value_from_errno(int value);
+
+Vector<AK::String> const& Implementation::arguments() const
+{
+    if (!cache.cached_arguments.has_value()) {
+        cache.cached_arguments.lazy_emplace([&] {
+            if (provide_arguments)
+                return provide_arguments();
+            return Vector<AK::String> {};
+        });
+    }
+
+    return *cache.cached_arguments;
+}
+
+Vector<AK::String> const& Implementation::environment() const
+{
+    if (!cache.cached_environment.has_value()) {
+        cache.cached_environment.lazy_emplace([&] {
+            if (provide_environment)
+                return provide_environment();
+            return Vector<AK::String> {};
+        });
+    }
+
+    return *cache.cached_environment;
+}
+
+Vector<Implementation::MappedPath> const& Implementation::preopened_directories() const
+{
+    if (!cache.cached_preopened_directories.has_value()) {
+        cache.cached_preopened_directories.lazy_emplace([&] {
+            if (provide_preopened_directories)
+                return provide_preopened_directories();
+            return Vector<MappedPath> {};
+        });
+    }
+
+    return *cache.cached_preopened_directories;
+}
+
+Implementation::Descriptor Implementation::map_fd(FD fd)
+{
+    u32 fd_value = fd.value();
+    if (auto* value = m_fd_map.find(fd_value))
+        return value->downcast<Descriptor>();
+
+    return UnmappedDescriptor(fd_value);
+}
+
+ErrorOr<Result<void>> Implementation::impl$args_get(Configuration& configuration, Pointer<Pointer<u8>> argv, Pointer<u8> argv_buf)
+{
+    UnderlyingPointerType raw_argv_buffer = argv_buf.value();
+    UnderlyingPointerType raw_argv = argv.value();
+
+    for (auto& entry : arguments()) {
+        auto ptr = Pointer<u8> { raw_argv_buffer };
+        auto byte_count = TRY(copy_string_including_terminating_null(configuration, entry.bytes_as_string_view(), ptr));
+        raw_argv_buffer += byte_count;
+
+        TRY(copy_typed_value_to(configuration, ptr, Pointer<Pointer<u8>> { raw_argv }));
+        raw_argv += sizeof(ptr);
+    }
+
+    return Result<void> {};
+}
+
+ErrorOr<Result<ArgsSizes>> Implementation::impl$args_sizes_get(Configuration&)
+{
+    size_t count = 0;
+    size_t total_size = 0;
+    for (auto& entry : arguments()) {
+        count += 1;
+        total_size += entry.bytes().size() + 1; // 1 extra byte for terminating null.
+    }
+
+    return Result<ArgsSizes>(ArgsSizes {
+        count,
+        total_size,
+    });
+}
+
+ErrorOr<Result<void>> Implementation::impl$environ_get(Configuration& configuration, Pointer<Pointer<u8>> environ, Pointer<u8> environ_buf)
+{
+    UnderlyingPointerType raw_environ_buffer = environ_buf.value();
+    UnderlyingPointerType raw_environ = environ.value();
+
+    for (auto& entry : environment()) {
+        auto ptr = Pointer<u8> { raw_environ_buffer };
+        auto byte_count = TRY(copy_string_including_terminating_null(configuration, entry.bytes_as_string_view(), ptr));
+        raw_environ_buffer += byte_count;
+
+        TRY(copy_typed_value_to(configuration, ptr, Pointer<Pointer<u8>> { raw_environ }));
+        raw_environ += sizeof(ptr);
+    }
+
+    return Result<void> {};
+}
+
+ErrorOr<Result<EnvironSizes>> Implementation::impl$environ_sizes_get(Configuration&)
+{
+    size_t count = 0;
+    size_t total_size = 0;
+    for (auto& entry : environment()) {
+        count += 1;
+        total_size += entry.bytes().size() + 1; // 1 extra byte for terminating null.
+    }
+
+    return Result<EnvironSizes>(EnvironSizes {
+        count,
+        total_size,
+    });
+}
+
+ErrorOr<Result<void>> Implementation::impl$proc_exit(Configuration&, ExitCode exit_code)
+{
+    exit(exit_code);
+}
+
+ErrorOr<Result<void>> Implementation::impl$fd_close(Configuration&, FD fd)
+{
+    return map_fd(fd).visit(
+        [&](u32 fd) -> Result<void> {
+            if (close(bit_cast<i32>(fd)) != 0)
+                return errno_value_from_errno(errno);
+            return {};
+        },
+        [&](PreopenedDirectoryDescriptor) -> Result<void> {
+            return errno_value_from_errno(EISDIR);
+        },
+        [&](UnmappedDescriptor) -> Result<void> {
+            return errno_value_from_errno(EBADF);
+        });
+}
+
+ErrorOr<Result<Size>> Implementation::impl$fd_write(Configuration& configuration, FD fd, Pointer<CIOVec> iovs, Size iovs_len)
+{
+    auto mapped_fd = map_fd(fd);
+    if (!mapped_fd.has<u32>())
+        return errno_value_from_errno(EBADF);
+
+    u32 fd_value = mapped_fd.get<u32>();
+    Size bytes_written = 0;
+    for (auto& iovec : TRY(copy_typed_array(configuration, iovs, iovs_len))) {
+        auto slice = TRY(slice_typed_memory(configuration, iovec.buf, iovec.buf_len));
+        auto result = write(fd_value, slice.data(), slice.size());
+        if (result < 0)
+            return errno_value_from_errno(errno);
+        bytes_written += static_cast<Size>(result);
+    }
+    return bytes_written;
+}
+
+ErrorOr<Result<PreStat>> Implementation::impl$fd_prestat_get(Configuration&, FD fd)
+{
+    auto& paths = preopened_directories();
+    return map_fd(fd).visit(
+        [&](UnmappedDescriptor unmapped_fd) -> Result<PreStat> {
+            // Map the new fd to the next available directory.
+            if (m_first_unmapped_preopened_directory_index >= paths.size())
+                return errno_value_from_errno(EBADF);
+
+            auto index = m_first_unmapped_preopened_directory_index++;
+            m_fd_map.insert(unmapped_fd.value(), PreopenedDirectoryDescriptor(index));
+            return PreStat {
+                .tag = 0,
+                .dir = PreStatDir {
+                    .pr_name_len = paths[index].mapped_path.string().bytes().size(),
+                },
+            };
+        },
+        [&](u32) -> Result<PreStat> {
+            return errno_value_from_errno(EBADF);
+        },
+        [&](PreopenedDirectoryDescriptor fd) -> Result<PreStat> {
+            return PreStat {
+                .tag = 0,
+                .dir = PreStatDir {
+                    .pr_name_len = paths[fd.value()].mapped_path.string().bytes().size(),
+                },
+            };
+        });
+}
+
+ErrorOr<Result<void>> Implementation::impl$fd_prestat_dir_name(Configuration& configuration, FD fd, Pointer<u8> path, Size path_len)
+{
+    auto mapped_fd = map_fd(fd);
+    if (!mapped_fd.has<PreopenedDirectoryDescriptor>())
+        return errno_value_from_errno(EBADF);
+
+    auto& entry = preopened_directories()[mapped_fd.get<PreopenedDirectoryDescriptor>().value()];
+    auto byte_count = TRY(copy_string_excluding_terminating_null(configuration, entry.mapped_path.string().view(), path, path_len));
+    if (byte_count < path_len.value())
+        return errno_value_from_errno(ENOBUFS);
+
+    return Result<void> {};
+}
+
+ErrorOr<Result<FileStat>> Implementation::impl$path_filestat_get(Configuration& configuration, FD fd, LookupFlags flags, ConstPointer<u8> path, Size path_len)
+{
+    int dir_fd = AT_FDCWD;
+
+    auto mapped_fd = map_fd(fd);
+    if (mapped_fd.has<PreopenedDirectoryDescriptor>()) {
+        auto& entry = preopened_directories()[mapped_fd.get<PreopenedDirectoryDescriptor>().value()];
+        dir_fd = entry.opened_fd.value_or_lazy_evaluated([&] {
+            DeprecatedString path = entry.host_path.string();
+            return open(path.characters(), O_DIRECTORY, 0755);
+        });
+        entry.opened_fd = dir_fd;
+    }
+
+    if (dir_fd < 0 && dir_fd != AT_FDCWD)
+        return errno_value_from_errno(errno);
+
+    int options = 0;
+    if (!flags.bits.symlink_follow)
+        options |= AT_SYMLINK_NOFOLLOW;
+
+    auto slice = TRY(slice_typed_memory(configuration, path, path_len));
+    auto null_terminated_string = DeprecatedString::copy(slice);
+
+    struct stat stat_buf;
+    if (fstatat(dir_fd, null_terminated_string.characters(), &stat_buf, options) < 0)
+        return errno_value_from_errno(errno);
+
+    constexpr auto file_type_of = [](struct stat const& buf) {
+        if (S_ISDIR(buf.st_mode))
+            return FileType::Directory;
+        if (S_ISCHR(buf.st_mode))
+            return FileType::CharacterDevice;
+        if (S_ISBLK(buf.st_mode))
+            return FileType::BlockDevice;
+        if (S_ISREG(buf.st_mode))
+            return FileType::RegularFile;
+        if (S_ISFIFO(buf.st_mode))
+            return FileType::Unknown; // no Pipe? :yakfused:
+        if (S_ISLNK(buf.st_mode))
+            return FileType::SymbolicLink;
+        if (S_ISSOCK(buf.st_mode))
+            return FileType::SocketDGram; // :shrug:
+        return FileType::Unknown;
+    };
+
+    return Result(FileStat {
+        .dev = stat_buf.st_dev,
+        .ino = stat_buf.st_ino,
+        .filetype = file_type_of(stat_buf),
+        .nlink = stat_buf.st_nlink,
+        .size = stat_buf.st_size,
+        .atim = stat_buf.st_atime,
+        .mtim = stat_buf.st_mtime,
+        .ctim = stat_buf.st_ctime,
+    });
+}
+
+ErrorOr<Result<void>> Implementation::impl$path_create_directory(Configuration& configuration, FD fd, Pointer<u8> path, Size path_len)
+{
+    int dir_fd = AT_FDCWD;
+
+    auto mapped_fd = map_fd(fd);
+    if (mapped_fd.has<PreopenedDirectoryDescriptor>()) {
+        auto& entry = preopened_directories()[mapped_fd.get<PreopenedDirectoryDescriptor>().value()];
+        dir_fd = entry.opened_fd.value_or_lazy_evaluated([&] {
+            DeprecatedString path = entry.host_path.string();
+            return open(path.characters(), O_DIRECTORY, 0755);
+        });
+        entry.opened_fd = dir_fd;
+    }
+
+    if (dir_fd < 0 && dir_fd != AT_FDCWD)
+        return errno_value_from_errno(errno);
+
+    auto slice = TRY(slice_typed_memory(configuration, path, path_len));
+    auto null_terminated_string = DeprecatedString::copy(slice);
+
+    if (mkdirat(dir_fd, null_terminated_string.characters(), 0755) < 0)
+        return errno_value_from_errno(errno);
+
+    return Result<void> {};
+}
+
+ErrorOr<Result<FD>> Implementation::impl$path_open(Configuration& configuration, FD fd, LookupFlags lookup_flags, Pointer<u8> path, Size path_len, OFlags o_flags, Rights, Rights, FDFlags fd_flags)
+{
+    int dir_fd = AT_FDCWD;
+
+    auto mapped_fd = map_fd(fd);
+    if (mapped_fd.has<PreopenedDirectoryDescriptor>()) {
+        auto& entry = preopened_directories()[mapped_fd.get<PreopenedDirectoryDescriptor>().value()];
+        dir_fd = entry.opened_fd.value_or_lazy_evaluated([&] {
+            DeprecatedString path = entry.host_path.string();
+            return open(path.characters(), O_DIRECTORY, 0755);
+        });
+        entry.opened_fd = dir_fd;
+    }
+
+    if (dir_fd < 0 && dir_fd != AT_FDCWD)
+        return errno_value_from_errno(errno);
+
+    // FIXME: What should we do with dsync/rsync?
+
+    int open_flags = 0;
+    if (fd_flags.bits.append)
+        open_flags |= O_APPEND;
+    if (fd_flags.bits.nonblock)
+        open_flags |= O_NONBLOCK;
+    if (fd_flags.bits.sync)
+        open_flags |= O_SYNC;
+
+    if (o_flags.bits.trunc)
+        open_flags |= O_TRUNC;
+    if (o_flags.bits.creat)
+        open_flags |= O_CREAT;
+    if (o_flags.bits.directory)
+        open_flags |= O_DIRECTORY;
+    if (o_flags.bits.excl)
+        open_flags |= O_EXCL;
+
+    if (!lookup_flags.bits.symlink_follow)
+        open_flags |= O_NOFOLLOW;
+
+    auto path_data = TRY(slice_typed_memory(configuration, path, path_len));
+    auto path_string = DeprecatedString::copy(path_data);
+
+    int opened_fd = openat(dir_fd, path_string.characters(), open_flags, 0644);
+    if (opened_fd < 0)
+        return errno_value_from_errno(errno);
+
+    // FIXME: Implement Rights and RightsInheriting.
+
+    return FD(opened_fd);
+}
+
+ErrorOr<Result<Timestamp>> Implementation::impl$clock_time_get(Configuration&, ClockID id, Timestamp precision)
+{
+    constexpr u64 nanoseconds_in_millisecond = 1000'000ull;
+    constexpr u64 nanoseconds_in_second = 1000'000'000ull;
+
+    clockid_t clock_id;
+    switch (id) {
+    case ClockID::Realtime:
+        if (precision >= nanoseconds_in_millisecond)
+            clock_id = CLOCK_REALTIME_COARSE;
+        else
+            clock_id = CLOCK_REALTIME;
+        break;
+    case ClockID::Monotonic:
+        if (precision >= nanoseconds_in_millisecond)
+            clock_id = CLOCK_MONOTONIC_COARSE;
+        else
+            clock_id = CLOCK_MONOTONIC;
+        break;
+    case ClockID::ProcessCPUTimeID:
+    case ClockID::ThreadCPUTimeID:
+        return Errno::NoSys;
+        break;
+    }
+
+    struct timespec ts;
+    if (clock_gettime(clock_id, &ts) < 0)
+        return errno_value_from_errno(errno);
+
+    return Result<Timestamp> { static_cast<u64>(ts.tv_sec) * nanoseconds_in_second + static_cast<u64>(ts.tv_nsec) };
+}
+
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wunused-parameter"
+
+ErrorOr<Result<Timestamp>> Implementation::impl$clock_res_get(Configuration&, ClockID id)
+{
+    return Errno::NoSys;
+}
+ErrorOr<Result<void>> Implementation::impl$fd_advise(Configuration&, FD, FileSize offset, FileSize len, Advice) { return Errno::NoSys; }
+ErrorOr<Result<void>> Implementation::impl$fd_allocate(Configuration&, FD, FileSize offset, FileSize len) { return Errno::NoSys; }
+ErrorOr<Result<void>> Implementation::impl$fd_datasync(Configuration&, FD) { return Errno::NoSys; }
+ErrorOr<Result<FDStat>> Implementation::impl$fd_fdstat_get(Configuration&, FD) { return Errno::NoSys; }
+ErrorOr<Result<void>> Implementation::impl$fd_fdstat_set_flags(Configuration&, FD, FDFlags) { return Errno::NoSys; }
+ErrorOr<Result<void>> Implementation::impl$fd_fdstat_set_rights(Configuration&, FD, Rights fs_rights_base, Rights fs_rights_inheriting) { return Errno::NoSys; }
+ErrorOr<Result<FileStat>> Implementation::impl$fd_filestat_get(Configuration&, FD) { return Errno::NoSys; }
+ErrorOr<Result<void>> Implementation::impl$fd_filestat_set_size(Configuration&, FD, FileSize) { return Errno::NoSys; }
+ErrorOr<Result<void>> Implementation::impl$fd_filestat_set_times(Configuration&, FD, Timestamp atim, Timestamp mtim, FSTFlags) { return Errno::NoSys; }
+ErrorOr<Result<Size>> Implementation::impl$fd_pread(Configuration&, FD, Pointer<IOVec> iovs, Size iovs_len, FileSize offset) { return Errno::NoSys; }
+ErrorOr<Result<Size>> Implementation::impl$fd_pwrite(Configuration&, FD, Pointer<CIOVec> iovs, Size iovs_len, FileSize offset) { return Errno::NoSys; }
+ErrorOr<Result<Size>> Implementation::impl$fd_read(Configuration&, FD, Pointer<IOVec> iovs, Size iovs_len) { return Errno::NoSys; }
+ErrorOr<Result<Size>> Implementation::impl$fd_readdir(Configuration&, FD, Pointer<u8> buf, Size buf_len, DirCookie cookie) { return Errno::NoSys; }
+ErrorOr<Result<void>> Implementation::impl$fd_renumber(Configuration&, FD from, FD to) { return Errno::NoSys; }
+ErrorOr<Result<FileSize>> Implementation::impl$fd_seek(Configuration&, FD, FileDelta offset, Whence whence) { return Errno::NoSys; }
+ErrorOr<Result<void>> Implementation::impl$fd_sync(Configuration&, FD) { return Errno::NoSys; }
+ErrorOr<Result<FileSize>> Implementation::impl$fd_tell(Configuration&, FD) { return Errno::NoSys; }
+ErrorOr<Result<void>> Implementation::impl$path_filestat_set_times(Configuration&, FD, LookupFlags, Pointer<u8> path, Size path_len, Timestamp atim, Timestamp mtim, FSTFlags) { return Errno::NoSys; }
+ErrorOr<Result<void>> Implementation::impl$path_link(Configuration&, FD, LookupFlags, Pointer<u8> old_path, Size old_path_len, FD, Pointer<u8> new_path, Size new_path_len) { return Errno::NoSys; }
+ErrorOr<Result<Size>> Implementation::impl$path_readlink(Configuration&, FD, LookupFlags, Pointer<u8> path, Size path_len, Pointer<u8> buf, Size buf_len) { return Errno::NoSys; }
+ErrorOr<Result<void>> Implementation::impl$path_remove_directory(Configuration&, FD, Pointer<u8> path, Size path_len) { return Errno::NoSys; }
+ErrorOr<Result<void>> Implementation::impl$path_rename(Configuration&, FD, Pointer<u8> old_path, Size old_path_len, FD, Pointer<u8> new_path, Size new_path_len) { return Errno::NoSys; }
+ErrorOr<Result<void>> Implementation::impl$path_symlink(Configuration&, Pointer<u8> old_path, Size old_path_len, FD, Pointer<u8> new_path, Size new_path_len) { return Errno::NoSys; }
+ErrorOr<Result<void>> Implementation::impl$path_unlink_file(Configuration&, FD, Pointer<u8> path, Size path_len) { return Errno::NoSys; }
+ErrorOr<Result<Size>> Implementation::impl$poll_oneoff(Configuration&, ConstPointer<Subscription> in, Pointer<Event> out, Size nsubscriptions) { return Errno::NoSys; }
+ErrorOr<Result<void>> Implementation::impl$proc_raise(Configuration&, Signal) { return Errno::NoSys; }
+ErrorOr<Result<void>> Implementation::impl$sched_yield(Configuration&) { return Errno::NoSys; }
+ErrorOr<Result<void>> Implementation::impl$random_get(Configuration&, Pointer<u8> buf, Size buf_len) { return Errno::NoSys; }
+ErrorOr<Result<FD>> Implementation::impl$sock_accept(Configuration&, FD fd, FDFlags fd_flags) { return Errno::NoSys; }
+ErrorOr<Result<SockRecvResult>> Implementation::impl$sock_recv(Configuration&, FD fd, Pointer<IOVec> ri_data, Size ri_data_len, RIFlags ri_flags) { return Errno::NoSys; }
+ErrorOr<Result<Size>> Implementation::impl$sock_send(Configuration&, FD fd, Pointer<CIOVec> si_data, Size si_data_len, SIFlags si_flags) { return Errno::NoSys; }
+ErrorOr<Result<void>> Implementation::impl$sock_shutdown(Configuration&, FD fd, SDFlags how) { return Errno::NoSys; }
+
+#pragma GCC diagnostic pop
+
+template<size_t N>
+static Array<Bytes, N> address_spans(Span<Value> values, Configuration& configuration)
+{
+    Array<Bytes, N> result;
+    auto memory = configuration.store().get(MemoryAddress { 0 })->data().span();
+    for (size_t i = 0; i < N; ++i)
+        result[i] = memory.slice(*values[i].to<i32>());
+    return result;
+}
+
+#define ENUMERATE_FUNCTION_NAMES(M) \
+    M(args_get)                     \
+    M(args_sizes_get)               \
+    M(environ_get)                  \
+    M(environ_sizes_get)            \
+    M(clock_res_get)                \
+    M(clock_time_get)               \
+    M(fd_advise)                    \
+    M(fd_allocate)                  \
+    M(fd_close)                     \
+    M(fd_datasync)                  \
+    M(fd_fdstat_get)                \
+    M(fd_fdstat_set_flags)          \
+    M(fd_fdstat_set_rights)         \
+    M(fd_filestat_get)              \
+    M(fd_filestat_set_size)         \
+    M(fd_filestat_set_times)        \
+    M(fd_pread)                     \
+    M(fd_prestat_get)               \
+    M(fd_prestat_dir_name)          \
+    M(fd_pwrite)                    \
+    M(fd_read)                      \
+    M(fd_readdir)                   \
+    M(fd_renumber)                  \
+    M(fd_seek)                      \
+    M(fd_sync)                      \
+    M(fd_tell)                      \
+    M(fd_write)                     \
+    M(path_create_directory)        \
+    M(path_filestat_get)            \
+    M(path_filestat_set_times)      \
+    M(path_link)                    \
+    M(path_open)                    \
+    M(path_readlink)                \
+    M(path_remove_directory)        \
+    M(path_rename)                  \
+    M(path_symlink)                 \
+    M(path_unlink_file)             \
+    M(poll_oneoff)                  \
+    M(proc_exit)                    \
+    M(proc_raise)                   \
+    M(sched_yield)                  \
+    M(random_get)                   \
+    M(sock_accept)                  \
+    M(sock_recv)                    \
+    M(sock_send)                    \
+    M(sock_shutdown)
+
+struct Names {
+#define NAME(x) FlyString x;
+    ENUMERATE_FUNCTION_NAMES(NAME)
+#undef NAME
+
+    static ErrorOr<Names> construct()
+    {
+        return Names {
+#define NAME(x) .x = TRY(FlyString::from_utf8(#x##sv)),
+            ENUMERATE_FUNCTION_NAMES(NAME)
+#undef NAME
+        };
+    }
+};
+
+ErrorOr<HostFunction> Implementation::function_by_name(StringView name)
+{
+    auto name_for_comparison = TRY(FlyString::from_utf8(name));
+    static auto names = TRY(Names::construct());
+
+#define IMPL(x)                         \
+    if (name_for_comparison == names.x) \
+        return invocation_of<&Implementation::impl$##x>(#x##sv);
+
+    ENUMERATE_FUNCTION_NAMES(IMPL)
+
+#undef IMPL
+
+    return Error::from_string_literal("No such host function");
+}
+
+namespace ABI {
+
+template<typename T>
+auto CompatibleValueType = IsOneOf<T, i8, i16, i32>
+    ? Wasm::ValueType(Wasm::ValueType::I32)
+    : Wasm::ValueType(Wasm::ValueType::I64);
+
+template<typename R, typename... Args, ErrorOr<Result<R>> (Implementation::*impl)(Configuration&, Args...)>
+struct InvocationOf<impl> {
+    HostFunction operator()(Implementation& self, StringView function_name)
+    {
+        Vector<ValueType> arguments_types { CompatibleValueType<typename ABI::ToCompatibleValue<Args>::Type>... };
+        if constexpr (!IsVoid<R>) {
+            if constexpr (requires { declval<typename R::SerializationComponents>(); }) {
+                for_each_type<typename R::SerializationComponents>([&]<typename T>(TypeWrapper<T>) {
+                    arguments_types.append(CompatibleValueType<typename ABI::ToCompatibleValue<Pointer<T>>::Type>);
+                });
+            } else {
+                arguments_types.append(CompatibleValueType<typename ABI::ToCompatibleValue<Pointer<R>>::Type>);
+            }
+        }
+
+        return HostFunction(
+            [&self, function_name](Configuration& configuration, Vector<Value>& arguments) -> Wasm::Result {
+                Tuple args = [&]<typename... Ts, auto... Is>(IndexSequence<Is...>)
+                {
+                    return Tuple { ABI::deserialize(ABI::to_compatible_value<Ts>(arguments[Is]))... };
+                }
+                .template operator()<Args...>(MakeIndexSequence<sizeof...(Args)>());
+
+                auto result = args.apply_as_args([&](auto&&... impl_args) { return (self.*impl)(configuration, impl_args...); });
+                dbgln_if(WASI_DEBUG, "WASI: Called {}", function_name);
+
+                if (result.is_error())
+                    return Wasm::Trap { DeprecatedString::formatted("Invalid call to {}() = {}", function_name, result.error()) };
+
+                auto value = result.release_value();
+                if (value.is_error())
+                    return Wasm::Result { Vector { Value { ValueType(ValueType::I32), static_cast<u64>(to_underlying(value.error().value())) } } };
+
+                if constexpr (!IsVoid<R>) {
+                    // Return values are passed as pointers, after the arguments
+                    if constexpr (requires { &R::serialize_into; }) {
+                        constexpr auto ResultCount = []<auto N>(void(R::*)(Array<Bytes, N>) const) { return N; }
+                        (&R::serialize_into);
+                        ABI::serialize(*value.result(), address_spans<ResultCount>(arguments.span().slice(sizeof...(Args)), configuration));
+                    } else {
+                        ABI::serialize(*value.result(), address_spans<1>(arguments.span().slice(sizeof...(Args)), configuration));
+                    }
+                }
+                // Return value is errno, we have nothing to return.
+                return Wasm::Result { Vector<Value> { Value(ValueType(ValueType::I32), 0ull) } };
+            },
+            FunctionType {
+                move(arguments_types),
+                { ValueType(ValueType::I32) },
+            });
+    }
+};
+
+};
+
+Errno errno_value_from_errno(int value)
+{
+    switch (value) {
+#ifdef ESUCCESS
+    case ESUCCESS:
+        return Errno::Success;
+#endif
+    case E2BIG:
+        return Errno::TooBig;
+    case EACCES:
+        return Errno::Access;
+    case EADDRINUSE:
+        return Errno::AddressInUse;
+    case EADDRNOTAVAIL:
+        return Errno::AddressNotAvailable;
+    case EAFNOSUPPORT:
+        return Errno::AFNotSupported;
+    case EAGAIN:
+        return Errno::Again;
+    case EALREADY:
+        return Errno::Already;
+    case EBADF:
+        return Errno::BadF;
+    case EBUSY:
+        return Errno::Busy;
+    case ECANCELED:
+        return Errno::Canceled;
+    case ECHILD:
+        return Errno::Child;
+    case ECONNABORTED:
+        return Errno::ConnectionAborted;
+    case ECONNREFUSED:
+        return Errno::ConnectionRefused;
+    case ECONNRESET:
+        return Errno::ConnectionReset;
+    case EDEADLK:
+        return Errno::Deadlock;
+    case EDESTADDRREQ:
+        return Errno::DestinationAddressRequired;
+    case EDOM:
+        return Errno::Domain;
+    case EEXIST:
+        return Errno::Exist;
+    case EFAULT:
+        return Errno::Fault;
+    case EFBIG:
+        return Errno::FBig;
+    case EHOSTUNREACH:
+        return Errno::HostUnreachable;
+    case EILSEQ:
+        return Errno::IllegalSequence;
+    case EINPROGRESS:
+        return Errno::InProgress;
+    case EINTR:
+        return Errno::Interrupted;
+    case EINVAL:
+        return Errno::Invalid;
+    case EIO:
+        return Errno::IO;
+    case EISCONN:
+        return Errno::IsConnected;
+    case EISDIR:
+        return Errno::IsDirectory;
+    case ELOOP:
+        return Errno::Loop;
+    case EMFILE:
+        return Errno::MFile;
+    case EMLINK:
+        return Errno::MLink;
+    case EMSGSIZE:
+        return Errno::MessageSize;
+    case ENAMETOOLONG:
+        return Errno::NameTooLong;
+    case ENETDOWN:
+        return Errno::NetworkDown;
+    case ENETRESET:
+        return Errno::NetworkReset;
+    case ENETUNREACH:
+        return Errno::NetworkUnreachable;
+    case ENFILE:
+        return Errno::NFile;
+    case ENOBUFS:
+        return Errno::NoBufferSpace;
+    case ENODEV:
+        return Errno::NoDevice;
+    case ENOENT:
+        return Errno::NoEntry;
+    case ENOEXEC:
+        return Errno::NoExec;
+    case ENOLCK:
+        return Errno::NoLock;
+    case ENOMEM:
+        return Errno::NoMemory;
+    case ENOPROTOOPT:
+        return Errno::NoProtocolOption;
+    case ENOSPC:
+        return Errno::NoSpace;
+    case ENOSYS:
+        return Errno::NoSys;
+    case ENOTCONN:
+        return Errno::NotConnected;
+    case ENOTDIR:
+        return Errno::NotDirectory;
+    case ENOTEMPTY:
+        return Errno::NotEmpty;
+    case ENOTRECOVERABLE:
+        return Errno::NotRecoverable;
+    case ENOTSOCK:
+        return Errno::NotSocket;
+    case ENOTSUP:
+        return Errno::NotSupported;
+    case ENOTTY:
+        return Errno::NoTTY;
+    case ENXIO:
+        return Errno::NXIO;
+    case EOVERFLOW:
+        return Errno::Overflow;
+    case EPERM:
+        return Errno::Permission;
+    case EPIPE:
+        return Errno::Pipe;
+    case EPROTO:
+        return Errno::Protocol;
+    case EPROTONOSUPPORT:
+        return Errno::ProtocolNotSupported;
+    case EPROTOTYPE:
+        return Errno::ProtocolType;
+    case ERANGE:
+        return Errno::Range;
+    case ESPIPE:
+        return Errno::SPipe;
+    case ESRCH:
+        return Errno::SRCH;
+    case ESTALE:
+        return Errno::Stale;
+    case ETIMEDOUT:
+        return Errno::TimedOut;
+    case ETXTBSY:
+        return Errno::TextBusy;
+    case EXDEV:
+        return Errno::XDev;
+    default:
+        return Errno::Invalid;
+    }
+}
+}
diff --git a/Userland/Libraries/LibWasm/Wasi.h b/Userland/Libraries/LibWasm/Wasi.h
new file mode 100644
index 0000000000..0dedce568d
--- /dev/null
+++ b/Userland/Libraries/LibWasm/Wasi.h
@@ -0,0 +1,943 @@
+/*
+ * Copyright (c) 2023, Ali Mohammad Pur <mpfard@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/DistinctNumeric.h>
+#include <AK/Endian.h>
+#include <AK/Function.h>
+#include <AK/LexicalPath.h>
+#include <AK/RedBlackTree.h>
+#include <AK/String.h>
+#include <AK/Vector.h>
+#include <LibWasm/AbstractMachine/AbstractMachine.h>
+#include <LibWasm/Forward.h>
+
+namespace Wasm::Wasi::ABI {
+
+// NOTE: The "real" ABI used in the wild is described by [api.h from libc-bottom-half](https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h)
+//       This is *not* the same ABI as the one described in the WASI spec, nor is it the same ABI as api.h on wasi-libc/master.
+// The highlights of the ABI are:
+// - (most) structs are passed as pointers to heap.
+// - arrays are fat pointers splat across two arguments
+// - return object locations are also passed as arguments, the number of arguments depends on the return type itself:
+//    - ArgsSizes / EnvironSizes / the return type of sock_recv use two arguments
+//    - everything else is passed like a normal struct
+
+template<auto impl>
+struct InvocationOf {
+    HostFunction operator()(Implementation&, StringView name);
+};
+
+template<typename T, size_t N>
+void serialize(T const&, Array<Bytes, N>);
+
+template<typename T, size_t N>
+T deserialize(Array<ReadonlyBytes, N> const&);
+
+template<typename T>
+struct ToCompatibleValue {
+    using Type = void;
+};
+
+template<typename T>
+struct CompatibleValue {
+    typename ToCompatibleValue<T>::Type value;
+
+    Wasm::Value to_wasm_value() const;
+};
+
+template<typename T>
+CompatibleValue<T> to_compatible_value(Wasm::Value const&);
+
+template<typename T>
+T deserialize(CompatibleValue<T> const&);
+
+}
+
+namespace Wasm::Wasi {
+
+// NOTE: This is a copy of LittleEndian from Endian.h,
+//       we can't use those because they have a packed attribute, and depend on it;
+//       but we want proper alignment on these types.
+template<typename T>
+class alignas(T) LittleEndian {
+public:
+    constexpr LittleEndian() = default;
+
+    constexpr LittleEndian(T value)
+        : m_value(AK::convert_between_host_and_little_endian(value))
+    {
+    }
+
+    constexpr operator T() const { return AK::convert_between_host_and_little_endian(m_value); }
+    constexpr T value() const { return AK::convert_between_host_and_little_endian(m_value); }
+
+    LittleEndian& operator+=(T other)
+    {
+        m_value = AK::convert_between_host_and_little_endian(AK::convert_between_host_and_little_endian(m_value) + other);
+        return *this;
+    }
+
+    // This returns the internal representation. In this case, that is the value stored in little endian format.
+    constexpr Bytes bytes() { return Bytes { &m_value, sizeof(m_value) }; }
+    constexpr ReadonlyBytes bytes() const { return ReadonlyBytes { &m_value, sizeof(m_value) }; }
+
+    void serialize_into(Array<Bytes, 1> bytes) const;
+    static LittleEndian read_from(Array<ReadonlyBytes, 1> const& bytes);
+
+private:
+    T m_value { 0 };
+};
+
+using Size = LittleEndian<u32>;
+using FileSize = LittleEndian<u64>;
+using Timestamp = LittleEndian<u64>;
+
+namespace Detail {
+template<typename>
+struct __Pointer_tag;
+template<typename>
+struct __ConstPointer_tag;
+}
+
+// NOTE: Might need to be updated if WASI ever supports memory64.
+using UnderlyingPointerType = u32;
+
+template<typename T>
+using Pointer = DistinctNumeric<LittleEndian<UnderlyingPointerType>, Detail::__Pointer_tag<T>, AK::DistinctNumericFeature::Comparison>;
+template<typename T>
+using ConstPointer = DistinctNumeric<LittleEndian<UnderlyingPointerType>, Detail::__ConstPointer_tag<T>, AK::DistinctNumericFeature::Comparison>;
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L70
+enum class ClockID : u32 {
+    Realtime,
+    Monotonic,
+    ProcessCPUTimeID,
+    ThreadCPUTimeID,
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L105
+enum class Errno : u16 {
+    Success,
+    TooBig,
+    Access,
+    AddressInUse,
+    AddressNotAvailable,
+    AFNotSupported,
+    Again,
+    Already,
+    BadF,
+    BadMessage,
+    Busy,
+    Canceled,
+    Child,
+    ConnectionAborted,
+    ConnectionRefused,
+    ConnectionReset,
+    Deadlock,
+    DestinationAddressRequired,
+    Domain,
+    DQuot, // Reserved, Unused.
+    Exist,
+    Fault,
+    FBig,
+    HostUnreachable,
+    IdentifierRemoved,
+    IllegalSequence,
+    InProgress,
+    Interrupted,
+    Invalid,
+    IO,
+    IsConnected,
+    IsDirectory,
+    Loop,
+    MFile,
+    MLink,
+    MessageSize,
+    MultiHop, // Reserved, Unused.
+    NameTooLong,
+    NetworkDown,
+    NetworkReset,
+    NetworkUnreachable,
+    NFile,
+    NoBufferSpace,
+    NoDevice,
+    NoEntry,
+    NoExec,
+    NoLock,
+    NoLink,
+    NoMemory,
+    NoMessage,
+    NoProtocolOption,
+    NoSpace,
+    NoSys,
+    NotConnected,
+    NotDirectory,
+    NotEmpty,
+    NotRecoverable,
+    NotSocket,
+    NotSupported,
+    NoTTY,
+    NXIO,
+    Overflow,
+    OwnerDead,
+    Permission,
+    Pipe,
+    Protocol,
+    ProtocolNotSupported,
+    ProtocolType,
+    Range,
+    ReadOnlyFS,
+    SPipe,
+    SRCH,
+    Stale,
+    TimedOut,
+    TextBusy,
+    XDev,
+    NotCapable,
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L498
+struct Rights {
+    using CompatibleType = u64;
+
+    struct Bits {
+        bool fd_datasync : 1;
+        bool fd_read : 1;
+        bool fd_seek : 1;
+        bool fd_fdstat_set_flags : 1;
+        bool fd_sync : 1;
+        bool fd_tell : 1;
+        bool fd_write : 1;
+        bool fd_advise : 1;
+        bool fd_allocate : 1;
+        bool path_create_directory : 1;
+        bool path_create_file : 1;
+        bool path_link_source : 1;
+        bool path_link_target : 1;
+        bool path_open : 1;
+        bool fd_readdir : 1;
+        bool path_readlink : 1;
+        bool path_rename_source : 1;
+        bool path_rename_target : 1;
+        bool path_filestat_get : 1;
+        bool path_filestat_set_size : 1;
+        bool path_filestat_set_times : 1;
+        bool fd_filestat_get : 1;
+        bool fd_filestat_set_size : 1;
+        bool fd_filestat_set_times : 1;
+        bool path_symlink : 1;
+        bool path_remove_directory : 1;
+        bool path_unlink_file : 1;
+        bool poll_fd_readwrite : 1;
+        bool sock_shutdown : 1;
+        bool sock_accept : 1;
+
+        u64 _unused : 34;
+    };
+
+    static_assert(sizeof(Bits) == sizeof(CompatibleType));
+
+    union {
+        Bits bits;
+        LittleEndian<CompatibleType> data;
+    };
+
+    void serialize_into(Array<Bytes, 1> bytes) const;
+    static Rights read_from(Array<ReadonlyBytes, 1> const& bytes);
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L663
+using FD = DistinctNumeric<LittleEndian<u32>, struct __FD_tag, AK::DistinctNumericFeature::Comparison>;
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L671
+struct IOVec {
+    Pointer<u8> buf;
+    Size buf_len;
+
+    static IOVec read_from(Array<ReadonlyBytes, 1> const& bytes);
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L692
+struct CIOVec {
+    ConstPointer<u8> buf;
+    Size buf_len;
+
+    static CIOVec read_from(Array<ReadonlyBytes, 1> const& bytes);
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L713
+using FileDelta = LittleEndian<i64>;
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L721
+enum class Whence : u8 {
+    Set,
+    Cur,
+    End,
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L746
+using DirCookie = LittleEndian<u64>;
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L754
+using DirNameLen = LittleEndian<u32>;
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L762
+using INode = LittleEndian<u64>;
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L770
+enum class FileType : u8 {
+    Unknown,
+    BlockDevice,
+    CharacterDevice,
+    Directory,
+    RegularFile,
+    SocketDGram,
+    SocketStream,
+    SymbolicLink,
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L818
+struct DirEnt {
+    DirCookie d_next;
+    INode d_ino;
+    DirNameLen d_namlen;
+    FileType d_type;
+    u8 _padding[3] { 0 }; // Not part of the API, but the struct is required to be 24 bytes - even though it has no explicit padding.
+};
+static_assert(sizeof(DirEnt) == 24);
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L851
+enum class Advice : u8 {
+    Normal,
+    Sequential,
+    Random,
+    WillNeed,
+    DontNeed,
+    NoReuse,
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L889
+struct FDFlags {
+    using CompatibleType = u16;
+
+    struct Bits {
+        bool append : 1;
+        bool dsync : 1;
+        bool nonblock : 1;
+        bool rsync : 1;
+        bool sync : 1;
+        u16 _unused : 11;
+    };
+    static_assert(sizeof(Bits) == sizeof(CompatibleType));
+
+    union {
+        Bits bits;
+        LittleEndian<CompatibleType> data;
+    };
+
+    void serialize_into(Array<Bytes, 1> bytes) const;
+    static FDFlags read_from(Array<ReadonlyBytes, 1> const& bytes);
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L924
+struct FDStat {
+    FileType fs_filetype;
+    FDFlags fs_flags;
+    Rights fs_rights_base;
+    Rights fs_rights_inheriting;
+
+    void serialize_into(Array<Bytes, 1> bytes) const;
+};
+static_assert(sizeof(FDStat) == 24);
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L959
+using Device = LittleEndian<u64>;
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L967
+struct FSTFlags {
+    using CompatibleType = u16;
+
+    struct Bits {
+        bool atim : 1;
+        bool atim_now : 1;
+        bool mtim : 1;
+        bool mtim_now : 1;
+        u16 _unused : 12;
+    };
+
+    static_assert(sizeof(Bits) == sizeof(CompatibleType));
+
+    union {
+        Bits bits;
+        LittleEndian<CompatibleType> data;
+    };
+
+    void serialize_into(Array<Bytes, 1> bytes) const;
+    static FSTFlags read_from(Array<ReadonlyBytes, 1> const& bytes);
+};
+static_assert(sizeof(FSTFlags) == 2);
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L995
+struct LookupFlags {
+    using CompatibleType = u32;
+
+    struct Bits {
+        bool symlink_follow : 1;
+        u32 _unused : 31;
+    };
+
+    static_assert(sizeof(Bits) == sizeof(CompatibleType));
+
+    union {
+        Bits bits;
+        LittleEndian<CompatibleType> data;
+    };
+
+    void serialize_into(Array<Bytes, 1> bytes) const;
+    static LookupFlags read_from(Array<ReadonlyBytes, 1> const& bytes);
+};
+static_assert(sizeof(LookupFlags) == 4);
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1008
+struct OFlags {
+    using CompatibleType = u16;
+
+    struct Bits {
+        bool creat : 1;
+        bool directory : 1;
+        bool excl : 1;
+        bool trunc : 1;
+
+        u16 _unused : 12;
+    };
+
+    static_assert(sizeof(Bits) == sizeof(CompatibleType));
+
+    union {
+        Bits bits;
+        LittleEndian<CompatibleType> data;
+    };
+
+    static OFlags read_from(Array<ReadonlyBytes, 1> const& bytes);
+};
+static_assert(sizeof(OFlags) == 2);
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1036
+using LinkCount = LittleEndian<u64>;
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1044
+struct FileStat {
+    Device dev;
+    INode ino;
+    FileType filetype;
+    LinkCount nlink;
+    FileSize size;
+    Timestamp atim;
+    Timestamp mtim;
+    Timestamp ctim;
+
+    void serialize_into(Array<Bytes, 1> bytes) const;
+};
+static_assert(sizeof(FileStat) == 64);
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1102
+using UserData = LittleEndian<u64>;
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1110
+enum class EventType : u8 {
+    Clock,
+    FDRead,
+    FDWrite,
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1137
+struct EventRWFlags {
+    using CompatibleType = u16;
+
+    struct Bits {
+        bool fd_readwrite_hangup : 1;
+
+        u16 _unused : 15;
+    };
+
+    static_assert(sizeof(Bits) == sizeof(CompatibleType));
+
+    union {
+        Bits bits;
+        LittleEndian<CompatibleType> data;
+    };
+
+    void serialize_into(Array<Bytes, 1> bytes) const;
+    static EventRWFlags read_from(Array<ReadonlyBytes, 1> const& bytes);
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1151
+struct EventFDReadWrite {
+    FileSize nbytes;
+    EventRWFlags flags;
+
+    void serialize_into(Array<Bytes, 1> bytes) const;
+    static EventFDReadWrite read_from(Array<ReadonlyBytes, 1> const& bytes);
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1186
+struct Event {
+    UserData userdata;
+    Errno errno_;
+    EventType type;
+    EventFDReadWrite fd_readwrite;
+
+    void serialize_into(Array<Bytes, 1> bytes) const;
+    static Event read_from(Array<ReadonlyBytes, 1> const& bytes);
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1220
+struct SubClockFlags {
+    using CompatibleType = u16;
+
+    struct Bits {
+        bool subscription_clock_abstime : 1;
+
+        u16 _unused : 15;
+    };
+    static_assert(sizeof(Bits) == sizeof(CompatibleType));
+
+    union {
+        Bits bits;
+        LittleEndian<CompatibleType> data;
+    };
+
+    void serialize_into(Array<Bytes, 1> bytes) const;
+    static SubClockFlags read_from(Array<ReadonlyBytes, 1> const& bytes);
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1237
+struct SubscriptionClock {
+    ClockID id;
+    Timestamp timeout;
+    Timestamp precision;
+    SubClockFlags flags;
+
+    void serialize_into(Array<Bytes, 1> bytes) const;
+    static SubscriptionClock read_from(Array<ReadonlyBytes, 1> const& bytes);
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1272
+struct SubscriptionFDReadWrite {
+    FD file_descriptor;
+
+    void serialize_into(Array<Bytes, 1> bytes) const;
+    static SubscriptionFDReadWrite read_from(Array<ReadonlyBytes, 1> const& bytes);
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1287
+struct SubscriptionU {
+    u8 tag;
+    union {
+        SubscriptionClock clock;
+        SubscriptionFDReadWrite fd_read;
+        SubscriptionFDReadWrite fd_write;
+    };
+
+    void serialize_into(Array<Bytes, 1> bytes) const;
+    static SubscriptionU read_from(Array<ReadonlyBytes, 1> const& bytes);
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1306
+struct Subscription {
+    UserData userdata;
+    SubscriptionU u;
+
+    void serialize_into(Array<Bytes, 1> bytes) const;
+    static Subscription read_from(Array<ReadonlyBytes, 1> const& bytes);
+};
+static_assert(sizeof(Subscription) == 48);
+static_assert(alignof(Subscription) == 8);
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1334
+using ExitCode = LittleEndian<u32>;
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1342
+enum class Signal : u8 {
+    None,
+    HUP,
+    INT,
+    QUIT,
+    ILL,
+    TRAP,
+    ABRT,
+    BUS,
+    FPE,
+    KILL,
+    USR1,
+    SEGV,
+    USR2,
+    PIPE,
+    ALRM,
+    TERM,
+    CHLD,
+    CONT,
+    STOP,
+    TSTP,
+    TTIN,
+    TTOU,
+    URG,
+    XCPU,
+    XFSZ,
+    VTALRM,
+    PROF,
+    WINCH,
+    POLL,
+    PWR,
+    SYS,
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1536
+struct RIFlags {
+    using CompatibleType = u16;
+
+    struct Bits {
+        bool recv_peek : 1;
+        bool recv_waitall : 1;
+
+        u16 _unused : 14;
+    };
+    static_assert(sizeof(Bits) == sizeof(CompatibleType));
+
+    union {
+        Bits bits;
+        LittleEndian<CompatibleType> data;
+    };
+
+    static RIFlags read_from(Array<ReadonlyBytes, 1> const& bytes);
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1554
+struct ROFlags {
+    using CompatibleType = u16;
+
+    struct Bits {
+        bool recv_data_truncated : 1;
+
+        u16 _unused : 15;
+    };
+    static_assert(sizeof(Bits) == sizeof(CompatibleType));
+
+    union {
+        Bits bits;
+        LittleEndian<CompatibleType> data;
+    };
+
+    void serialize_into(Array<Bytes, 1> bytes) const;
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1568
+using SIFlags = LittleEndian<u16>;
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1576
+struct SDFlags {
+    using CompatibleType = u8;
+
+    struct Bits {
+        bool rd : 1;
+        bool wr : 1;
+
+        u8 _unused : 6;
+    };
+    static_assert(sizeof(Bits) == sizeof(CompatibleType));
+
+    union {
+        Bits bits;
+        LittleEndian<CompatibleType> data;
+    };
+
+    void serialize_into(Array<Bytes, 1> bytes) const;
+    static SDFlags read_from(Array<ReadonlyBytes, 1> const& bytes);
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1594
+enum class PreOpenType : u8 {
+    Dir,
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1607
+struct PreStatDir {
+    Size pr_name_len;
+
+    void serialize_into(Array<Bytes, 1> bytes) const;
+    static PreStatDir read_from(Array<ReadonlyBytes, 1> const& bytes);
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1636
+struct PreStat {
+    u8 tag;
+    union {
+        PreStatDir dir;
+    };
+
+    void serialize_into(Array<Bytes, 1> bytes) const;
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1676
+struct ArgsSizes {
+    Size count;
+    Size size;
+
+    using SerializationComponents = TypeList<Size, Size>;
+
+    void serialize_into(Array<Bytes, 2> bytes) const;
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L1708
+struct EnvironSizes {
+    Size count;
+    Size size;
+
+    using SerializationComponents = TypeList<Size, Size>;
+
+    void serialize_into(Array<Bytes, 2> bytes) const;
+};
+
+// https://github.com/WebAssembly/wasi-libc/blob/2c2fc9a2fddd0927a66f1c142e65c8dab6f5c5d7/libc-bottom-half/headers/public/wasi/api.h#L2664
+struct SockRecvResult {
+    Size size;
+    ROFlags roflags;
+
+    using SerializationComponents = TypeList<Size, ROFlags>;
+
+    void serialize_into(Array<Bytes, 2> bytes) const;
+};
+
+template<typename TResult, typename Tag = u32>
+struct Result {
+    Result(TResult&& result)
+        : bits {}
+        , tag(0)
+    {
+        new (&bits) TResult(move(result));
+    }
+
+    Result(Errno&& error)
+        : bits {}
+        , tag(1)
+    {
+        new (&bits) Errno(error);
+    }
+
+    Optional<TResult&> result() const
+    {
+        if (tag == 0)
+            return *bit_cast<TResult*>(&bits[0]);
+        return {};
+    }
+
+    Optional<Errno&> error() const
+    {
+        if (tag == 1)
+            return *bit_cast<Errno*>(&bits[0]);
+        return {};
+    }
+
+    bool is_error() const { return tag == 1; }
+
+    template<size_t N>
+    Errno serialize_into(Array<Bytes, N>&& spans) const
+    {
+        if (tag == 1)
+            return error().value();
+
+        ABI::serialize(*result(), move(spans));
+        return Errno::Success;
+    }
+
+private:
+    alignas(max(alignof(TResult), alignof(Errno))) u8 bits[max(sizeof(TResult), sizeof(Errno))];
+    LittleEndian<Tag> tag;
+};
+
+template<typename Tag>
+struct Result<void, Tag> {
+    Result()
+        : error_bits {}
+        , tag(0)
+    {
+    }
+
+    Result(Errno&& error)
+        : error_bits {}
+        , tag(1)
+    {
+        new (&error_bits) Errno(error);
+    }
+
+    Optional<Empty> result() const
+    {
+        if (tag == 0)
+            return { Empty {} };
+        return {};
+    }
+    Optional<Errno&> error() const
+    {
+        if (tag == 1)
+            return *bit_cast<Errno*>(&error_bits[0]);
+        return {};
+    }
+    bool is_error() const { return tag == 1; }
+
+private:
+    alignas(Errno) u8 error_bits[sizeof(Errno)];
+    LittleEndian<Tag> tag;
+};
+
+struct Implementation {
+    struct MappedPath {
+        LexicalPath host_path;
+        LexicalPath mapped_path;
+        mutable Optional<int> opened_fd {};
+    };
+
+    struct Details {
+        Function<Vector<AK::String>()> provide_arguments;
+        Function<Vector<AK::String>()> provide_environment;
+        Function<Vector<MappedPath>()> provide_preopened_directories;
+    };
+
+    explicit Implementation(Details&& details)
+        : provide_arguments(move(details.provide_arguments))
+        , provide_environment(move(details.provide_environment))
+        , provide_preopened_directories(move(details.provide_preopened_directories))
+    {
+        // Map all of std{in,out,err} by default.
+        m_fd_map.insert(0, 0);
+        m_fd_map.insert(1, 1);
+        m_fd_map.insert(2, 2);
+    }
+
+    ErrorOr<HostFunction> function_by_name(StringView);
+
+private:
+    template<auto impl>
+    HostFunction invocation_of(StringView name) { return ABI::InvocationOf<impl> {}(*this, name); }
+
+    ErrorOr<Result<void>> impl$args_get(Configuration&, Pointer<Pointer<u8>> argv, Pointer<u8> argv_buf);
+    ErrorOr<Result<ArgsSizes>> impl$args_sizes_get(Configuration&);
+    ErrorOr<Result<void>> impl$environ_get(Configuration&, Pointer<Pointer<u8>> environ, Pointer<u8> environ_buf);
+    ErrorOr<Result<EnvironSizes>> impl$environ_sizes_get(Configuration&);
+    ErrorOr<Result<Timestamp>> impl$clock_res_get(Configuration&, ClockID id);
+    ErrorOr<Result<Timestamp>> impl$clock_time_get(Configuration&, ClockID id, Timestamp precision);
+    ErrorOr<Result<void>> impl$fd_advise(Configuration&, FD, FileSize offset, FileSize len, Advice);
+    ErrorOr<Result<void>> impl$fd_allocate(Configuration&, FD, FileSize offset, FileSize len);
+    ErrorOr<Result<void>> impl$fd_close(Configuration&, FD);
+    ErrorOr<Result<void>> impl$fd_datasync(Configuration&, FD);
+    ErrorOr<Result<FDStat>> impl$fd_fdstat_get(Configuration&, FD);
+    ErrorOr<Result<void>> impl$fd_fdstat_set_flags(Configuration&, FD, FDFlags);
+    ErrorOr<Result<void>> impl$fd_fdstat_set_rights(Configuration&, FD, Rights fs_rights_base, Rights fs_rights_inheriting);
+    ErrorOr<Result<FileStat>> impl$fd_filestat_get(Configuration&, FD);
+    ErrorOr<Result<void>> impl$fd_filestat_set_size(Configuration&, FD, FileSize);
+    ErrorOr<Result<void>> impl$fd_filestat_set_times(Configuration&, FD, Timestamp atim, Timestamp mtim, FSTFlags);
+    ErrorOr<Result<Size>> impl$fd_pread(Configuration&, FD, Pointer<IOVec> iovs, Size iovs_len, FileSize offset);
+    ErrorOr<Result<PreStat>> impl$fd_prestat_get(Configuration&, FD);
+    ErrorOr<Result<void>> impl$fd_prestat_dir_name(Configuration&, FD, Pointer<u8> path, Size path_len);
+    ErrorOr<Result<Size>> impl$fd_pwrite(Configuration&, FD, Pointer<CIOVec> iovs, Size iovs_len, FileSize offset);
+    ErrorOr<Result<Size>> impl$fd_read(Configuration&, FD, Pointer<IOVec> iovs, Size iovs_len);
+    ErrorOr<Result<Size>> impl$fd_readdir(Configuration&, FD, Pointer<u8> buf, Size buf_len, DirCookie cookie);
+    ErrorOr<Result<void>> impl$fd_renumber(Configuration&, FD from, FD to);
+    ErrorOr<Result<FileSize>> impl$fd_seek(Configuration&, FD, FileDelta offset, Whence whence);
+    ErrorOr<Result<void>> impl$fd_sync(Configuration&, FD);
+    ErrorOr<Result<FileSize>> impl$fd_tell(Configuration&, FD);
+    ErrorOr<Result<Size>> impl$fd_write(Configuration&, FD, Pointer<CIOVec> iovs, Size iovs_len);
+    ErrorOr<Result<void>> impl$path_create_directory(Configuration&, FD, Pointer<u8> path, Size path_len);
+    ErrorOr<Result<FileStat>> impl$path_filestat_get(Configuration&, FD, LookupFlags, ConstPointer<u8> path, Size path_len);
+    ErrorOr<Result<void>> impl$path_filestat_set_times(Configuration&, FD, LookupFlags, Pointer<u8> path, Size path_len, Timestamp atim, Timestamp mtim, FSTFlags);
+    ErrorOr<Result<void>> impl$path_link(Configuration&, FD, LookupFlags, Pointer<u8> old_path, Size old_path_len, FD, Pointer<u8> new_path, Size new_path_len);
+    ErrorOr<Result<FD>> impl$path_open(Configuration&, FD, LookupFlags, Pointer<u8> path, Size path_len, OFlags, Rights fs_rights_base, Rights fs_rights_inheriting, FDFlags fd_flags);
+    ErrorOr<Result<Size>> impl$path_readlink(Configuration&, FD, LookupFlags, Pointer<u8> path, Size path_len, Pointer<u8> buf, Size buf_len);
+    ErrorOr<Result<void>> impl$path_remove_directory(Configuration&, FD, Pointer<u8> path, Size path_len);
+    ErrorOr<Result<void>> impl$path_rename(Configuration&, FD, Pointer<u8> old_path, Size old_path_len, FD, Pointer<u8> new_path, Size new_path_len);
+    ErrorOr<Result<void>> impl$path_symlink(Configuration&, Pointer<u8> old_path, Size old_path_len, FD, Pointer<u8> new_path, Size new_path_len);
+    ErrorOr<Result<void>> impl$path_unlink_file(Configuration&, FD, Pointer<u8> path, Size path_len);
+    ErrorOr<Result<Size>> impl$poll_oneoff(Configuration&, ConstPointer<Subscription> in, Pointer<Event> out, Size nsubscriptions);
+    ErrorOr<Result<void>> impl$proc_exit(Configuration&, ExitCode); // Note: noreturn.
+    ErrorOr<Result<void>> impl$proc_raise(Configuration&, Signal);
+    ErrorOr<Result<void>> impl$sched_yield(Configuration&);
+    ErrorOr<Result<void>> impl$random_get(Configuration&, Pointer<u8> buf, Size buf_len);
+    ErrorOr<Result<FD>> impl$sock_accept(Configuration&, FD fd, FDFlags fd_flags);
+    ErrorOr<Result<SockRecvResult>> impl$sock_recv(Configuration&, FD fd, Pointer<IOVec> ri_data, Size ri_data_len, RIFlags ri_flags);
+    ErrorOr<Result<Size>> impl$sock_send(Configuration&, FD fd, Pointer<CIOVec> si_data, Size ri_data_len, SIFlags si_flags);
+    ErrorOr<Result<void>> impl$sock_shutdown(Configuration&, FD fd, SDFlags how);
+
+    Vector<AK::String> const& arguments() const;
+    Vector<AK::String> const& environment() const;
+    Vector<MappedPath> const& preopened_directories() const;
+
+    using PreopenedDirectoryDescriptor = DistinctNumeric<LittleEndian<size_t>, struct PreopenedDirectoryDescriptor_tag, AK::DistinctNumericFeature::Comparison, AK::DistinctNumericFeature::CastToUnderlying, AK::DistinctNumericFeature::Increment>;
+    using UnmappedDescriptor = DistinctNumeric<LittleEndian<size_t>, struct UnmappedDescriptor_tag, AK::DistinctNumericFeature::Comparison, AK::DistinctNumericFeature::CastToUnderlying>;
+    using MappedDescriptor = Variant<u32, PreopenedDirectoryDescriptor>;
+    using Descriptor = Variant<u32, PreopenedDirectoryDescriptor, UnmappedDescriptor>;
+
+    Descriptor map_fd(FD);
+
+public:
+    Function<Vector<AK::String>()> provide_arguments;
+    Function<Vector<AK::String>()> provide_environment;
+    Function<Vector<MappedPath>()> provide_preopened_directories;
+
+private:
+    struct Cache {
+        Optional<Vector<AK::String>> cached_arguments;
+        Optional<Vector<AK::String>> cached_environment;
+        Optional<Vector<MappedPath>> cached_preopened_directories;
+    };
+
+    mutable Cache cache {};
+
+    RedBlackTree<u32, MappedDescriptor> m_fd_map;
+    size_t m_first_unmapped_preopened_directory_index { 0 };
+};
+
+#undef IMPL
+
+}
+
+namespace Wasm::Wasi::ABI {
+
+template<typename T, typename... Args>
+struct ToCompatibleValue<DistinctNumeric<T, Args...>> {
+    using Type = typename ToCompatibleValue<T>::Type;
+};
+
+template<typename T>
+struct ToCompatibleValue<LittleEndian<T>> {
+    using Type = MakeSigned<T>;
+};
+
+template<typename T>
+requires(requires { declval<typename T::CompatibleType>(); })
+struct ToCompatibleValue<T> {
+    using Type = MakeSigned<typename T::CompatibleType>;
+};
+
+template<Integral T>
+struct ToCompatibleValue<T> {
+    using Type = MakeSigned<T>;
+};
+
+template<Enum T>
+struct ToCompatibleValue<T> {
+    using Type = MakeSigned<UnderlyingType<T>>;
+};
+
+}
+
+template<typename T>
+struct AK::Formatter<Wasm::Wasi::LittleEndian<T>> : AK::Formatter<T> {
+    ErrorOr<void> format(FormatBuilder& builder, Wasm::Wasi::LittleEndian<T> value)
+    {
+        return Formatter<T>::format(builder, value.operator T());
+    }
+};