Kernel: Implement bind mounts

You can now bind-mount files and directories. This essentially exposes an
existing part of the file system in another place, and can be used as an
alternative to symlinks or hardlinks.

Here's an example of doing this:

    # mkdir /tmp/foo
    # mount /home/anon/myfile.txt /tmp/foo -o bind
    # cat /tmp/foo
    This is anon's file.

3 files changed, 29 insertions, 0 deletions

diff --git a/Kernel/FileSystem/VirtualFileSystem.cpp b/Kernel/FileSystem/VirtualFileSystem.cpp
index ccdf193182..6ed65df5e4 100644
--- a/Kernel/FileSystem/VirtualFileSystem.cpp
+++ b/Kernel/FileSystem/VirtualFileSystem.cpp
@@ -49,6 +49,16 @@ KResult VFS::mount(FS& file_system, Custody& mount_point, int flags)
     return KSuccess;
 }
 
+KResult VFS::bind_mount(Custody& source, Custody& mount_point)
+{
+    dbg() << "VFS: Bind-mounting " << source.absolute_path() << " at " << mount_point.absolute_path();
+    // FIXME: check that this is not already a mount point
+    Mount mount { source.inode(), mount_point };
+    m_mounts.append(move(mount));
+    mount_point.did_mount_on({});
+    return KSuccess;
+}
+
 KResult VFS::unmount(InodeIdentifier guest_inode_id)
 {
     LOCKER(m_lock);
@@ -617,6 +627,14 @@ VFS::Mount::Mount(FS& guest_fs, Custody* host_custody, int flags)
 {
 }
 
+VFS::Mount::Mount(Inode& source, Custody& host_custody)
+    : m_guest(source.identifier())
+    , m_guest_fs(source.fs())
+    , m_host_custody(host_custody)
+    , m_flags(MS_BIND)
+{
+}
+
 String VFS::Mount::absolute_path() const
 {
     if (!m_host_custody)
diff --git a/Kernel/FileSystem/VirtualFileSystem.h b/Kernel/FileSystem/VirtualFileSystem.h
index 1a7f580997..c062d7bba1 100644
--- a/Kernel/FileSystem/VirtualFileSystem.h
+++ b/Kernel/FileSystem/VirtualFileSystem.h
@@ -48,6 +48,7 @@ public:
     class Mount {
     public:
         Mount(FS&, Custody* host_custody, int flags);
+        Mount(Inode& source, Custody& host_custody);
 
         InodeIdentifier host() const;
         InodeIdentifier guest() const { return m_guest; }
@@ -73,6 +74,7 @@ public:
 
     bool mount_root(FS&);
     KResult mount(FS&, Custody& mount_point, int flags);
+    KResult bind_mount(Custody& source, Custody& mount_point);
     KResult unmount(InodeIdentifier guest_inode_id);
 
     KResultOr<NonnullRefPtr<FileDescription>> open(StringView path, int options, mode_t mode, Custody& base, Optional<UidAndGid> = {});
diff --git a/Kernel/Process.cpp b/Kernel/Process.cpp
index 81e382e86e..38b6933edd 100644
--- a/Kernel/Process.cpp
+++ b/Kernel/Process.cpp
@@ -3602,6 +3602,15 @@ int Process::sys$mount(const Syscall::SC_mount_params* user_params)
 
     RefPtr<FS> fs;
 
+    if (params.flags & MS_BIND) {
+        // We're doing a bind mount.
+        auto source_or_error = VFS::the().resolve_path(source, current_directory());
+        if (source_or_error.is_error())
+            return source_or_error.error();
+        auto& source_custody = source_or_error.value();
+        return VFS::the().bind_mount(source_custody, target_custody);
+    }
+
     if (fs_type == "ext2" || fs_type == "Ext2FS") {
         auto metadata_or_error = VFS::the().lookup_metadata(source, current_directory());
         if (metadata_or_error.is_error())