Kernel+LibC: Make sys$getcwd truncate the result silently

This gives us the superpower of knowing the ideal buffer length if it fails.
See also https://github.com/SerenityOS/serenity/discussions/4357

2 files changed, 9 insertions, 8 deletions

diff --git a/Kernel/Process.h b/Kernel/Process.h
index 946aee0594..3d8bfba55b 100644
--- a/Kernel/Process.h
+++ b/Kernel/Process.h
@@ -263,7 +263,7 @@ public:
     int sys$select(const Syscall::SC_select_params*);
     int sys$poll(Userspace<const Syscall::SC_poll_params*>);
     ssize_t sys$get_dir_entries(int fd, void*, ssize_t);
-    int sys$getcwd(Userspace<char*>, ssize_t);
+    int sys$getcwd(Userspace<char*>, size_t);
     int sys$chdir(Userspace<const char*>, size_t);
     int sys$fchdir(int fd);
     int sys$sleep(unsigned seconds);
diff --git a/Kernel/Syscalls/chdir.cpp b/Kernel/Syscalls/chdir.cpp
index 46cf3a2459..23555e2cff 100644
--- a/Kernel/Syscalls/chdir.cpp
+++ b/Kernel/Syscalls/chdir.cpp
@@ -61,17 +61,18 @@ int Process::sys$fchdir(int fd)
     return 0;
 }
 
-int Process::sys$getcwd(Userspace<char*> buffer, ssize_t size)
+int Process::sys$getcwd(Userspace<char*> buffer, size_t size)
 {
     REQUIRE_PROMISE(rpath);
-    if (size < 0)
-        return -EINVAL;
+
     auto path = current_directory().absolute_path();
-    if ((size_t)size < path.length() + 1)
-        return -ERANGE;
-    if (!copy_to_user(buffer, path.characters(), path.length() + 1))
+
+    size_t ideal_size = path.length() + 1;
+    auto size_to_copy = min(ideal_size, size);
+    if (!copy_to_user(buffer, path.characters(), size_to_copy))
         return -EFAULT;
-    return 0;
+    // Note: we return the whole size here, not the copied size.
+    return ideal_size;
 }
 
 }