kernel::fs::file::LocalFile should be marked as repr(transparent)

[pekkarr]

Unlike File which uses repr(transparent), LocalFile uses the implicit repr(Rust) representation which has an unspecified layout. This can break unsafe code that assumes that it has the same layout as bindings::file.

For example, the LocalFile::from_raw_file function casts *const bindings::file to *const LocalFile:

linux/rust/kernel/fs/file.rs

Lines 274 to 280 in f264495

	pub unsafe fn from_raw_file<'a>(ptr: *const bindings::file) -> &'a LocalFile {
	// SAFETY: The caller guarantees that the pointer is not dangling and stays valid for the
	// duration of 'a. The cast is okay because `File` is `repr(transparent)`.
	//
	// INVARIANT: The caller guarantees that there are no problematic `fdget_pos` calls.
	unsafe { &*ptr.cast() }
	}

Note that the safety comment here should be talking about LocalFile instead of File (probably a copy-paste error). Some other comments also refer to the wrong type (lines 227 and 238).

In practice, the default representation most likely uses the same layout as with repr(transparent), but it's best to not rely on that since it's unspecified.

[ojeda]

Sounds good to me.

Cc: @Darksonn @y86-dev

[y86-dev]

yup this should be changed to be repr(transparent) and also update the safety comments. @pekkarr do you want to take a shot at this?

[pekkarr]

Sure! I'll send a patch to the mailing list.

[ojeda]

Thanks!