Re: [PATCH] vfs: relax linkat() AT_EMPTY_PATH - aka flink() - requirements

[Linus Torvalds <torvalds@linux-foundation.org>]

On Sat, 13 Apr 2024 at 08:16, Christian Brauner <brauner@kernel.org> wrote:
>
> I think it should be ok to allow AT_EMPTY_PATH with NULL because
> userspace can detect whether the kernel allows that by passing
> AT_EMPTY_PATH with a NULL path argument and they would get an error back
> that would tell them that this kernel doesn't support NULL paths.

Yeah, it should return -1 / EFAULT on  older kernels.

> I'd like to try a patch for this next week. It's a good opportunity to
> get into some of the more gritty details of this area.
>
> From a rough first glance most AT_EMPTY_PATH users should be covered by
> adapting getname_flags() accordingly.
>
> Imho, this could likely be done by introducing a single struct filename
> null_filename.

It's probably better to try to special-case it entirely.

See commit 9013c51c630a ("vfs: mostly undo glibc turning 'fstat()'
into 'fstatat(AT_EMPTY_PATH)'") and the numbers in there in
particular.

That still leaves performance on the table exactly because it has to
do that extra "get_user()" to check for an empty path, but it avoids
not only the pathname allocation, but also the setup for the pathname
walk.

If we had a NULL case there, I'd expect that fstatat() and fstat()
would perform the same (modulo a couple of instructions).

Of course, the performance of get_user() will vary depending on
microarchitecture. If you don't have SMAP, it's cheap. It's the
STAC/CLAC that is most of the cost, and the exact cost of those will
then depend on implementations - they *could* be much faster than they
are.

              Linus