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

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Date: Fri, 12 Apr 2024 09:45:54 +0200
From: Christian Brauner <brauner@...nel.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Charles Mirabile <cmirabil@...hat.com>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Andrew Lutomirski <luto@...nel.org>, Peter Anvin <hpa@...or.com>
Subject: Re: [PATCH] vfs: relax linkat() AT_EMPTY_PATH - aka flink() -
 requirements

On Thu, Apr 11, 2024 at 12:34:52PM -0700, Linus Torvalds wrote:
> On Thu, 11 Apr 2024 at 11:13, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > So while I understand your motivation, I actually think it's actively
> > wrong to special-case __O_TMPFILE, because it encourages a pattern
> > that is bad.
> 
> Just to clarify: I think the ns_capable() change is a good idea and
> makes sense. The whole "limited to global root" makes no sense if the
> file was opened within a namespace, and I think it always just came
> from the better check not being obvious at the point where
> AT_EMPTY_PATH was checked for.
> 
> Similarly, while the FMODE_PATH test _looks_ very similar to an
> O_TMPFILE check, I think it's fundamentally different in a conceptual
> sense: not only is FMODE_PATH filesystem-agnostic, a FMODE_PATH file
> is *only* useful as a pathname (ie no read/write semantics).

But the annoying part imho is that we also allow stuff like fsetxattr()
to work on O_PATH file descriptors. Something that I really dislike.

> 
> And so if a FMODE_PATH file descriptor is passed in from the outside,
> I feel like the "you cannot use this to create a path" is kind of a
> fundamentally nonsensical rule.

Hm, I would like to avoid adding an exception for O_PATH. While it is
useful for lookup O_PATH is also often used simply as an opaque handle
to a file. O_PATH is often used to simply identify or compare files, or
for execveat(). Especially for cross-container interactions it would be
nice to not generally allow O_PATH to be linkat()ed. The current
behavior proposed in this patch should be enough.