linux-kernel - Re: [GIT PULL] execve updates for v6.13-rc1 (take 2)

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Message-ID: <20241128020558.GF3387508@ZenIV>
Date: Thu, 28 Nov 2024 02:05:58 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Kees Cook <kees@...nel.org>, linux-kernel@...r.kernel.org,
	Christophe JAILLET <christophe.jaillet@...adoo.fr>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Nir Lichtman <nir@...htman.org>,
	Tycho Andersen <tandersen@...flix.com>,
	Vegard Nossum <vegard.nossum@...cle.com>
Subject: Re: [GIT PULL] execve updates for v6.13-rc1 (take 2)

On Wed, Nov 27, 2024 at 05:59:53PM -0800, Linus Torvalds wrote:
> On Wed, 27 Nov 2024 at 16:53, Kees Cook <kees@...nel.org> wrote:
> >
> > On a related note, what do you think of using execveat's "pathname"
> > argument as "comm" if AT_EMPTY_PATH is set? That'll give process
> > launchers control over comm (which is what they want), and we can keep
> > the dentry name fallback as proposed too?
> 
> That's not actually how AT_EMPTY_PATH works.
> 
> Yes, it's how AT_EMPTY_PATH *should* work, but despite the name,
> AT_EMPTYH_PATH does not mean "path is empty".
> 
> It means "path *may* be empty - but if path isn't empty, it's a regular path".
> 
> IOW, what is going on is that POSIX required that an empty path be an
> error. And AT_EMPTY_PATH is basically a "don't error out on an empty
> path" flag, not a "path *is* empty" flag.
> 
> So if pathname exists and isn't empty, AT_EMPTY_PATH does nothing.

... so let's tie that to pathname _being_ empty - it's not as if it
had been hard to check.

What's more, let's allow userland pointer to be NULL - use getname_maybe_null()
and treat NULL returned by it as "we have an empty pathname".