linux-kernel - Re: [PATCH v3 1/2] exec: fix up /proc/pid/comm in the execveat(AT_EMPTY

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Message-ID: <20241001.184625-homely.tarps.selfish.shotgun-bJjSBGdHDLKb@cyphar.com>
Date: Tue, 1 Oct 2024 20:48:06 +0200
From: Aleksa Sarai <cyphar@...har.com>
To: Tycho Andersen <tycho@...ho.pizza>
Cc: Alexander Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, Eric Biederman <ebiederm@...ssion.com>, 
	Kees Cook <kees@...nel.org>, linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	Tycho Andersen <tandersen@...flix.com>, Zbigniew Jędrzejewski-Szmek <zbyszek@...waw.pl>
Subject: Re: [PATCH v3 1/2] exec: fix up /proc/pid/comm in the
 execveat(AT_EMPTY_PATH) case

On 2024-10-01, Aleksa Sarai <cyphar@...har.com> wrote:
> On 2024-10-01, Tycho Andersen <tycho@...ho.pizza> wrote:
> > From: Tycho Andersen <tandersen@...flix.com>
> > 
> > Zbigniew mentioned at Linux Plumber's that systemd is interested in
> > switching to execveat() for service execution, but can't, because the
> > contents of /proc/pid/comm are the file descriptor which was used,
> > instead of the path to the binary. This makes the output of tools like
> > top and ps useless, especially in a world where most fds are opened
> > CLOEXEC so the number is truly meaningless.
> > 
> > Change exec path to fix up /proc/pid/comm in the case where we have
> > allocated one of these synthetic paths in bprm_init(). This way the actual
> > exec machinery is unchanged, but cosmetically the comm looks reasonable to
> > admins investigating things.
> 
> While I still think the argv[0] solution was semantically nicer, it
> seems this is enough to fix the systemd problem for most cases and so we
> can revisit the argv[0] discussion in another 10 years. :D

Of course, this assumes the busybox problem I mentioned really is not an
issue. But at least this option is "less wrong" than using the fd
number. I suspect we will eventually need the argv[0] thing.

> Reviewed-by: Aleksa Sarai <cyphar@...har.com>
> 
> > Signed-off-by: Tycho Andersen <tandersen@...flix.com>
> > Suggested-by: Zbigniew Jędrzejewski-Szmek <zbyszek@...waw.pl>
> > CC: Aleksa Sarai <cyphar@...har.com>
> > Link: https://github.com/uapi-group/kernel-features#set-comm-field-before-exec
> > ---
> > v2: * drop the flag, everyone :)
> >     * change the rendered value to f_path.dentry->d_name.name instead of
> >       argv[0], Eric
> > v3: * fix up subject line, Eric
> > ---
> >  fs/exec.c | 13 ++++++++++++-
> >  1 file changed, 12 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/exec.c b/fs/exec.c
> > index dad402d55681..9520359a8dcc 100644
> > --- a/fs/exec.c
> > +++ b/fs/exec.c
> > @@ -1416,7 +1416,18 @@ int begin_new_exec(struct linux_binprm * bprm)
> >  		set_dumpable(current->mm, SUID_DUMP_USER);
> >  
> >  	perf_event_exec();
> > -	__set_task_comm(me, kbasename(bprm->filename), true);
> > +
> > +	/*
> > +	 * If fdpath was set, execveat() made up a path that will
> > +	 * probably not be useful to admins running ps or similar.
> > +	 * Let's fix it up to be something reasonable.
> > +	 */
> > +	if (bprm->fdpath) {
> > +		BUILD_BUG_ON(TASK_COMM_LEN > DNAME_INLINE_LEN);
> > +		__set_task_comm(me, bprm->file->f_path.dentry->d_name.name, true);
> > +	} else {
> > +		__set_task_comm(me, kbasename(bprm->filename), true);
> > +	}
> >  
> >  	/* An exec changes our domain. We are no longer part of the thread
> >  	   group */
> > 
> > base-commit: baeb9a7d8b60b021d907127509c44507539c15e5
> > -- 
> > 2.34.1
> > 
> 
> -- 
> Aleksa Sarai
> Senior Software Engineer (Containers)
> SUSE Linux GmbH
> <https://www.cyphar.com/>



-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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