linux-kernel - Re: [PATCH v2 1/2] exec: add a flag for "reasonable" execveat() comm

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Message-ID: <87ikuhw155.fsf@email.froward.int.ebiederm.org>
Date: Fri, 27 Sep 2024 10:45:58 -0500
From: "Eric W. Biederman" <ebiederm@...ssion.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>,  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>,  Aleksa Sarai <cyphar@...har.com>
Subject: Re: [PATCH v2 1/2] exec: add a flag for "reasonable" execveat() comm

Tycho Andersen <tycho@...ho.pizza> writes:

> 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.

Perhaps change the subject to match the code.

> 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
> ---
>  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);

We can just do this regardless of bprm->fdpath.

It will be a change of behavior on when executing symlinks and possibly
mount points but I don't think we care.  If we do then we can add make
it conditional with "if (bprm->fdpath)"

At the very least using the above version unconditionally ought to flush
out any bugs.

It should be 99% application invisible as all an application can see
is argv0.  So it is only ps and friends where the comm value is visible.

Eric