Re: [PATCH v3 1/2] exec: fix up /proc/pid/comm in the execveat(AT_EMPTY_PATH) case

[Aleksa Sarai]

On 2024-10-01, Tycho Andersen <tycho@tycho.pizza> wrote:
> From: Tycho Andersen <tandersen@xxxxxxxxxxx>
> 
> 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

Reviewed-by: Aleksa Sarai <cyphar@xxxxxxxxxx>

> Signed-off-by: Tycho Andersen <tandersen@xxxxxxxxxxx>
> Suggested-by: Zbigniew Jędrzejewski-Szmek <zbyszek@xxxxxxxxx>
> CC: Aleksa Sarai <cyphar@xxxxxxxxxx>
> 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/>

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