Re: [RFC] exec: add a flag for "reasonable" execveat() comm

[Tycho Andersen <tycho@tycho.pizza>]

On Wed, Sep 25, 2024 at 09:09:18PM -0500, Eric W. Biederman wrote:
> Tycho Andersen <tycho@tycho.pizza> writes:
> 
> > Yep, I did this for the test above, and it worked fine:
> >
> >         if (bprm->fdpath) {
> >                 /*
> >                  * 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.
> >                  */
> >                 struct path root;
> >                 char *path, buf[1024];
> >
> >                 get_fs_root(current->fs, &root);
> >                 path = __d_path(&bprm->file->f_path, &root, buf, sizeof(buf));
> >
> >                 __set_task_comm(me, kbasename(path), true);
> >         } else {
> >                 __set_task_comm(me, kbasename(bprm->filename), true);
> >         }
> >
> > obviously we don't want a stack allocated buffer, but triggering on
> > ->fdpath != NULL seems like the right thing, so we won't need a flag
> > either.
> >
> > The question is: argv[0] or __d_path()?
> 
> You know.  I think we can just do:
> 
> 	BUILD_BUG_ON(DNAME_INLINE_LEN >= TASK_COMM_LEN);
> 	__set_task_comm(me, bprm->file->f_path.dentry->d_name.name, true);
> 
> Barring cache misses that should be faster and more reliable than what
> we currently have and produce the same output in all of the cases we
> like, and produce better output in all of the cases that are a problem
> today.
> 
> Does anyone see any problem with that?

Nice, this works great. We need to drop the BUILD_BUG_ON() since it is
violated in today's tree, but I think this is safe to do anyway since
__set_task_comm() does strscpy_pad(tsk->comm, buf, sizeof(tsk->comm)).

I will respin with this and dropping the flag.

Tycho