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

[Tycho Andersen <tycho@tycho.pizza>]

On Fri, Sep 27, 2024 at 09:43:22AM -0500, Eric W. Biederman wrote:
> Tycho Andersen <tycho@tycho.pizza> writes:
> 
> > 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)).
> 
> Doh.  I simply put the conditional in the wrong order.  That should have
> been:
> 	BUILD_BUG_ON(TASK_COMM_LEN > DNAME_INLINE_LEN);
> 
> Sorry I was thinking of the invariant that needs to be preserved rather
> than the bug that happens.

Thanks, I will include that. Just for my own education: this is still
*safe* to do, because of _pad, it's just that it is a userspace
visible break if TASK_COMM_LEN > DNAME_INLINE_LEN is ever true?

Tycho