Re: [PATCH 1/2] debugfs: Only clobber mode/uid/gid on remount if asked

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Sep 01, 2022 at 03:32:18PM -0700, Brian Norris wrote:
> On Thu, Sep 01, 2022 at 05:58:14PM +0200, Greg Kroah-Hartman wrote:
> > On Fri, Aug 26, 2022 at 05:44:16PM -0700, Brian Norris wrote:
> > > Users may have explicitly configured their debugfs permissions; we
> > > shouldn't overwrite those just because a second mount appeared.
> > 
> > What userspace mounts debugfs twice?
> 
> I'll admit, my particular userspace in question (Chrom{e,ium}OS) does
> not. There are several debugfs mounts, but they are bind mounts, which
> don't hit this problem.
> 
> But Steven hits the nail on the head for most of my reasoning; my main
> motivation is for tracefs (patch 2), whose automount makes this very
> surprising. I included patch 1 for consistency (tracefs essentially
> copy/pasted debugfs). I could drop patch 1 if that helps somehow, but
> I'd still like to consider the automount difficulties in patch 2.
> 
> > > Only clobber if the options were provided at mount time.
> > > 
> > >   # Don't change /sys/kernel/debug/ permissions.
> > >   mount -t debugfs none /mnt/foo
> > > 
> > >   # Change /sys/kernel/debug/ mode and uid, but not gid.
> > >   mount -t debugfs -o uid=bar,mode=0750 none /mnt/baz
> > 
> > So what happens today with this change?  Without it?
> 
> Sorry, this was probably a bit too implied -- the gid changes to its
> default (0 if we never set it in a mount option before; or it will reset
> to any previous gid= mount setting).
> 
> # ls -ld /sys/kernel/debug/.
> drwxr-x---. 45 root debugfs-access 0 Dec 31  1969 /sys/kernel/debug/.
> # chown root:power /sys/kernel/debug/
> # ls -ld /sys/kernel/debug/.
> drwxr-x---. 45 root power 0 Dec 31  1969 /sys/kernel/debug/.
> # mount -t debugfs -o uid=power,mode=0750 none /tmp/mnt
> # ls -ld /sys/kernel/debug/.
> drwxr-x---. 45 power debugfs-access 0 Dec 31  1969 /sys/kernel/debug/.
> # mount | grep '\/sys\/kernel\/debug '
> debugfs on /sys/kernel/debug type debugfs (rw,nosuid,nodev,noexec,relatime,seclabel,uid=228,gid=605,mode=750)
> 
> I can include more before/after examples in the commit message if you
> want. Honestly, that's kind of why I offered to write test cases; test
> cases show what's happening better than narrative descriptions.

before/after in the changelog comments are very good, thanks.  And yes,
tests are also good, I'd gladly take that too if you do a v2 of this
patch.

And make it independant of the tracefs change please, these would go
through two different trees as they have different maintainers.

thanks,

greg k-h