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From: Christoph Hellwig on 11 Feb 2010 12:30
On Wed, Feb 10, 2010 at 12:15:53PM +0100, Miklos Szeredi wrote:
> - renamed flag to UMOUNT_NOFOLLOW
> - added UMOUNT_UNUSED for feature detection

Umm, why? MNT_ certainly isn't the best naming for unmount flags,
but switching convention after we had a few doesn't make any sense.

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From: Miklos Szeredi on 11 Feb 2010 13:10
On Thu, 11 Feb 2010, Christoph Hellwig wrote:
> On Wed, Feb 10, 2010 at 12:15:53PM +0100, Miklos Szeredi wrote:
> > - renamed flag to UMOUNT_NOFOLLOW
> > - added UMOUNT_UNUSED for feature detection
>
> Umm, why? MNT_ certainly isn't the best naming for unmount flags,
> but switching convention after we had a few doesn't make any sense.

It's not just bad naming, having another set of unrelated flags with
the same prefix *and* in close proximity to each other is asking for
trouble.

We could rename all the umount flags, and leave userspace do #defines
for backward compat. It's not as if this was some widely used or
portable API.

Thanks,
Miklos
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From: Al Viro on 20 Feb 2010 21:10
On Thu, Feb 11, 2010 at 12:21:00PM -0500, Christoph Hellwig wrote:
> On Wed, Feb 10, 2010 at 12:15:53PM +0100, Miklos Szeredi wrote:
> > - renamed flag to UMOUNT_NOFOLLOW
> > - added UMOUNT_UNUSED for feature detection
>
> Umm, why? MNT_ certainly isn't the best naming for unmount flags,
> but switching convention after we had a few doesn't make any sense.

Actually, I've got more interesting question: what's being attempted
there? Is that just a "let's protect ourselves against somebody feeding
us an untrusted symlink"? I'm not sure if it makes much sense; if we
are dealing with pathnames on untrusted fs, there's nothing to stop the
attacker from having /mnt/foo/dir (originally containing a mountpoint
at /mnt/foo/dir/usr) killed and replaced with a symlink to /, making any
code that does umount() on such pathnames vulnerable as hell anyway.

Lack of LOOKUP_FOLLOW affects only the last pathname component. So what
is that patch trying to make safe?
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From: Miklos Szeredi on 22 Feb 2010 15:40
On Sun, 21 Feb 2010, Al Viro wrote:
> On Thu, Feb 11, 2010 at 12:21:00PM -0500, Christoph Hellwig wrote:
> > On Wed, Feb 10, 2010 at 12:15:53PM +0100, Miklos Szeredi wrote:
> > > - renamed flag to UMOUNT_NOFOLLOW
> > > - added UMOUNT_UNUSED for feature detection
> >
> > Umm, why? MNT_ certainly isn't the best naming for unmount flags,
> > but switching convention after we had a few doesn't make any sense.
>
> Actually, I've got more interesting question: what's being attempted
> there? Is that just a "let's protect ourselves against somebody feeding
> us an untrusted symlink"? I'm not sure if it makes much sense; if we
> are dealing with pathnames on untrusted fs, there's nothing to stop the
> attacker from having /mnt/foo/dir (originally containing a mountpoint
> at /mnt/foo/dir/usr) killed and replaced with a symlink to /, making any
> code that does umount() on such pathnames vulnerable as hell anyway.

It is trivial to check the path up to the mountpoint (chdir + getcwd).
But doing that on the mountpoint will make it busy, so NOFOLLOW is
really needed there.

Thanks,
Miklos
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