On Sun, Mar 19, 2017 at 06:46:01PM -0700, Linus Torvalds wrote:
> On Sun, Mar 19, 2017 at 10:24 AM, Al Viro <v...@zeniv.linux.org.uk> wrote:
> >         Bringing back an old conversation - what do you think about the
> > potential usefulness of the following ...at() option:
> >         * no mountpoint crossings allowed (mount --bind included)
> >         * only relative symlinks traversals are allowed
> >         * starting point acts as a chroot boundary (IOW, .. does not lead
> > out of it)
> Hmm. The original use of this was iirc apache (or maybe samba), that
> simply wanted to be sure that when you did a pathname lookup, it
> didn't escape out of the subtree that the lookup was done from (eg
> some public_html directory or whatever).
> But I had that discussions *long* ago, and I don't even remember who
> it might have been with. My reaction at the time was simply that it
> would have been easy for us to have a "no symlink traversal at all"
> mode, because their solution was to just walk each path component
> carefully by hand.
> Maybe somebody who knows apache (or samba) can pipe up and say "yeah,
> we still do that", or "we solved it with our own filename cache, so we
> don't care".

The last posting of the O_BENEATH flag with very similar semantics
indicated that there were sandboxing usecases that still want this [1].
I think it's a good idea; the kernel is in the best place to do this
correctly instead of having a bunch of half-baked implementations in
userspace everywhere.

1: http://marc.info/?l=linux-fsdevel&m=143945842819081&w=2

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