From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org>
Subject: Re: new ...at() flag: AT_NO_JUMPS
Date: Sun, 30 Apr 2017 00:25:04 +0100
Message-ID: <20170429232504.GU29622@ZenIV.linux.org.uk>
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To: Andy Lutomirski <luto-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
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On Sat, Apr 29, 2017 at 04:17:18PM -0700, Andy Lutomirski wrote:
> On Sat, Apr 29, 2017 at 3:04 PM, Al Viro <viro-RmSDqhL/yNMiFSDQTTA3OLVCufUGDwFn@public.gmane.org> wrote:
> > New AT_... flag - AT_NO_JUMPS
> >
> > Semantics: pathname resolution must not involve
> >         * traversals of absolute symlinks
> >         * traversals of procfs-style symlinks
> >         * traversals of mountpoints (including bindings, referrals, etc.)
> >         * traversal of .. in the starting point of pathname resolution.
> 
> Can you clarify this last one?  I assume that ".." will be rejected,
> but what about "a/../.."?  How about "b" if b is a symlink to ".."?
> How about "a/b" if a is a directory and b is a symlink to "../.."?

All of those will be rejected - in each of those cases pathname traversal
leads back into the starting point with .. being the next component to
handle.