linux-kernel - Re: new ...at() flag: AT_NO

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Message-ID: <20170429232504.GU29622@ZenIV.linux.org.uk>
Date:   Sun, 30 Apr 2017 00:25:04 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Linux API <linux-api@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: new ...at() flag: AT_NO_JUMPS

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@...iv.linux.org.uk> 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.