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

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Message-ID: <20170430043822.GE27790@bombadil.infradead.org>
Date:   Sat, 29 Apr 2017 21:38:22 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Andy Lutomirski <luto@...nel.org>,
        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 Sun, Apr 30, 2017 at 12:25:04AM +0100, Al Viro wrote:
> 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.

It sounds more like AT_NO_ESCAPE ... or AT_BELOW, or something.  Perhaps
some example usages in the changelog?