linux-kernel - Re: [PATCH v4 2/5] fs: Add fchmodat2()

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Date:   Fri, 28 Jul 2023 14:42:12 -0400
From:   "dalias@...c.org" <dalias@...c.org>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Aleksa Sarai' <cyphar@...har.com>,
        Alexey Gladkov <legion@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
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        Palmer Dabbelt <palmer@...ive.com>
Subject: Re: [PATCH v4 2/5] fs: Add fchmodat2()

On Fri, Jul 28, 2023 at 08:43:58AM +0000, David Laight wrote:
> ....
> > FWIW, I agree with Christian that these behaviours are not ideal (and
> > I'm working on a series that might allow for these things to be properly
> > blocked in the future) but there's also the consistency argument -- I
> > don't think fchownat() is much safer to allow in this way than
> > fchmodat() and (again) this behaviour is already possible through
> > procfs.
> 
> If the 'through procfs' involves readlink("/proc/self/fd/n") and
> accessing through the returned path then the permission checks
> are different.
> Using the returned path requires search permissions on all the
> directories.

That's *not* how "through procfs" works. The "magic symlinks" in
/proc/*/fd are not actual symlinks that get dereferenced to the
contents they readlink() to, but special-type objects that dereference
directly to the underlying file associated with the open file
description.

Rich