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

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Message-ID: <20230727-zerrt-leitmotiv-9e8b60abf690@brauner>
Date:   Thu, 27 Jul 2023 19:02:53 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Andreas Schwab <schwab@...ux-m68k.org>
Cc:     David Laight <David.Laight@...LAB.COM>,
        'Aleksa Sarai' <cyphar@...har.com>,
        Alexey Gladkov <legion@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
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        Palmer Dabbelt <palmer@...ive.com>
Subject: Re: [PATCH v4 2/5] fs: Add fchmodat2()

On Thu, Jul 27, 2023 at 06:28:53PM +0200, Andreas Schwab wrote:
> On Jul 27 2023, David Laight wrote:
> 
> > From: Aleksa Sarai
> >> Sent: 25 July 2023 17:36
> > ...
> >> We almost certainly want to support AT_EMPTY_PATH at the same time.
> >> Otherwise userspace will still need to go through /proc when trying to
> >> chmod a file handle they have.
> >
> > That can't be allowed.
> 
> IIUC, fchmodat2(fd, "", m, AT_EMPTY_PATH) is equivalent to fchmod(fd,
> m).  With that, new architectures only need to implement the fchmodat2
> syscall to cover all chmod variants.

There's a difference though as fchmod() doesn't work with O_PATH file
descriptors while AT_EMPTY_PATH does. Similar to how fchown() doesn't
work with O_PATH file descriptors.

However, we do allow AT_EMPTY_PATH with fchownat() so there's no reason
to not allow it for fchmodat2().

But it's a bit of a shame that O_PATH looks less and less like O_PATH.
It came from can-do-barely-anything to can-do-quite-a-lot-of-things over
the years.

In any case, AT_EMPTY_PATH for fchmodat2() can be an additional patch on
top.