linux-kernel - Re: procfs: open("/proc/self/fd/...") allows bypassing O

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Message-ID: <CAJfpegs4GVirNVtf4OqunzNwbXQywZVkxpGPtpN=ZonHU2SpiA@mail.gmail.com>
Date:   Thu, 12 May 2022 14:56:22 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Simon Ser <contact@...rsion.fr>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: procfs: open("/proc/self/fd/...") allows bypassing O_RDONLY

On Thu, 12 May 2022 at 14:41, Simon Ser <contact@...rsion.fr> wrote:
>
> On Thursday, May 12th, 2022 at 12:37, Simon Ser <contact@...rsion.fr> wrote:
>
> > what would be a good way to share a FD to another
> > process without allowing it to write to the underlying file?
>
> (I'm reminded that memfd + seals exist for this purpose. Still, I'd be
> interested to know whether that O_RDONLY/O_RDWR behavior is intended,
> because it's pretty surprising. The motivation for using O_RDONLY over
> memfd seals is that it isn't Linux-specific.)

Yes, this is intended.   The /proc/$PID/fd/$FD file represents the
inode pointed to by $FD.   So the open flags for $FD are irrelevant
when operating on the proc fd file.

Thanks,
Miklos