Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <lGo7a4qQABKb-u_xsz6p-QtLIy2bzciBLTUJ7-ksv7ppK3mRrJhXqFmCFU4AtQf6EyrZUrYuSLDMBHEUMe5st_iT9VcRuyYPMU_jVpSzoWg=@emersion.fr>
 <03l0hfZIzD9KwSxSntGcmfFhvbIKiK45poGUhXtR7Qi0Av0-ZnqnSBPAP09GGpSrKGZWZNCTvme_Gpiuz0Bcg6ewDIXSH24SBx_tvfyZSWU=@emersion.fr>
In-Reply-To: <03l0hfZIzD9KwSxSntGcmfFhvbIKiK45poGUhXtR7Qi0Av0-ZnqnSBPAP09GGpSrKGZWZNCTvme_Gpiuz0Bcg6ewDIXSH24SBx_tvfyZSWU=@emersion.fr>
From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Thu, 12 May 2022 14:56:22 +0200
Message-ID: <CAJfpegs4GVirNVtf4OqunzNwbXQywZVkxpGPtpN=ZonHU2SpiA@mail.gmail.com>
Subject: Re: procfs: open("/proc/self/fd/...") allows bypassing O_RDONLY
To:     Simon Ser <contact@emersion.fr>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Thu, 12 May 2022 at 14:41, Simon Ser <contact@emersion.fr> wrote:
>
> On Thursday, May 12th, 2022 at 12:37, Simon Ser <contact@emersion.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