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From:   Amir Goldstein <amir73il@gmail.com>
Date:   Fri, 13 May 2022 12:36:44 +0300
Message-ID: <CAOQ4uxiw0gaa+=afk=Avnr+8+DiyP9CRgUNUDK6NYZbo+Z7dOQ@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>
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On Thu, May 12, 2022 at 3:38 PM Simon Ser <contact@emersion.fr> wrote:
>
> On Thursday, May 12th, 2022 at 14:30, Amir Goldstein <amir73il@gmail.com> wrote:
>
> > Clients can also readlink("/proc/self/fd/<fd>") to get the path of the file
> > and open it from its path (if path is accessible in their mount namespace).
>
> What the compositor does is:
>
> - shm_open with O_RDWR
> - Write the kyeboard keymap
> - shm_open again the same file with O_RDONLY
> - shm_unlink
> - Send the O_RDONLY FD to clients
>
> Thus, the file doesn't exist anymore when clients get the FD.

>From system POV, a readonly bind mount of /dev/shm
could be created (e.g. at /dev/shm-ro) and then wayland could open
the shm rdonly file from that path.

If wayland cannot rely on system to create the bind mount for it,
it could also clone its own mount namespace and create the
bind mount in its own namespace for opening the rdonly file.

But that implementation would be Linux specific an Linux has many
other APIs that were suggested on the linked gitlab issue.
You did not mention them in your question and did not say why
those solutions are not a good enough for your needs.

Thanks,
Amir.