Re: [Bug 221881] New: Documenting if both /dev/ptmx and /dev/pts/ptmx can be accessed at the same time.

[Alejandro Colomar <alx@kernel.org>]

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Hi Paolo,

> Date: 2026-08-13 14:20:33+0000
> From: bugzilla-daemon@kernel.org
>
> https://bugzilla.kernel.org/show_bug.cgi?id=221881
> 
>             Bug ID: 221881
>            Summary: Documenting if both /dev/ptmx and /dev/pts/ptmx can be
>                     accessed at the same time.
>            Product: Documentation
>            Version: unspecified
>           Hardware: All
>                 OS: Linux
>             Status: NEW
>           Severity: low
>           Priority: P3
>          Component: man-pages
>           Assignee: documentation_man-pages@kernel-bugs.osdl.org
>           Reporter: dedo.paolo.24+linux@proton.me
>         Regression: No
> 
> Traditionally pseudoterminals are created through the device file /dev/ptmx.
> With the introduction of the devpts virtual filesystem, new slave
> pseudoterminals are created through /dev/pts/ptmx device file and allocated in
> /dev/pts/*. In order to mantain backward compatibility, the device file
> /dev/ptmx has not been dropped byt instead requests to /dev/ptmx are forwarded
> to /dev/pts/ptmx.
> 
> However, this is not preserved by bind mounts to /dev/ptmx: for example, after
> launching
> 
> "# mount --bind /dev/ptmx /dev/ptmx"
> 
> any attempt to opening /dev/ptmx will fail. Instead, bind mounting
> /dev/pts/ptmx works as expected (if devptsfs has previously been mounted with
> ptmxmode=0666). This is quite problematic if one wants to give access to
> /dev/ptmx inside a mount namespace. The solution provided in
> https://docs.kernel.org/filesystems/devpts.html is to remount devptsfs with the
> mount option ptmxmode=0666, then remove /dev/ptmx and finally place a bind
> mount or a symlink pointing to /dev/pts/ptmx.
> 
> The issues involving documentations are the following:
> 
> 1. man 4 pts only describes the old interface /dev/ptmx and the newer defice
> file /dev/pts/ptmx is never mentioned. /dev/pts/ptmx should at least be
> mentioned as an alternative for /dev/ptmx both to help software maintainers
> moving to the new interface and to provide access to pseudoterminals inside
> mount namespaces;
> 
> 2. nowhere inside the documentation is explained if it is safe to provide
> access to both /dev/ptmx and /dev/pts/ptmx at the same time. All provided
> scenarios give access to exactly one of /dev/ptmx and /dev/pts/ptmx device
> files. If we have to avoid giving full access to both /dev/ptmx and
> /dev/pts/ptmx through ptmxmode=0666 (to avoid race conditions) then it should
> be explicitly stated. On the contrary, a note that says there are no problems
> in giving full access to both these device files at the same time should be
> added.

If you know how to solve those issues, would you mind sending a patch?
If so, plese have a look at
<https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING>
and
<https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING.d/>.

If you don't know these, you could maybe ask through email to the kernel
maintainers of those features, and CC the linux-man@ mailing list, so we
learn that information.  (The links above will also help you.)


Have a lovely day!
Alex

-- 
<https://www.alejandro-colomar.es>

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