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

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: [email protected]
          Reporter: [email protected]
        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.

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