Information about bind mounts and simultaneous access to /dev/ptmx and /dev/pts/ptmx

Dear Alexander Viro,

I have noticed that trying to open /dev/ptmx device file through a bind mount will always fail. More precisely, calling

    open("/dev/ptmx", O_RDWR)

usually succeeds, however it will always fail if you previously launched the following command:

   # mount --bind /dev/ptmx /dev/ptmx

I think the root cause of this is the relationship between /dev/ptmx and the devpts virtual filesystem, usually mounted at /dev/pts since all operations on /dev/ptmx are forwarded to /dev/pts/ptmx. This issue makes unfeasible to share access to /dev/ptmx through a mount namespace, however bind mounts work as expected with /dev/pts/ptmx (if devpts has been mounted with the option ptmxmode=0666).

My question is if it is safe to give full access to both /dev/ptmx and /dev/pts/ptmx (through ptmxmode=0666) at the same time, or if there are some racing conditions that would make simultaneous access insecure. Documentation at https://docs.kernel.org/filesystems/devpts.html and man 4 pts say nothing about this scenario, but I think this information will be very helpful for all people that needs having pseudoterminal access inside a mount namespace.

Bests,
Paolo