Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance

[Eric W. Biederman]

ebiederm@xxxxxxxxxxxx (Eric W. Biederman) writes:

>>> In that system ptys simply did not work after boot when I tested
>>> associating /dev/ptmx with the first mount of the devpts filesystem.
>>
>> Assuming userspace isn't broken by that patch, is a fixed association
>> with first mount otherwise an acceptable solution for magic /dev/ptmx
>> (where /dev/ptmx is not a symlink to /dev/pts/ptmx)?
>
> I do not believe a fixed association with the first mount is an
> acceptable solution for implementing  /dev/ptmx in association with
> a change to cause mount of devpts to be an independent filesystem.
> Such an association fails to be backwards compatible with existing
> userspace, and it is extremely fragile.

Ugh.  After reviewing the userspace code that mounts devpts we have
to do use a magic /dev/ptmx to solve the issue we are trying to solve.

The fragility of detecting the primary system devpts seems solvable.

CentOS5 and openwrt-15.05 mount devpts, unmount devpts,
then mount devpts again.  So a rule of mouting the internal devpts if it
isn't mounted would work for those.

CentOS6 uses switch_root and moves it's early mount of devpts onto the
primary root, and then because devpts is also in /etc/fstab tries and
fails to mount devpts once more at the same location.  Implying
newinstance will make that mounting devpts twice.  That sounds solvable
but I don't see a clean way of detecting that case yet.

Ugh.

I am going to pound my head up against what is needed to find the
primary system mount of devpts for a bit more and see if I can solve
that.  Otherwise this exercise is pointless.

Eric

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