Re: [PATCH 01/13] devpts: Teach /dev/ptmx to find the associated devpts via path lookup

[Eric W. Biederman]

"H. Peter Anvin" <hpa@xxxxxxxxx> writes:

> On April 9, 2016 6:09:09 AM PDT, One Thousand Gnomes <gnomes@xxxxxxxxxxxxxxxxxxx> wrote:
>>
>>> If anyone has a better idea on how userspace should connect the
>>master
>>> pty file descriptor the slave file descriptor, I would be willing to
>>> implement that instead.
>>
>>If we are willing to go away from the existing mess of a tty interface
>>inflicted on us by BSD and then mashed up by POSIX then a syscall of
>>
>>  int err = ptypair(int fd[2], int perms, int flags);
>>
>>[where flags is the O_ ones we usually need to cover (CLOEXEC etc) and
>>maybe even some kind of "private" flag to say don't even expose it via
>>devpts).
>>
>>would do remarkably sane things to the majoirty of use cases as it
>>breaks
>>the dependence on grantpt and also the historic screwup that pty pairs
>>aren't allocated atomically with both file handles returned as pipe()
>>does.
>>
>>Alan
>
> We don't even need to do that if we'd be willing to change the user
> space interface... if we could rely on the POSIX interface then
> posix_openpt() could simply open /dev/pts/ptmx and everything would
> just work.

At a quick skim it does look like userspace uses posix_openpt for the
most part.  Certainly portable apps that can run on FreeBSD do.
And just grepping through binaries all of the ones I have checked so far
are calling posix_openpt.

Peter if you or someone could start updating the userspace version of
posix_openpt to use /dev/pts/ptmx when available over /dev/ptmx in
parallel to the kernel work to always allow mount of devpts to give
distinct instances that would be great.

> The trick here is how to make it work in the presence of some
> extremely bad practices in existing userspace.

Yeah.  I am going to look and see if I can move this controversial bit
to a separate patch so we can discuss it more conviniently.

Eric