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

[H. Peter Anvin]

On April 9, 2016 7:45:46 AM PDT, ebiederm@xxxxxxxxxxxx wrote:
>"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

On the flipside, if we were to allow ourselves to break userspace, at this point I would suggest making /dev/pts/ptmx have a different device number and make the legacy /dev/ptmx print a warning message, after which it can at least eventually be deleted.

This might not be a bad idea anyway.
-- 
Sent from my Android device with K-9 Mail. Please excuse brevity and formatting.