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

[Andy Lutomirski]

On Fri, Dec 11, 2015 at 2:35 PM, Eric W. Biederman
<ebiederm@xxxxxxxxxxxx> wrote:
> Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:
>
>> On Fri, Dec 11, 2015 at 2:07 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>>> On 12/11/15 13:48, Andy Lutomirski wrote:
>>>> On Fri, Dec 11, 2015 at 1:11 PM, Eric W. Biederman
>>>> <ebiederm@xxxxxxxxxxxx> wrote:
>>>>> Al Viro <viro@xxxxxxxxxxxxxxxxxx> writes:
>>>>>
>>>>>> On Fri, Dec 11, 2015 at 01:40:40PM -0600, Eric W. Biederman wrote:
>>>>>>
>>>>>>> +    inode = path.dentry->d_inode;
>>>>>>> +    filp->f_path = path;
>>>>>>> +    filp->f_inode = inode;
>>>>>>> +    filp->f_mapping = inode->i_mapping;
>>>>>>> +    path_put(&old);
>>>>>>
>>>>>> Don't.  You are creating a fairly subtle constraint on what the code in
>>>>>> fs/open.c and fs/namei.c can do, for no good reason.  You can bloody
>>>>>> well maintain the information you need without that.
>>>>>
>>>>> There is a good reason.  We can not write a race free version of ptsname
>>>>> without it.
>>>>
>>>> As long as this is for new userspace code, would it make sense to just
>>>> add a new ioctl to ask "does this ptmx fd match this /dev/pts fd?"
>>>>
>>>
>>> For the newinstance case st_dev should match between the master and the
>>> slave.  Unfortunately this is not the case for a legacy ptmx, as a
>>> stat() on the master descriptor still returns the st_dev, st_rdev, and
>>> st_ino for the ptmx device node.
>>
>> Sure, but I'm not talking about stat.  I'm saying that we could add a
>> new ioctl that works on any ptmx fd (/dev/ptmx or /dev/pts/ptmx) that
>> answers the question "does this ptmx logically belong to the given
>> devpts filesystem".
>>
>> Since it's not stat, we can make it do whatever we want, including
>> following a link to the devpts instance that isn't f_path or f_inode.
>
> The useful ioctl to add in my opinion would be one that actually opens
> the slave, at which point ptsname could become ttyname, and that closes
> races in grantpt.

Unfortunately, ptsname is POSIX, so we can't get rid of it.  It's a
bad idea, but it's in the standard.

>
> I even posted an implementation earlier in the discussion and no one was
> interested.
>
> Honestly the more weird special cases we add to devpts the less likely
> userspace will be to get things right.  We have been trying since 1998
> and devpts is still a poor enough design we have not been able to get
> rid of /usr/lib/pt_chown.  Adding another case where we have to sand on
> one foot and touch our nose does not seem to likely to achieve
> widespread adoption.  How many version of libc are there now?

Old libc can stay buggy, I think.  Given that this mess is partially a
libc mis-design, I don't see why it needs to be fixed entirely in the
kernel.

For new systems, it would be really nice if we can make a /dev/ptmx
symlink be 100% functional.

In any event, this is semi-moot.  If we actually want to retire the
newinstance=0 thing from the kernel, we apparently need a magic
/dev/ptmx node.  That doesn't mean that new userspace needs to *use*
that magic node.  So why not implement the magic node without fiddling
with f_path?

--Andy
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