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Subject: Re: [PATCH 01/13] devpts: Teach /dev/ptmx to find the associated devpts via path lookup
From: "H. Peter Anvin" <hpa@zytor.com>
Date: Sat, 09 Apr 2016 17:06:52 -0700
To: Linus Torvalds <torvalds@linux-foundation.org>
CC: "Eric W. Biederman" <ebiederm@xmission.com>,
        One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>,
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On April 9, 2016 5:01:27 PM PDT, Linus Torvalds <torvalds@linux-foundation.org> wrote:
>On Sat, Apr 9, 2016 at 3:37 PM, H. Peter Anvin <hpa@zytor.com> wrote:
>>
>> 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.
>
>You don't need a different device number.
>
>The /dev/pts/ptmx file may look like it's the same node as /dev/ptmx,
>but it is trivial to recognize as the pts one:
>
>     if (dentry->d_sb->s_magic == DEVPTS_SUPER_MAGIC)
>
>and you're done.
>
>But nobody actually uses /dev/pts/ptmx, because it has never had sane
>permissions.
>
>So the fact is, /dev/ptmx is what people use, and we're not breaking
>userspace.
>
>But when we fix bad semantics (and always just looking up the initial
>pts mount really is crazy semantics) that doesn't mean that we have to
>bend over backwards to not make the changed semantics visible. We
>don't _break_ user space, but we also don't care about some random
>test-program that checks for particular semantics.
>
>And I can pretty much _guarantee_ that nobody has ever done the "let's
>bind-mount a 'ptmx' node in a /dev directory, and then expect that to
>bind to some _other_ pts thing than the one in /dev/pts/".
>
>Except as a test-program, or possibly as a "why the f*ck doesn't this
>work? Oh, I need to use the single-instance thing because the
>multi-instance pts thing is broken. Damn shitty implementation".
>
>                Linus

Fixing the default permissions is trivial, of course.  The intent from the beginning was to make a ptmx -> pts/ptmx, but user space never did...
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