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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 16 Aug 2017 11:26:37 -0700
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Subject: Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name
To: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Al Viro <viro@zeniv.linux.org.uk>
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On Wed, Aug 16, 2017 at 10:12 AM, Christian Brauner
<christian.brauner@ubuntu.com> wrote:
>
> Recently the kernel has implemented the TIOCGPTPEER ioctl() call which allows
> users to retrieve an fd for the slave side of a pty solely based on the
> corresponding fd for the master side. The ioctl()-based fd retrieval however
> causes the "/proc/<pid>/fd/<pty-slave-fd>" symlink to point to the wrong dentry.
> Currently, it will always point to "/". The following simple program can be used
> to illustrate the problem when run on a system that implements the TIOCGPTPEER
> ioctl() call.

I think your patch is wrong - we need to actually use the *right*
path, rather than hardcode "/dev/pts/%d" in there.

Hardcoding "/dev/pts/%d" is something that user space can already do.
The kernel can and should do better.

That "dynamic_dname()" helper is for things that don't really have a
real pathname at all, so things like pipes and other file descriptors
that were opened without an associated entry in a filesystem (sockets,
other "special" inodes with no filesystem backing).

For things like pts slaves, we actually *do* have a real pathname -
and we should expose it. If the devpts filesystem is mounted somewhere
else than /dev/pts, it should give that correct pathname.

I also think your test program is a bit buggy, and silly:

>         ret = snprintf(path, 4096, "/proc/self/fd/%d", slave);
>         if (ret < 0 || ret >= 4096)
>                 goto on_error;
>
>         ret = readlink(path, buf, sizeof(buf));
>         if (ret < 0)
>                 goto on_error;
>         printf("I point to \"%s\"\n", buf);

This just smells wrong to me. And not just because you don't
NUL-terminate the readlink() return value (or just use "%.*s" with
ret, buf) .

We should just give people a better way to get the pathname than that
readlink on /proc/self/fd/<fd> thing. It's kind of ridiculous that we
don't. We already have a "getcwd()" system call that only does it for
cwd.

So I think we could/should just add a system call for 'fdname()' or similar.

                Linus