linux-kernel - Re: [PATCH 0/1] devpts: use dynamic

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Message-ID: <CA+55aFyy1RU2emO6giU9gNVb_doYP+K_y61ij2cumgs1_VMMmA@mail.gmail.com>
Date:   Wed, 16 Aug 2017 11:48:48 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name

On Wed, Aug 16, 2017 at 11:26 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> Hardcoding "/dev/pts/%d" is something that user space can already do.
> The kernel can and should do better.

Put another way: there's no point in applying the patch as-is, since
existing glibc ptsname() does the same thing better and faster
entirely in user space.

Also, we already do special things to get a path for this, but it
clearly isn't working. See the

        /* We need to cache a fake path for TIOCGPTPEER. */

comment in ptmx_open(). Why doesn't the file d_path get filled in
correctly there, I wonder.

Because The regular

        readlink("/proc/self/fd/0", ...)

that 'tty' does works correctly.  I think we've done something
incorrect in pty_open_peer(), which means that the fd path hasn't been
fully filled in.

Fixing that *should* fix the readlink() automatically, since it
clearly works for the 'tty' binary.

I'm wondering why it's not working as-is. "vfs_open()" does that

        file->f_path = *path;

thing. Why aren't we getting the right path? The ptmx_open() code
looks ok to me.

Al, do you see what the issue is, and why we don't get a proper path
on that readlink?

            Linus