Regression in 2.6.30 - /dev/pts - mount options - bisected

Regression in 2.6.30 - /dev/pts - mount options - bisected

[Marc Dionne]

I ran into a regression from 2.6.29 to 2.6.30 on a newly installed
fedora 11 machine.  The symptom is that things that require /dev/pts
(gnome-terminal, konsole, xterm, etc.) don't work for a regular user,
but run fine as root.  Everything is fine with the fedora 2.6.29
kernel or with a self-compiled 2.6.29, but fail with a recent
2.6.30-rc.

Bisection pinpoints this commit:
http://git.kernel.org/linus/1bd7903560f1f713e85188a5aaf4d2428b6c8b50
as the culprit, although I think that's a bit misleading.

The commit moves code around, and along the way replaces this:

  memcpy(&fsi->mount_opts, opts, sizeof(opts))

in one function where opts is a (struct pts_mount_opts *) - 8 bytes,
clearly an error, with

  memcpy(&(DEVPTS_SB(s))->mount_opts, &opts, sizeof(opts));

in a different function where opts is an actual structure (24 bytes),
which looks like what was intended.  So fixing the copy of the mount
options triggers the problem, and the real cause is probably
neighbouring commits in the same patch set.  I confirmed that using
sizeof(&opts) in the above memcpy makes the problem go away (but is
obviously not a solution).

Note that I have CONFIG_DEVPTS_MULTIPLE_INSTANCES=y.  It may be that
user space is doing something unusual, but it passes the test of
"works with 2.6.29, fails with 2.6.30".

I wasn't able to reproduce on an other system with a similar configuration.

Marc

Re: Regression in 2.6.30 - /dev/pts - mount options - bisected

[Geert Uytterhoeven]

On Fri, 8 May 2009, Marc Dionne wrote:
> I ran into a regression from 2.6.29 to 2.6.30 on a newly installed
> fedora 11 machine.  The symptom is that things that require /dev/pts
> (gnome-terminal, konsole, xterm, etc.) don't work for a regular user,
> but run fine as root.  Everything is fine with the fedora 2.6.29
> kernel or with a self-compiled 2.6.29, but fail with a recent
> 2.6.30-rc.

When investigating a different issue on Fedora 11 on PS3, I ran into the same
problem with a 2.6.30-rc4 Fedora kernel, and with plain 2.6.30-rc4 kernels
built using a config based on the Fedora config.

> Bisection pinpoints this commit:
> http://git.kernel.org/linus/1bd7903560f1f713e85188a5aaf4d2428b6c8b50
> as the culprit, although I think that's a bit misleading.

I didn't go a far as bisecting, but strace did show it failed on opening
/dev/pts/something.

> Note that I have CONFIG_DEVPTS_MULTIPLE_INSTANCES=y.  It may be that
> user space is doing something unusual, but it passes the test of
> "works with 2.6.29, fails with 2.6.30".

Yes, it works fine with 2.6.29, so it's a regression.

> I wasn't able to reproduce on an other system with a similar configuration.

Originally, I couldn't reproduce the problem with kernels built using my
default config, which has CONFIG_DEVPTS_MULTIPLE_INSTANCES=n.

And indeed, disabling CONFIG_DEVPTS_MULTIPLE_INSTANCES in the Fedora-based
config makes the problem go away, and brings back my terminal-emulators.

With kind regards,

Geert Uytterhoeven
Software Architect
Techsoft Centre

Technology and Software Centre Europe
The Corporate Village · Da Vincilaan 7-D1 · B-1935 Zaventem · Belgium

Phone:    +32 (0)2 700 8453
Fax:      +32 (0)2 700 8622
E-mail:   Geert.Uytterhoeven@sonycom.com
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Re: Regression in 2.6.30 - /dev/pts - mount options - bisected

[Sukadev Bhattiprolu]

Geert Uytterhoeven [Geert.Uytterhoeven@sonycom.com] wrote:

| And indeed, disabling CONFIG_DEVPTS_MULTIPLE_INSTANCES in the Fedora-based
| config makes the problem go away, and brings back my terminal-emulators.

Yes, a patch is available for this at

	http://lkml.org/lkml/2009/5/11/586

The patch has been merged into -mm.

Thanks,

Sukadev

Re: Regression in 2.6.30 - /dev/pts - mount options - bisected

[Geert Uytterhoeven]

On Wed, 13 May 2009, Sukadev Bhattiprolu wrote:
> Geert Uytterhoeven [Geert.Uytterhoeven@sonycom.com] wrote:
> 
> | And indeed, disabling CONFIG_DEVPTS_MULTIPLE_INSTANCES in the Fedora-based
> | config makes the problem go away, and brings back my terminal-emulators.
> 
> Yes, a patch is available for this at
> 
> 	http://lkml.org/lkml/2009/5/11/586
> 
> The patch has been merged into -mm.

Thanks, I missed that one, as there were no replies to Marc's email.

With kind regards,

Geert Uytterhoeven
Software Architect
Techsoft Centre

Technology and Software Centre Europe
The Corporate Village · Da Vincilaan 7-D1 · B-1935 Zaventem · Belgium

Phone:    +32 (0)2 700 8453
Fax:      +32 (0)2 700 8622
E-mail:   Geert.Uytterhoeven@sonycom.com
Internet: http://www.sony-europe.com/

A division of Sony Europe (Belgium) N.V.
VAT BE 0413.825.160 · RPR Brussels
Fortis · BIC GEBABEBB · IBAN BE41293037680010