Bug 2431

Summary:

Crash in int10 module with linuxthreads/TLS glibc

Product:

xorg

Reporter:

J. Scott Berg <jsberg>

Component:

Server/DDX/Xorg

Assignee:

Xorg Project Team <xorg-team>

Status:

RESOLVED FIXED

QA Contact:

Severity:

critical

Priority:

medium

CC:

ajax, eich

Version:

6.8.1

Hardware:

x86 (IA32)

OS:

Linux (All)

Whiteboard:

i915 platform:

i915 features:

Attachments:

Description	Flags
Patch to save the ds register before the vm86old syscall	none

Description J. Scott Berg 2005-01-31 07:27:05 UTC

First of all, I have a bit of an unusual config, I think: 

Linux, kernel 2.4.29
glibc-2.3.4, built with
../glibc-2.3.4/configure " '--prefix=/usr'
'--enable-add-ons=linuxthreads,libidn' '--enable-kernel=2.4.18'
This means that glibc is built with linuxthreads and TLS.

Basically, the X server segfaults within the libint10.a code, or just after
(hard to tell, of course).

I tracked this down, and think I know what is going on.  The problem is that the
vm86old syscall which is executed by vm86_rep clobbers the gs segment register.
 But TLS (at least with linuxthreads) uses the gs register to keep track of
where the thread local storage is (which is evidently used even when you don't
use threads...).  Thus, the next time malloc is called, we segfault and die.  My
simple solution was to push the gs register before the int $0x80 call and
restore it after (gcc didn't seem to recognize the gs register for a clobber spec).

From looking on Google, there is at least one other person with this same
problem, so my crazy configuration is not completely unique...

Comment 1 J. Scott Berg 2005-01-31 07:28:20 UTC

Created attachment 1797 [details] [review]
Patch to save the ds register before the vm86old syscall

Comment 2 Adam Jackson 2005-01-31 11:23:22 UTC

wow, good catch.  i'll take a look at this one.

Comment 3 Egbert Eich 2005-02-07 09:09:23 UTC

The patch looks sane. 
Ajax, I've already prepared everything in my tree to commit this.
If you haven't done that already I can commit it.

Comment 4 Adam Jackson 2005-02-07 09:33:44 UTC

go for it.

Comment 5 Egbert Eich 2005-02-07 10:16:40 UTC

Committed to HEAD

2005-02-07  Egbert Eich  <eich-at-freedesktop-dot-org>

        * programs/Xserver/hw/xfree86/os-support/linux/int10/vm86/linux_vm86.c: 
(vm86_rep):
        Save gs register before calling the vm86_old syscall thru
        int 0x80. This is required for linuxthreads as the TLS uses
        this register to keep track of local thread storage
        (Bugzilla #2431, J. Scott Berg).

Use of freedesktop.org services, including Bugzilla, is subject to our Code of Conduct. How we collect and use information is described in our Privacy Policy.