Re: Regression in 2.6.25-rc3: s2ram segfaults before suspending

Ingo Molnar
2008-02-28T09:17:22Z

* Klaus S. Madsen <ksm@hjernemadsen.org> wrote:

> Hi all,
>
> I have a Thinkpad T61p, which I'm able to suspend with s2ram
> on Linux 2.6.24.3. However when I try to suspend it on 2.6.25-rc3,
> s2ram dies after changing to vt1, with a segfault. I'm using s2ram
> from cvs, and libx86 version 0.99 from
> http://www.codon.org.uk/~mjg59/libx86/.
>
> Some details about the segfault:
>
> $ sudo gdb ./s2ram
> (gdb) run
> Starting program: /home/ksm/downloads/suspend/s2ram
> Switching from vt7 to vt1
> Calling get_mode
>
> Program received signal SIGSEGV, Segmentation fault.
> 0xb7facf4a in run_vm86 () at lrmi.c:526
> 526 asm volatile (
> (gdb) list
> 521 static int
> 522 lrmi_vm86(struct vm86_struct *vm)
> 523 {
> 524 int r;
> 525 #ifdef __PIC__
> 526 asm volatile (
> 527 "pushl %%ebx\n\t"
> 528 "movl %2, %%ebx\n\t"
> 529 "int $0x80\n\t"
> 530 "popl %%ebx"
> (gdb) bt
> #0 0xb7facf4a in run_vm86 () at lrmi.c:526
> #1 0xb7fad61b in LRMI_int (i=16, r=0xbffca670) at lrmi.c:844
> #2 0x0804acfc in do_vbe_service (AX=20227, BX=0, regs=0xbffca670)
> at vbetool/vbetool.c:158
> #3 0x0804af7e in __get_mode () at vbetool/vbetool.c:453
> #4 0x0804a30f in s2ram_hacks () at s2ram-x86.c:268
> #5 0x0804954f in main (argc=1, argv=0x0) at s2ram-main.c:92
>
> I have tried to bisect the problem, and it fingered the following
> commit:
>
> commit 82bc03fc158e28c90d7ed9919410776039cb4e14
> Author: Ingo Molnar <mingo@elte.hu>
>
> x86: add PWT to NOCACHE flags
>
> Reverting this commit in the bisected tree (by executing git show
> 82bc03fc158e28c90d7ed9919410776039cb4e14 | patch -R -p1), makes the
> segfault go away. I've run make clean between each kernel compile, to
> be sure the tree was correctly compiled.

thanks for tracking this down. It would be nice to figure out why this
change made a difference. Perhaps VM86 mode has some restrictions in
what type of pagetables it can operate in - and the CPU just refuses to
properly emulate those 16-bit instructions? (this would be very weird).
We are trying to execute 16-bit BIOS code here, right?

which instruction is the segfault coming from - the int $0x80? So in
vm86 mode we generated a #GPF which shows up as a SIGSEGV?

Ingo