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From: Frederic Weisbecker on 8 Apr 2010 07:00
On Thu, Apr 08, 2010 at 11:57:20AM +0200, Eric Dumazet wrote:
> Hello
>
> Current linux-2.6 tree panics on my dev machine
>
> 64 bit kernel, 32bit user land
> CONFIG_FRAME_POINTER=y
>
> perf timechart record &
>
> Instant crash
>
> Call Trace:
> perf_trace_sched_switch+0xd5/0x120
> schedule+0x6b5/0x860
> retint_careful+0xd/0x21
>
> RIP ffffffff81010955 perf_arch_fetch_caller_regs+0x15/0x40
> CR2: 00000000d21f1422
>
>
> rewind_frame_pointer() is probably wrong.
>
> No test performed to check frame is in current stack, or
> that (!user_mode_vm(regs))



user_mode_vm() can not work here as we are actually filling
regs from scratch.

But we indeed need to have a safe dereference to avoid such
crashes. A simple probe_kernel_address() should do the trick.

This API is going to change for the next cycle as it won't need
to rewind further than the first caller. So I'm going to do a
rough probe_kernel_address() fix for the current version. The next
one won't have this problem.



>
>
> static inline unsigned long rewind_frame_pointer(int n)
> {
> struct stack_frame *frame;
>
> get_bp(frame);
>
> #ifdef CONFIG_FRAME_POINTER
> while (n--)
> frame = frame->next_frame;
> #endif
>
> return (unsigned long)frame;
> }
>
>
>

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