Re: perf_fuzzer compiled for x32 causes reboot

[Vince Weaver]

On Sun, 23 Feb 2014, H. Peter Anvin wrote:

> So we do a write to the buffer rather immediately before this happens,
> and in particular that will update the head:
> 
> 	rb->user_page->data_head = head;
> 
> However, that doesn't explain what is going on and in particular the
> write to whatever address was in %rbp.  The rest pretty much seems to be
> the page fault logic.

It turns out you don't even have to over-write rb->user_page->data_head.
Just touching the mmap page with a write of a single byte (it doesn't 
matter where) is enough to trigger the bug.

This is a pain to track down, it would be easier if I could get a 
replayable syscall trace, but even though the segfault is very 
reproducible with my fuzzer, it's very sensitive to extra syscalls in the 
trace path and the fuzzer logger/replayer path has a different number of 
write syscalls and won't trigger the problem.

> Incidentally, I doubt that this is x32-related in any way; there seems
> to be absolutely no difference between x86-64 perf and x32 perf; more
> likely it just makes the error more reproducible because the address
> space is so much smaller.

quite possibly.  I only began chasing the problem because when compiled 
for x32 this bug apparently will reboot the machine now and then (not just 
segfault the program).  I never saw that failure mode with x86_64, but 
again maybe it's just easier to hit with the reduced address space as you 
say.

Vince
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