linux-kernel - Re: perf_fuzzer compiled for x32 causes reboot

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Date:	Sun, 23 Feb 2014 21:22:38 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Vince Weaver <vincent.weaver@...ne.edu>
CC:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, "H.J. Lu" <hjl.tools@...il.com>
Subject: Re: perf_fuzzer compiled for x32 causes reboot

On 02/23/2014 07:02 PM, Vince Weaver wrote:
> On Sun, 23 Feb 2014, Vince Weaver wrote:
>>
>> and as far as I can tell nothing touches rbp again until the segfault.
>> Nothing in _memset_sse2 does as far as I can tell.
> 
> I only know enough about ftrace to be dangerous, but here is what I think 
> is the trace of the problem:
> 
>      perf_fuzzer-11492 [000] 197077.488420: function:                perf_output_put_handle
>      perf_fuzzer-11492 [000] 197077.488421: function:             __do_page_fault

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.

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.

	-hpa

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