linux-kernel - Re: [PATCH] perf,x86: fix uninitialized pt_regs in intel_pmu_drain_bts

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Date:	Tue, 19 Mar 2013 13:50:08 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>, Jiri Olsa <jolsa@...hat.com>,
	Salman Qazi <sqazi@...gle.com>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>
Subject: Re: [PATCH] perf,x86: fix uninitialized pt_regs in intel_pmu_drain_bts_buffer()

On Tue, Mar 19, 2013 at 1:43 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, 2013-03-18 at 14:46 +0100, Stephane Eranian wrote:
>>
>> This patch fixes an uninitialized pt_regs struct in drain BTS
>> function. The pt_regs struct is propagated all the way to the
>> code_get_segment() function from perf_instruction_pointer()
>> and may get garbage.
>>
>> We cannot simply inherit the actual pt_regs from the interrupt
>> because BTS must be flushed on context-switch or when the associated
>> event is disabled. And there we do not have a pt_regs handy.
>>
>> Setting pt_regs to all zeroes may not be the best option but it is
>> not clear what else to do given where the drain_bts_buffer() is called
>> from.
>>
>> Signed-off-by: Stephane Eranian <eranian@...gle.com>
>> ---
>>
>> diff --git a/arch/x86/kernel/cpu/perf_event_intel_ds.c b/arch/x86/kernel/cpu/perf_event_intel_ds.c
>> index b05a575..208f0c8 100644
>> --- a/arch/x86/kernel/cpu/perf_event_intel_ds.c
>> +++ b/arch/x86/kernel/cpu/perf_event_intel_ds.c
>> @@ -302,6 +302,8 @@ int intel_pmu_drain_bts_buffer(void)
>>       struct perf_sample_data data;
>>       struct pt_regs regs;
>>
>> +     memset(&regs, 0, sizeof(regs));
>> +
>>       if (!event)
>>               return 0;
>>
>
> Should we not replace:
>
>         regs.ip     = 0;
>
> with that memset? It avoids the memset work in a few cases and removes
> the then superfluous clearing of the IP field.
>
We could drop it because it's covered by the memset().

The issue here was that you eventually end up in code_segment_base() which
looks at other uninitialized fields in pt_regs and may cause breakage.
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