linux-kernel - Re: [PATCH] perf/core: Fix the mask in perf_output_sample

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Date:	Tue, 16 Aug 2016 10:59:05 +0530
From:	Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	Yury Norov <ynorov@...iumnetworks.com>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH] perf/core: Fix the mask in perf_output_sample_regs



On Thursday 11 August 2016 05:57 PM, Peter Zijlstra wrote:
> Sorry, found it in my inbox while clearing out backlog..
>
> On Sun, Jul 03, 2016 at 11:31:58PM +0530, Madhavan Srinivasan wrote:
>> When decoding the perf_regs mask in perf_output_sample_regs(),
>> we loop through the mask using find_first_bit and find_next_bit functions.
>> While the exisitng code works fine in most of the case,
>> the logic is broken for 32bit kernel (Big Endian).
>> When reading u64 mask using (u32 *)(&val)[0], find_*_bit() assumes it gets
>> lower 32bits of u64 but instead gets upper 32bits which is wrong.
>> Proposed fix is to swap the words of the u64 to handle this case.
>> This is _not_ endianness swap.
> But it looks an awful lot like it..
Hit this issue when testing my perf_arch_regs patchset. Yep exactly
the reason for adding that comment in the commit message.


>
>> +++ b/kernel/events/core.c
>> @@ -5205,8 +5205,10 @@ perf_output_sample_regs(struct perf_output_handle *handle,
>>   			struct pt_regs *regs, u64 mask)
>>   {
>>   	int bit;
>> +	DECLARE_BITMAP(_mask, 64);
>>   
>> -	for_each_set_bit(bit, (const unsigned long *) &mask,
>> +	bitmap_from_u64(_mask, mask);
>> +	for_each_set_bit(bit, _mask,
>>   			 sizeof(mask) * BITS_PER_BYTE) {
>>   		u64 val;
>> +++ b/lib/bitmap.c
>> +void bitmap_from_u64(unsigned long *dst, u64 mask)
>> +{
>> +	dst[0] = mask & ULONG_MAX;
>> +
>> +	if (sizeof(mask) > sizeof(unsigned long))
>> +		dst[1] = mask >> 32;
>> +}
>> +EXPORT_SYMBOL(bitmap_from_u64);
> Looks small enough for an inline.
>
> Alternatively you can go all the way and add bitmap_from_u64array(), but
> that seems massive overkill.

Ok will make it inline and resend.

Maddy

>
> Tedious stuff.. I can't come up with anything prettier :/
>