linux-kernel - Re: [PATCH 3/3] Fix insn.c misaligned address error

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Date:   Mon, 29 Jul 2019 11:22:34 +0300
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Numfor Mbiziwo-Tiapo <nums@...gle.com>, peterz@...radead.org,
        mingo@...hat.com, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, namhyung@...nel.org, songliubraving@...com,
        mbd@...com, linux-kernel@...r.kernel.org, irogers@...gle.com,
        eranian@...gle.com
Subject: Re: [PATCH 3/3] Fix insn.c misaligned address error

On 27/07/19 12:46 PM, Masami Hiramatsu wrote:
> On Fri, 26 Jul 2019 16:38:06 -0300
> Arnaldo Carvalho de Melo <acme@...nel.org> wrote:
> 
>> Em Wed, Jul 24, 2019 at 11:45:12AM -0700, Numfor Mbiziwo-Tiapo escreveu:
>>> The ubsan (undefined behavior sanitizer) version of perf throws an
>>> error on the 'x86 instruction decoder - new instructions' function
>>> of perf test.
>>>
>>> To reproduce this run:
>>> make -C tools/perf USE_CLANG=1 EXTRA_CFLAGS="-fsanitize=undefined"
>>>
>>> then run: tools/perf/perf test 62 -v
>>>
>>> The error occurs in the __get_next macro (line 34) where an int is
>>> read from a potentially unaligned address. Using memcpy instead of
>>> assignment from an unaligned pointer.
>>
>> Since this came from the kernel, don't we have to fix it there as well?
>> Masami, Adrian?
> 
> I guess we don't need it, since x86 can access "unaligned address" and
> x86 insn decoder in kernel runs only on x86. I'm not sure about perf's
> that part. Maybe if we run it on other arch as cross-arch application,
> it may cause unaligned pointer issue.

Yes, theoretically Intel PT decoding can be done on any arch.

But the memcpy is probably sub-optimal for x86, so the patch as it stands
does not seem suitable.  I notice the kernel has get_unaligned() and
put_unaligned().

Obviously it would be better for a patch to be accepted to
arch/x86/lib/insn.c also.

> 
> Thank you,
> 
>>
>> [acme@...co perf]$ find . -name insn.c
>> ./arch/x86/lib/insn.c
>> ./arch/arm/kernel/insn.c
>> ./arch/arm64/kernel/insn.c
>> ./tools/objtool/arch/x86/lib/insn.c
>> ./tools/perf/util/intel-pt-decoder/insn.c
>> [acme@...co perf]$ diff -u ./tools/perf/util/intel-pt-decoder/insn.c ./arch/x86/lib/insn.c
>> --- ./tools/perf/util/intel-pt-decoder/insn.c	2019-07-06 16:59:05.734265998 -0300
>> +++ ./arch/x86/lib/insn.c	2019-07-06 16:59:01.369202998 -0300
>> @@ -10,8 +10,8 @@
>>  #else
>>  #include <string.h>
>>  #endif
>> -#include "inat.h"
>> -#include "insn.h"
>> +#include <asm/inat.h>
>> +#include <asm/insn.h>
>>
>>  /* Verify next sizeof(t) bytes can be on the same instruction */
>>  #define validate_next(t, insn, n)	\
>> [acme@...co perf]$
>>
>>
>> - Arnaldo
>>  
>>> Signed-off-by: Numfor Mbiziwo-Tiapo <nums@...gle.com>
>>> ---
>>>  tools/perf/util/intel-pt-decoder/insn.c | 3 ++-
>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/tools/perf/util/intel-pt-decoder/insn.c b/tools/perf/util/intel-pt-decoder/insn.c
>>> index ca983e2bea8b..de1944c60aa9 100644
>>> --- a/tools/perf/util/intel-pt-decoder/insn.c
>>> +++ b/tools/perf/util/intel-pt-decoder/insn.c
>>> @@ -31,7 +31,8 @@
>>>  	((insn)->next_byte + sizeof(t) + n <= (insn)->end_kaddr)
>>>  
>>>  #define __get_next(t, insn)	\
>>> -	({ t r = *(t*)insn->next_byte; insn->next_byte += sizeof(t); r; })
>>> +	({ t r; memcpy(&r, insn->next_byte, sizeof(t)); \
>>> +		insn->next_byte += sizeof(t); r; })
>>>  
>>>  #define __peek_nbyte_next(t, insn, n)	\
>>>  	({ t r = *(t*)((insn)->next_byte + n); r; })
>>> -- 
>>> 2.22.0.657.g960e92d24f-goog
>>
>> -- 
>>
>> - Arnaldo
> 
>