linux-kernel - Re: [PATCH] s390/test_unwind: use raw opcode instead of invalid instruction

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Date:   Fri, 19 Nov 2021 10:39:15 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Ilie Halip <ilie.halip@...il.com>
Cc:     linux-kernel@...r.kernel.org, Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Mete Durlu <meted@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        linux-s390@...r.kernel.org, llvm@...ts.linux.dev,
        Ulrich Weigand <Ulrich.Weigand@...ibm.com>
Subject: Re: [PATCH] s390/test_unwind: use raw opcode instead of invalid
 instruction



Am 19.11.21 um 02:10 schrieb Nick Desaulniers:
> On Wed, Nov 17, 2021 at 9:48 AM Ilie Halip <ilie.halip@...il.com> wrote:
>>
>> Building with clang & LLVM_IAS=1 leads to an error:
>>      arch/s390/lib/test_unwind.c:179:4: error: invalid register pair
>>                          "       mvcl    %%r1,%%r1\n"
>>                          ^
>>
>> The test creates an invalid instruction that would trap at runtime, but the
>> LLVM inline assembler tries to validate it at compile time too.
>>
>> Use the raw instruction opcode instead.
>>
>> Link: https://github.com/ClangBuiltLinux/linux/issues/1421
>> Reported-by: Nick Desaulniers <ndesaulniers@...gle.com>
>> Signed-off-by: Ilie Halip <ilie.halip@...il.com>
> 
> Ilie, thanks for the patch!
> 
> So if I understand
> https://sourceware.org/binutils/docs/as/s390-Directives.html#s390-Directives
> https://sourceware.org/binutils/docs/as/s390-Formats.html
> that `e,` prefix is for 16B opcodes?

e is an instruction format as specified by the architecture.
See http://publibfp.dhe.ibm.com/epubs/pdf/a227832c.pdf
without any parameters.
Normally RR would be the right thing for MVCL, but since
we try to build an invalid opcode without the assembler
noticing (ab)using e seem like a safer approach.

> 
> LGTM, thanks again.
> Suggested-by: Ulrich Weigand <Ulrich.Weigand@...ibm.com>
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

added those and added my RB. applied to the s390 tree. Thanks


> 
> I triple checked that GAS, clang, and GNU objdump are in agreement in
> terms of encoding here.
> 
>> ---
>>   arch/s390/lib/test_unwind.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/s390/lib/test_unwind.c b/arch/s390/lib/test_unwind.c
>> index cfc5f5557c06..d342bc884b94 100644
>> --- a/arch/s390/lib/test_unwind.c
>> +++ b/arch/s390/lib/test_unwind.c
>> @@ -176,7 +176,7 @@ static noinline int unwindme_func4(struct unwindme *u)
>>                   * trigger specification exception
>>                   */
>>                  asm volatile(
>> -                       "       mvcl    %%r1,%%r1\n"
>> +                       "       .insn e,0x0e11\n"       /* mvcl %%r1,%%r1" */
>>                          "0:     nopr    %%r7\n"
>>                          EX_TABLE(0b, 0b)
>>                          :);
>> --
>> 2.25.1
>>
> 
>