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

[Christian Borntraeger]

Am 19.11.21 um 02:10 schrieb Nick Desaulniers:
> On Wed, Nov 17, 2021 at 9:48 AM Ilie Halip <ilie.halip@xxxxxxxxx> 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@xxxxxxxxxx>
> > Signed-off-by: Ilie Halip <ilie.halip@xxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

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