Re: [PATCH 1/4] glibc: Perform rseq(2) registration at C startup and thread creation (v7)

[Carlos O'Donell]

On 3/27/19 5:16 AM, Martin Schwidefsky wrote:
> On Mon, 25 Mar 2019 11:54:32 -0400 (EDT)
> Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>
> > > > +++ b/sysdeps/unix/sysv/linux/s390/bits/rseq.h
> > [...]
> > > > +
> > > > +/* Signature required before each abort handler code.  */
> > > > +#define RSEQ_SIG 0x53053053
> > >
> > > Why not a s390 specific value here?
> >
> > s390 also has the abort handler in a __rseq_failure section:
> >
> > #define RSEQ_ASM_DEFINE_ABORT(label, teardown, abort_label)             \
> >                  ".pushsection __rseq_failure, \"ax\"\n\t"               \
> >                  ".long " __rseq_str(RSEQ_SIG) "\n\t"                    \
> >                  __rseq_str(label) ":\n\t"                               \
> >                  teardown                                                \
> >                  "j %l[" __rseq_str(abort_label) "]\n\t"                 \
> >                  ".popsection\n\t"
> >
> > Same question applies as powerpc: since disassemblers will try to decode
> > that instruction, would it be better to define it as a valid one ?
> >
> > [...]
>
> A 4-byte sequence starting with 0x53 is decoded as a "diebr" instruction.
> And please replace that "j %l[...]" with a "jg %l[...]", the branch target
> range of the "j" instruction is 64K, not enough for the general case.

Why was this particular operated selected?
 
So on s390 the RSEQ_SIG will show up as an unexpected "divide to integer"
instruction that can't be reached by any control flow?

Can we use a NOP with a unique value in an immediate operand?

The goal being to have something that won't confuse during a debug
session, or that the debugger can ignore (like constant pools on Arm)

--
Cheers,
Carlos.