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

[Martin Schwidefsky]

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.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.