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

[Michael Ellerman]

Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> writes:
> Hi Carlos,
>
> ----- On Mar 22, 2019, at 4:09 PM, Carlos O'Donell codonell@xxxxxxxxxx wrote:
...
>
> [...]
>>> +++ b/sysdeps/unix/sysv/linux/powerpc/bits/rseq.h
> [...]
>>> +/* Signature required before each abort handler code.  */
>>> +#define RSEQ_SIG 0x53053053
>> 
>> Why isn't this an opcode specific to power?
>
> On powerpc 32/64, the abort is placed in a __rseq_failure executable section:
>
> #define RSEQ_ASM_DEFINE_ABORT(label, abort_label)                               \
>                 ".pushsection __rseq_failure, \"ax\"\n\t"                       \
>                 ".long " __rseq_str(RSEQ_SIG) "\n\t"                            \
>                 __rseq_str(label) ":\n\t"                                       \
>                 "b %l[" __rseq_str(abort_label) "]\n\t"                         \
>                 ".popsection\n\t"
>
> That section only contains snippets of those trampolines. Arguably, it would be
> good if disassemblers could find valid instructions there. Boqun Feng could perhaps
> shed some light on this signature choice ? Now would be a good time to decide
> once and for all whether a valid instruction would be a better choice.

I'm a bit vague on what we're trying to do here.

But it seems like you want some sort of "eye catcher" prior to the branch?

That value is a valid instruction on current CPUs (rlwimi.
r5,r24,6,1,9), and even if it wasn't it could become one in future.

If you change it to 0x8053530 that is both a valid instruction and is a
nop (conditional trap immediate but with no conditions set).

cheers