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

[Carlos O'Donell]

On 4/2/19 2:02 AM, Michael Ellerman wrote:
> 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).

The s390 IBM team needs to respond to this and I want to make sure they ACK
the NOP being used here because it impacts them directly.

I'd like to see Martin's opinion on this.

--
Cheers,
Carlos.