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

[Carlos O'Donell]

On 4/2/19 3:08 AM, Florian Weimer wrote:
> * Michael Ellerman:
>
> > 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).
>
> I think we need something that is very unlikely to appear in the
> instruction stream.  It's just a marker.  The instruction will never be
> executed, and it does not have to be a trap, either (I believe that a
> standard trap instruction would be a bad choice).

I assume you want to avoid a standard trap instruction because it would
be common, and so not meet the intent of the RSEQ_SIG choice as being something
that is *uncommon* right?

It is valuable that it be a trap, particularly for constant pools because
it means that a jump into the constant pool will trap.

--
Cheers,
Carlos.