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Re: [PATCH][RFA/RFC] Stack clash mitigation 0/9
- From: Segher Boessenkool <segher at kernel dot crashing dot org>
- To: Jeff Law <law at redhat dot com>
- Cc: gcc-patches <gcc-patches at gcc dot gnu dot org>
- Date: Wed, 12 Jul 2017 17:44:29 -0500
- Subject: Re: [PATCH][RFA/RFC] Stack clash mitigation 0/9
- Authentication-results: sourceware.org; auth=none
- References: <ed1f2daf-c45b-e642-65a3-b88b803b0f89@redhat.com>
On Tue, Jul 11, 2017 at 03:19:36PM -0600, Jeff Law wrote:
> Examples of implicit probes include
> 2. ABI mandates that *sp always contain a backchain pointer (ppc)
In the ELFv2 ABI a backchain is not required. GCC still always has
one afaik. I'll find out more.
> To get a sense of overhead, just 1.5% of routines in glibc need probing
> in their prologues (x86) in the testing I performed. IIRC each and
> every one of those routines needed just 1-4 inlined probes.
>
> Significantly more functions need alloca space probed (IIRC ~5%), but
> given the amazingly inefficient alloca code, I can't believe anyone will
> ever notice the probing overhead.
That is quite a lot of functions IMO, but it's just one stor per page
(or per alloca), and supposedly you'll store to that stack anyway (or
it is stupid slow code in the first place). Did you measure any real
timings?
Segher