Re: [PATCH v3 5/6] x86/alternative: Use a single access in text_poke() where possible

[Alexandre Chartre]

On 01/15/2019 05:19 PM, Steven Rostedt wrote:
> On Tue, 15 Jan 2019 12:10:19 +0100
> Alexandre Chartre <alexandre.chartre@xxxxxxxxxx> wrote:
>
> > Thinking more about it (and I've probably missed something or I am just being
> > totally stupid because this seems way too simple), can't we just replace the
> > "call" with "push+jmp" and patch the jmp instruction?
> >
> > Instead of having:
> >
> >      call target
> >
> > Have:
> >
> >      push $done
> > static_call:
> >      jmp target
> > done:
>
> But how do you implement it? Inline assembly()? Then you need to be
> able to do that for any type of function parameters (there will be
> users that have 13 parameters!)
>
> I believe people have mentioned having a gcc plugin that would do it
> for us, which was one of the suggested solutions.

Ah okay, I think I get it now (hopefully; I probably lost track of the
discussion at some point), so Linus' latest proposal avoids the gcc
plugin by keeping the call as is, and deals with it in the int3 handler
+ thunk.

Thanks, and sorry for the noise.

alex.

> > Then we can safely patch the "jmp" instruction to jump to a new target
> > with text_poke_bp(), using the new target as the text_poke_bp() handler:
> >
> >    new_jmp_code = opcode of "jmp new_target"
> >
> >    text_poke_bp(static_call, new_jmp_code, new_jmp_code_size, new_target);
> >
> > Problems come with patching a call instruction, but there's no issue with patching
> > a jmp, no? (that's what jump labels do).
> >
> > No change to the int3 handler, no thunk, this seems really too simple... :-)