Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions

From: Peter Zijlstra
Date: Tue May 07 2019 - 13:39:47 EST

On Tue, May 07, 2019 at 10:08:50AM -0700, Linus Torvalds wrote:
> On Tue, May 7, 2019 at 9:34 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > Would you consider my approach later on, under the guise of unification?
> WHY?
> The *only* advantage of your patch is that trivial "look up kernel stack" macro.
> Seriously. There's absolutely nothing else.

The ftrace_regs_caller, the kprobe tramplines, the unwinder, they all
have 'funny' bits because pt_regs isn't 'right'.

> So the whole "let's clean up x86-32 to look like x86-64, which got
> things right" is to me a completely bogus argument. x86-64 got the
> "yes, push ss/sp unconditionally" part right, but got a lot of other
> things horribly wrong. So this is all just one small detail that
> differs, across two architectures that are similar but have very
> different warts.

It's a detail that leaks into the C code. Yes SWAPGS is horrible crap,
but C code doesn't much care. The partial pt_regs thing otoh comes up a
fair number of times.

Anyway; I think we're at the point where we'll have to agree to
disagree (or maybe slightly past it).