Re: [PATCH] x86/ftrace: Don't bother preserving/restoring R10/R11

[Ard Biesheuvel <ardb@kernel.org>]

On Wed, 9 Oct 2024 at 18:31, Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Wed, 9 Oct 2024 18:25:25 +0200
> Ard Biesheuvel <ardb@kernel.org> wrote:
>
> > > Say we have code of:
> > >
> > >         pushq   r10
> > >         pushq   r11
> > >         call    foo
> > >         popq    r11
> > >         popq    r10
> > >
> > > Where we add a kprobe to the start of foo, the callback should be able to
> > > see what r10 and r11 were.
> >
> > Why exactly is that? The contents of R10 and R11 have no purpose going
> > forward, so is it just to see what some previous code may have left in
> > them?
>
> Because the probe is on the call. Unless they were used between the push
> and the call, they still have the value you may be looking for.
>

Right. So putting a probe on foo() is a way to inspect the register
values during the execution if its caller. Fair enough.

> >
> > > But the restore part is for the function foo to
> > > see. It shouldn't care about r10 or r11 and if a kprobe updates them, it
> > > should not have any effect.
> > >
> > > What does restoring r10 and r11 give us?
> > >
> >
> > Nothing. Which is why I don't understand why you would need to record
> > them in the first place.
>
> As I mentioned above. Unless they are used after they are pushed, you still
> have access to them on the call (or the kprobe attached to ftrace).
>

OK. I just didn't imagine this usage mode, where you probe foo() to
capture the values of dead registers in its callers.

I'll send a v2 and drop the first hunk.