Re: [PATCH 1/2] uprobes/x86: Add support to emulate nop5 instruction

[Oleg Nesterov <oleg@redhat.com>]

On 04/09, Jiri Olsa wrote:
>
> On Wed, Apr 09, 2025 at 01:28:39PM +0200, Oleg Nesterov wrote:
> > On 04/08, Jiri Olsa wrote:
> > >
> > > --- a/arch/x86/kernel/uprobes.c
> > > +++ b/arch/x86/kernel/uprobes.c
> > > @@ -608,6 +608,16 @@ static void riprel_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
> > >  		*sr = utask->autask.saved_scratch_register;
> > >  	}
> > >  }
> > > +
> > > +static int is_nop5_insn(uprobe_opcode_t *insn)
> > > +{
> > > +	return !memcmp(insn, x86_nops[5], 5);
> > > +}
> > > +
> > > +static bool emulate_nop5_insn(struct arch_uprobe *auprobe)
> > > +{
> > > +	return is_nop5_insn((uprobe_opcode_t *) &auprobe->insn);
> > > +}
> >
> > Why do we need 2 functions? Can't branch_setup_xol_ops() just use
> > is_nop5_insn(insn->kaddr) ?
>
> I need is_nop5_insn in other changes I have in queue, so did not want
> to introduce extra changes

But I didn't suggest to remove is_nop5_insn(), I meant that
branch_setup_xol_ops() can do

	if (is_nop5_insn(insn->kaddr))
		goto setup;
or
	if (is_nop5_insn(auprobe->insn))
		goto setup;

this even looks more readable to me. but I won't insist.

> > For the moment, lets forget about compat tasks on a 64-bit kernel, can't
> > we simply do something like below?
>
> I sent similar change (CONFIG_X86_64 only) in this thread:
>   https://lore.kernel.org/bpf/Z_O0Z1ON1YlRqyny@krava/T/#m59c430fb5a30cb9faeb9587fd672ea0adbf3ef4f
>
> uprobe won't attach on nop9/10/11 atm,

Ah, OK, I didn't know. But this means that nop9/10/11 will be rejected
by uprobe_init_insn() -> is_prefix_bad() before branch_setup_xol_ops() is
called, right? So I guess it is safe to use ASM_NOP_MAX. Nevermind.

> also I don't have practical justification
> for doing that.. nop5 seems to have future, because of the optimization

OK, I won't insist, up to you.

Just it looks a bit strange to me. Even if we do not have a use-case
for other nops, why we can't emulate them all just for consistency?

And given that emulate_nop5_insn() compares the whole insn with
x86_nops[5], I guess we don't even need to check OPCODE1(insn)...
Nevermind.

So, once again, I won't argue.

Oleg.