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

[Jiri Olsa <olsajiri@gmail.com>]

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

> 
> >  #else /* 32-bit: */
> >  /*
> >   * No RIP-relative addressing on 32-bit
> > @@ -621,6 +631,10 @@ static void riprel_pre_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
> >  static void riprel_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
> >  {
> >  }
> > +static bool emulate_nop5_insn(struct arch_uprobe *auprobe)
> > +{
> > +	return false;
> > +}
> 
> Hmm, why? I mean, why we can't emulate x86_nops[5] if !CONFIG_X86_64 ?

ok, so the following uprobe optimization is for CONFIG_X86_64 only, so I followed
that, but I guess we might emulate nop5 for !CONFIG_X86_64

> 
> OTOH. What if the kernel is 64-bit, but the probed task is 32-bit and it
> uses the 64-bit version of BYTES_NOP5?
> 
> Perhaps this is fine, I simply don't know, so let me ask...

hum, did not think of that, let me try it

> 
> > @@ -852,6 +866,8 @@ static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
> >  		break;
> >
> >  	case 0x0f:
> > +		if (emulate_nop5_insn(auprobe))
> > +			goto setup;
> 
> I think this will work, but if we want to emulate nop5, then perhaps
> we can do the same for other nops?
> 
> 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, also I don't have practical justification
for doing that.. nop5 seems to have future, because of the optimization

thanks,
jirka


> 
> Oleg.
> ---
> 
> diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> index 9194695662b2..76d2cceca6c4 100644
> --- a/arch/x86/kernel/uprobes.c
> +++ b/arch/x86/kernel/uprobes.c
> @@ -840,12 +840,16 @@ static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
>  	insn_byte_t p;
>  	int i;
>  
> +	/* prefix* + nop[i]; same as jmp with .offs = 0 */
> +	for (i = 1; i <= ASM_NOP_MAX; ++i) {
> +		if (!memcmp(insn->kaddr, x86_nops[i], i))
> +			goto setup;
> +	}
> +
>  	switch (opc1) {
>  	case 0xeb:	/* jmp 8 */
>  	case 0xe9:	/* jmp 32 */
>  		break;
> -	case 0x90:	/* prefix* + nop; same as jmp with .offs = 0 */
> -		goto setup;
>  
>  	case 0xe8:	/* call relative */
>  		branch_clear_offset(auprobe, insn);
>