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);
>
