Re: [PATCH RFCv3 10/23] uprobes/x86: Add support to emulate nop5 instruction

Jiri Olsa
2025-04-08T20:22:02Z

On Mon, Apr 07, 2025 at 01:07:26PM +0200, Jiri Olsa wrote:
> On Fri, Apr 04, 2025 at 01:33:11PM -0700, Andrii Nakryiko wrote:
> > On Thu, Mar 20, 2025 at 4:43 AM Jiri Olsa <jolsa@kernel.org> wrote:
> > >
> > > Adding support to emulate nop5 as the original uprobe instruction.
> > >
> > > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > > ---
> > > arch/x86/kernel/uprobes.c | 16 ++++++++++++++++
> > > 1 file changed, 16 insertions(+)
> > >
> >
> > This optimization is independent from the sys_uprobe, right? Maybe
> > send it as a stand-alone patch and let's land it sooner?
>
> ok, will send it separately
>
> > Also, how hard would it be to do the same for other nopX instructions?
>
> will check, might be easy

we can't do all at the moment, nop1-nop8 are fine, but uprobe won't
attach on nop9/10/11 due unsupported prefix.. I guess insn decode
would need to be updated first

I'll send the nop5 emulation change, because of above and also I don't
see practical justification to emulate other nops

jirka

---
diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index 9194695662b2..6616cc9866cc 100644
--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -608,6 +608,21 @@ static void riprel_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
*sr = utask->autask.saved_scratch_register;
}
}
+
+static bool emulate_nop_insn(struct arch_uprobe *auprobe)
+{
+ unsigned int i;
+
+ /*
+ * Uprobe is only allowed to be attached on nop1 through nop8. Further nop
+ * instructions have unsupported prefix and uprobe fails to attach on them.
+ */
+ for (i = 1; i < 9; i++) {
+ if (!memcmp(&auprobe->insn, x86_nops[i], i))
+ return true;
+ }
+ return false;
+}
#else /* 32-bit: */
/*
* No RIP-relative addressing on 32-bit
@@ -621,6 +636,10 @@ 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_nop_insn(struct arch_uprobe *auprobe)
+{
+ return false;
+}
#endif /* CONFIG_X86_64 */

struct uprobe_xol_ops {
@@ -840,6 +859,9 @@ static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
insn_byte_t p;
int i;

+ if (emulate_nop_insn(auprobe))
+ goto setup;
+
switch (opc1) {
case 0xeb: /* jmp 8 */
case 0xe9: /* jmp 32 */