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

[Jiri Olsa <olsajiri@gmail.com>]

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 */