[PATCH bpf-next 07/13] uprobes/x86: Add support to emulate nop5 instruction

Jiri Olsa
2024-12-11T13:35:25Z

Adding support to emulate nop5 as the original uprobe instruction.

This speeds up uprobes on top of nop5 instructions:
(results from benchs/run_bench_uprobes.sh)

current:

uprobe-nop : 3.252 ± 0.019M/s
uprobe-push : 3.097 ± 0.002M/s
uprobe-ret : 1.116 ± 0.001M/s
--> uprobe-nop5 : 1.115 ± 0.001M/s
uretprobe-nop : 1.731 ± 0.016M/s
uretprobe-push : 1.673 ± 0.023M/s
uretprobe-ret : 0.843 ± 0.009M/s
--> uretprobe-nop5 : 1.124 ± 0.001M/s

after the change:

uprobe-nop : 3.281 ± 0.003M/s
uprobe-push : 3.085 ± 0.003M/s
uprobe-ret : 1.130 ± 0.000M/s
--> uprobe-nop5 : 3.276 ± 0.007M/s
uretprobe-nop : 1.716 ± 0.016M/s
uretprobe-push : 1.651 ± 0.017M/s
uretprobe-ret : 0.846 ± 0.006M/s
--> uretprobe-nop5 : 3.279 ± 0.002M/s

Strangely I can see uretprobe-nop5 is now much faster compared to
uretprobe-nop, while perf profiles for both are almost identical.
I'm still checking on that.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
arch/x86/kernel/uprobes.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index 23e4f2821cff..cdea97f8cd39 100644
--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -909,6 +909,11 @@ static const struct uprobe_xol_ops push_xol_ops = {
.emulate = push_emulate_op,
};

+static int is_nop5_insn(uprobe_opcode_t *insn)
+{
+ return !memcmp(insn, x86_nops[5], 5);
+}
+
/* Returns -ENOSYS if branch_xol_ops doesn't handle this insn */
static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
{
@@ -928,6 +933,8 @@ static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
break;

case 0x0f:
+ if (is_nop5_insn((uprobe_opcode_t *) &auprobe->insn))
+ goto setup;
if (insn->opcode.nbytes != 2)
return -ENOSYS;
/*
--
2.47.0