Re: [PATCHv2 07/11] selftests/bpf: Change uprobe syscall tests to use nop10

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 3 potential issue(s) to consider:
- [Medium] Missing update to USDT_NOP macro causes `subtest_optimized_attach` test failure.
- [Medium] Inadequate search bounds in `find_instr()` may cause false test failures.
- [Low] Misleading benchmark and function names referencing `nop5`.
--

commit 1996488ddbe422eb1d441da9b06aafe6bd47e6d1
Author: Jiri Olsa <jolsa@kernel.org>

selftests/bpf: Change uprobe syscall tests to use nop10

Optimized uprobes are now on top of 10-bytes nop instructions,
reflect that in existing tests.

> diff --git a/tools/testing/selftests/bpf/benchs/bench_trigger.c b/tools/testing/selftests/bpf/benchs/bench_trigger.c
> index 2f22ec61667b9..bcc4820c802e5 100644
> --- a/tools/testing/selftests/bpf/benchs/bench_trigger.c
> +++ b/tools/testing/selftests/bpf/benchs/bench_trigger.c
> @@ -398,7 +398,7 @@ static void *uprobe_producer_ret(void *input)
>  #ifdef __x86_64__
>  __nocf_check __weak void uprobe_target_nop5(void)
>  {
> -	asm volatile (".byte 0x0f, 0x1f, 0x44, 0x00, 0x00");
> +	asm volatile (".byte 0x66, 0x66, 0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00");
>  }
>  
>  static void *uprobe_producer_nop5(void *input)

This isn't a bug, but should the function and benchmark names also be updated?

The benchmark exposes names like uprobe-nop5 and uretprobe-nop5, but they are
now testing a 10-byte nop.

[ ... ]

> diff --git a/tools/testing/selftests/bpf/prog_tests/usdt.c b/tools/testing/selftests/bpf/prog_tests/usdt.c
> index 69759b27794d7..a160d7c4fa0d3 100644
> --- a/tools/testing/selftests/bpf/prog_tests/usdt.c
> +++ b/tools/testing/selftests/bpf/prog_tests/usdt.c
> @@ -252,7 +252,7 @@ extern void usdt_1(void);
>  extern void usdt_2(void);
>  
>  static unsigned char nop1[1] = { 0x90 };
> -static unsigned char nop1_nop5_combo[6] = { 0x90, 0x0f, 0x1f, 0x44, 0x00, 0x00 };
> +static unsigned char nop1_nop10_combo[11] = { 0x90, 0x66, 0x66, 0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00 };

Is the loop in find_instr() adequate to find this instruction sequence?

tools/testing/selftests/bpf/prog_tests/usdt.c:find_instr() {
	for (i = 0; i < 10; i++) {
		if (!memcmp(instr, fn + i, cnt))
			return fn + i;
	}
	return NULL;
}

In uprobe_syscall.c, find_nop10() was updated to search up to 128 bytes to
account for compiler-generated prologues. If a modern compiler generates a
prologue longer than 9 bytes for usdt_2(), this will prematurely terminate and
cause the subtest_optimized_attach() test to fail.

[ ... ]

> @@ -271,17 +271,17 @@ static void subtest_optimized_attach(void)
>  	__u8 *addr_1, *addr_2;
>  
>  	/* usdt_1 USDT probe has single nop instruction */
> -	addr_1 = find_instr(usdt_1, nop1_nop5_combo, 6);
> -	if (!ASSERT_NULL(addr_1, "usdt_1_find_nop1_nop5_combo"))
> +	addr_1 = find_instr(usdt_1, nop1_nop10_combo, 11);
> +	if (!ASSERT_NULL(addr_1, "usdt_1_find_nop1_nop10_combo"))
>  		return;
>  
>  	addr_1 = find_instr(usdt_1, nop1, 1);
>  	if (!ASSERT_OK_PTR(addr_1, "usdt_1_find_nop1"))
>  		return;
>  
> -	/* usdt_2 USDT probe has nop,nop5 instructions combo */
> -	addr_2 = find_instr(usdt_2, nop1_nop5_combo, 6);
> -	if (!ASSERT_OK_PTR(addr_2, "usdt_2_find_nop1_nop5_combo"))
> +	/* usdt_2 USDT probe has nop,nop10 instructions combo */
> +	addr_2 = find_instr(usdt_2, nop1_nop10_combo, 11);
> +	if (!ASSERT_OK_PTR(addr_2, "usdt_2_find_nop1_nop10_combo"))

Does this test fail because USDT_NOP was not updated in usdt.h?

The test expects to find the 11-byte nop1_nop10_combo, but usdt_2.c relies on
the default definition of USDT_NOP from tools/testing/selftests/bpf/usdt.h.

Because usdt.h was not updated in this patch, usdt_2.c will compile with the
6-byte nop sequence, causing find_instr() to return NULL here and the
ASSERT_OK_PTR to fail.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260518105957.123445-8-jolsa@kernel.org?part=1