Re: [PATCH bpf v2 2/2] selftests/bpf: Add test for usdt probe with page-crossing nop10

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The search loop limit `2 * page_sz` for locating the USDT probe can cause the test to falsely fail if the compiler generates a prologue of 11 bytes or larger.
--

--- Patch [2]: [PATCH bpf v2 2/2] selftests/bpf: Add test for usdt probe with page-crossing nop10 ---
commit 3ab03ec0febc59ef7c61c7f1449047ef827d5e7b
Author: Jiayuan Chen <jiayuan.chen@linux.dev>

selftests/bpf: Add test for usdt probe with page-crossing nop10

This commit adds a USDT probe layout where the nop10 crosses a page boundary.
It verifies that attaching to it succeeds and the probe fires without the
uprobe being shifted onto the nop10 incorrectly.

> diff --git a/tools/testing/selftests/bpf/prog_tests/usdt.c b/tools/testing/selftests/bpf/prog_tests/usdt.c
> index 8004c9568ffa5..eff1e57ab13c0 100644
> --- a/tools/testing/selftests/bpf/prog_tests/usdt.c
> +++ b/tools/testing/selftests/bpf/prog_tests/usdt.c
[ ... ]
> @@ -342,6 +343,57 @@ static void subtest_optimized_attach(void)
>  	test_usdt__destroy(skel);
>  }
>  
> +/*
> + * Test attachment to a USDT probe whose nop10 crosses a page boundary.
> + * The kernel can't optimize such nop10, so libbpf keeps the uprobe on
> + * the preceding 1-byte nop. Do not assume any particular placement
> + * here, though: however the probe ends up attached, the attachment
> + * must succeed and the probe must fire.
> + */
> +static void subtest_optimized_attach_cross_page(void)
> +{
> +	long page_sz = sysconf(_SC_PAGESIZE);
> +	struct test_usdt *skel;
> +	__u8 *addr = NULL;
> +	long i;
> +
> +	/* combo is placed up to a page of padding after the function start */
> +	for (i = 0; i < 2 * page_sz; i++) {

[Severity: Medium]
Will this loop bound of 2 * page_sz (8192) cause the test to falsely fail on
builds with larger compiler prologues (like ASAN builds or those with frame
pointers enabled)?

If the compiler generates a prologue of size P, the offset to the probe is
P + align_padding (up to 4095) + skip_padding (4086) = P + 8181.

If P is 11 bytes or larger, the offset reaches 8192, which would exceed
this i < 2 * page_sz search bound and cause a spurious ASSERT_OK_PTR
failure.

> +		if (!memcmp((void *)usdt_2_cross_page + i, nop1_nop10_combo, 11)) {
> +			addr = (void *)usdt_2_cross_page + i;
> +			break;
> +		}
> +	}
> +	if (!ASSERT_OK_PTR(addr, "find_nop1_nop10_combo"))
> +		return;

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260825150444.31603-1-jiayuan.chen@linux.dev?part=2