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

> diff --git a/tools/testing/selftests/bpf/prog_tests/usdt.c b/tools/testing/selftests/bpf/prog_tests/usdt.c
> index 8004c9568ffa..eff1e57ab13c 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++) {

This isn't a bug, but should the comment say up to two pages to match the
2 * page_sz scan bound? With the .balign 4096 fill plus the 4086-byte
.skip in usdt_2_cross_page(), the combo can land roughly 8150 bytes past
the function start when the compiler emits a prologue.

> +		if (!memcmp((void *)usdt_2_cross_page + i, nop1_nop10_combo, 11)) {

This isn't a bug, but could find_instr() take a search range so this
subtest reuses it rather than open-coding the same memcmp scan?
find_instr() already implements this loop a hundred lines above, only
with a hardcoded 10-byte window instead of 2 * page_sz.

> +			addr = (void *)usdt_2_cross_page + i;
> +			break;
> +		}
> +	}
> +	if (!ASSERT_OK_PTR(addr, "find_nop1_nop10_combo"))
> +		return;
> +
> +	/* layout sanity check: the nop10 must cross the page boundary */
> +	if (!ASSERT_GT((unsigned long)(addr + 1) % page_sz + 10, page_sz,
> +		       "nop10_crosses_page"))
> +		return;
> +
> +	skel = test_usdt__open_and_load();
> +	if (!ASSERT_OK_PTR(skel, "test_usdt__open_and_load"))
> +		return;
> +
> +	skel->bss->my_pid = getpid();
> +
> +	skel->links.usdt0 = bpf_program__attach_usdt(skel->progs.usdt0,
> +						     0 /*self*/, "/proc/self/exe",
> +						     "optimized_attach",
> +						     "usdt_2_cross_page", NULL);
> +	if (!ASSERT_OK_PTR(skel->links.usdt0, "bpf_program__attach_usdt"))
> +		goto cleanup;
> +
> +	usdt_2_cross_page();
> +	usdt_2_cross_page();
> +
> +	ASSERT_EQ(skel->bss->usdt0_called, 2, "usdt0_called");
> +
> +cleanup:
> +	test_usdt__destroy(skel);
> +}

[ ... ]


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/32712703327