Re: [PATCH bpf-next v8 2/2] selftests/bpf: Add an usdt_o2 test case in selftests to cover SIB handling logic

[Jiri Olsa <olsajiri@gmail.com>]

On Thu, Aug 14, 2025 at 06:45:04AM +0000, Jiawei Zhao wrote:
> When using GCC on x86-64 to compile an usdt prog with -O1 or higher
> optimization, the compiler will generate SIB addressing mode for global
> array and PC-relative addressing mode for global variable,
> e.g. "1@-96(%rbp,%rax,8)" and "-1@4+t1(%rip)".
> 
> In this patch:
> - add usdt_o2 test case to cover SIB addressing usdt argument spec
>   handling logic

hi,
on my setup (gcc15) the test generates ust register argument:

  stapsdt              0x0000002a       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt1
    Location: 0x00000000007677ce, Base: 0x00000000035bc728, Semaphore: 0x0000000000000000
    Arguments: 8@%rax


  7677c6:       48 8b 04 c5 20 49 9c    mov    0x39c4920(,%rax,8),%rax
  7677cd:       03
  7677ce:       90                      nop


I'm not sure if there's reliable solution to generate SIB argument from gcc,
maybe we could generate all in assembly, but that might get complicated

jirka


> 
> Signed-off-by: Jiawei Zhao <phoenix500526@163.com>
> ---
>  tools/testing/selftests/bpf/Makefile          |  1 +
>  .../selftests/bpf/prog_tests/usdt_o2.c        | 69 +++++++++++++++++++
>  .../selftests/bpf/progs/test_usdt_o2.c        | 37 ++++++++++
>  3 files changed, 107 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/usdt_o2.c
>  create mode 100644 tools/testing/selftests/bpf/progs/test_usdt_o2.c
> 
> diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
> index 4863106034df..24ff1a329625 100644
> --- a/tools/testing/selftests/bpf/Makefile
> +++ b/tools/testing/selftests/bpf/Makefile
> @@ -760,6 +760,7 @@ TRUNNER_BPF_BUILD_RULE := $$(error no BPF objects should be built)
>  TRUNNER_BPF_CFLAGS :=
>  $(eval $(call DEFINE_TEST_RUNNER,test_maps))
>  
> +
>  # Define test_verifier test runner.
>  # It is much simpler than test_maps/test_progs and sufficiently different from
>  # them (e.g., test.h is using completely pattern), that it's worth just
> diff --git a/tools/testing/selftests/bpf/prog_tests/usdt_o2.c b/tools/testing/selftests/bpf/prog_tests/usdt_o2.c
> new file mode 100644
> index 000000000000..f02dcf5188ab
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/usdt_o2.c
> @@ -0,0 +1,69 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2025 Jiawei Zhao <phoenix500526@163.com>. */
> +#include <test_progs.h>
> +
> +#include "../sdt.h"
> +#include "test_usdt_o2.skel.h"
> +
> +#if defined(__GNUC__) && !defined(__clang__)
> +__attribute__((optimize("O2")))
> +#endif
> +
> +#define test_value 0xFEDCBA9876543210ULL
> +#define SEC(name) __attribute__((section(name), used))
> +
> +int lets_test_this(int);
> +static volatile __u64 array[1] = {test_value};
> +
> +static __always_inline void trigger_func(void)
> +{
> +	/* Base address + offset + (index * scale) */
> +	for (volatile int i = 0; i <= 0; i++)
> +		STAP_PROBE1(test, usdt1, array[i]);
> +}
> +
> +static void basic_sib_usdt(void)
> +{
> +	LIBBPF_OPTS(bpf_usdt_opts, opts);
> +	struct test_usdt_o2 *skel;
> +	struct test_usdt_o2__bss *bss;
> +	int err;
> +
> +	skel = test_usdt_o2__open_and_load();
> +	if (!ASSERT_OK_PTR(skel, "skel_open"))
> +		return;
> +
> +	bss = skel->bss;
> +	bss->my_pid = getpid();
> +
> +	err = test_usdt_o2__attach(skel);
> +	if (!ASSERT_OK(err, "skel_attach"))
> +		goto cleanup;
> +
> +	/* usdt1 won't be auto-attached */
> +	opts.usdt_cookie = 0xcafedeadbeeffeed;
> +	skel->links.usdt1 = bpf_program__attach_usdt(skel->progs.usdt1,
> +						     0 /*self*/, "/proc/self/exe",
> +						     "test", "usdt1", &opts);
> +	if (!ASSERT_OK_PTR(skel->links.usdt1, "usdt1_link"))
> +		goto cleanup;
> +
> +	trigger_func();
> +
> +	ASSERT_EQ(bss->usdt1_called, 1, "usdt1_called");
> +	ASSERT_EQ(bss->usdt1_cookie, 0xcafedeadbeeffeed, "usdt1_cookie");
> +	ASSERT_EQ(bss->usdt1_arg_cnt, 1, "usdt1_arg_cnt");
> +	ASSERT_EQ(bss->usdt1_arg, test_value, "usdt1_arg");
> +	ASSERT_EQ(bss->usdt1_arg_ret, 0, "usdt1_arg_ret");
> +	ASSERT_EQ(bss->usdt1_arg_size, sizeof(array[0]), "usdt1_arg_size");
> +
> +cleanup:
> +	test_usdt_o2__destroy(skel);
> +}
> +
> +
> +
> +void test_usdt_o2(void)
> +{
> +	basic_sib_usdt();
> +}
> diff --git a/tools/testing/selftests/bpf/progs/test_usdt_o2.c b/tools/testing/selftests/bpf/progs/test_usdt_o2.c
> new file mode 100644
> index 000000000000..14602aa54578
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/test_usdt_o2.c
> @@ -0,0 +1,37 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2022 Meta Platforms, Inc. and affiliates. */
> +
> +#include "vmlinux.h"
> +#include <bpf/bpf_helpers.h>
> +#include <bpf/usdt.bpf.h>
> +
> +int my_pid;
> +
> +int usdt1_called;
> +u64 usdt1_cookie;
> +int usdt1_arg_cnt;
> +int usdt1_arg_ret;
> +u64 usdt1_arg;
> +int usdt1_arg_size;
> +
> +SEC("usdt")
> +int usdt1(struct pt_regs *ctx)
> +{
> +	long tmp;
> +
> +	if (my_pid != (bpf_get_current_pid_tgid() >> 32))
> +		return 0;
> +
> +	__sync_fetch_and_add(&usdt1_called, 1);
> +
> +	usdt1_cookie = bpf_usdt_cookie(ctx);
> +	usdt1_arg_cnt = bpf_usdt_arg_cnt(ctx);
> +
> +	usdt1_arg_ret = bpf_usdt_arg(ctx, 0, &tmp);
> +	usdt1_arg = (u64)tmp;
> +	usdt1_arg_size = bpf_usdt_arg_size(ctx, 0);
> +
> +	return 0;
> +}
> +
> +char _license[] SEC("license") = "GPL";
> -- 
> 2.43.0
> 
>