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Message-ID: <d438bf3a-a9c9-4d34-b814-63f2e9bb3a85@linux.dev>
Date: Tue, 26 Aug 2025 09:51:08 -0700
From: Yonghong Song <yonghong.song@...ux.dev>
To: Jiawei Zhao <phoenix500526@....com>, ast@...nel.org
Cc: daniel@...earbox.net, andrii@...nel.org, bpf@...r.kernel.org,
linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v15 2/2] selftests/bpf: Enrich subtest_basic_usdt
case in selftests to cover SIB handling logic
On 8/23/25 12:18 AM, 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@...1(%rip)".
You can remove "-1@...1(%rip)" which is not relavent to this patch.
Also, please also mention usage of STAP_PROBE_ASM so the test works
for both gcc and clang.
I tested the patch with gcc/clang for x86_64 and it works fine.
I didn't test 32-bit x86 arch.
>
> In this patch:
> - enrich subtest_basic_usdt test case to cover SIB addressing usdt argument spec
> handling logic
>
> Signed-off-by: Jiawei Zhao <phoenix500526@....com>
> ---
> tools/testing/selftests/bpf/prog_tests/usdt.c | 62 ++++++++++++++++++-
> tools/testing/selftests/bpf/progs/test_usdt.c | 32 ++++++++++
> 2 files changed, 92 insertions(+), 2 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/usdt.c b/tools/testing/selftests/bpf/prog_tests/usdt.c
> index 9057e983cc54..4b264f7d3324 100644
> --- a/tools/testing/selftests/bpf/prog_tests/usdt.c
> +++ b/tools/testing/selftests/bpf/prog_tests/usdt.c
> @@ -10,6 +10,26 @@
>
> int lets_test_this(int);
>
> +#if defined(__x86_64__) || defined(__i386__)
> +/*
> + * SIB (Scale-Index-Base) addressing format:
> + * "size@(base_reg, index_reg, scale)"
> + * - 'size' is the size in bytes of the array element, and its sign indicates
> + * whether the type is signed (negative) or unsigned (positive).
> + * - 'base_reg' is the register holding the base address, normally rdx or edx
> + * - 'index_reg' is the register holding the index, normally rax or eax
> + * - 'scale' is the scaling factor (typically 1, 2, 4, or 8), which matches the
> + * size of the element type.
> + *
> + * For example, for an array of 'short' (signed 2-byte elements), the SIB spec would be:
> + * - size: -2 (negative because 'short' is signed)
> + * - scale: 2 (since sizeof(short) == 2)
> + * The resulting SIB format: "-2@(%%rdx,%%rax,2)"
> + */
> +static volatile short array[] = {-1, -2, -3, -4};
> +#define USDT_SIB_ARG_SPEC -2@(%%rdx,%%rax,2)
> +#endif
> +
> static volatile int idx = 2;
> static volatile __u64 bla = 0xFEDCBA9876543210ULL;
> static volatile short nums[] = {-1, -2, -3, -4};
> @@ -25,6 +45,10 @@ unsigned short test_usdt0_semaphore SEC(".probes");
> unsigned short test_usdt3_semaphore SEC(".probes");
> unsigned short test_usdt12_semaphore SEC(".probes");
>
> +#if defined(__x86_64__) || defined(__i386__)
> +unsigned short test_usdt_sib_semaphore SEC(".probes");
> +#endif
> +
> static void __always_inline trigger_func(int x) {
> long y = 42;
>
> @@ -40,12 +64,27 @@ static void __always_inline trigger_func(int x) {
> }
> }
>
> +#if defined(__x86_64__) || defined(__i386__)
> +static void trigger_sib_spec(void)
> +{
> + /* Base address + offset + (index * scale) */
> + /* Force SIB addressing with inline assembly */
> + asm volatile(
> + STAP_PROBE_ASM(test, usdt_sib, USDT_SIB_ARG_SPEC)
> + :
> + : "d"(array), "a"(0)
> + : "memory"
> + );
> +}
> +#endif
> +
> static void subtest_basic_usdt(void)
> {
> LIBBPF_OPTS(bpf_usdt_opts, opts);
> struct test_usdt *skel;
> struct test_usdt__bss *bss;
> int err, i;
> + const __u64 expected_cookie = 0xcafedeadbeeffeed;
>
> skel = test_usdt__open_and_load();
> if (!ASSERT_OK_PTR(skel, "skel_open"))
> @@ -59,20 +98,29 @@ static void subtest_basic_usdt(void)
> goto cleanup;
>
> /* usdt0 won't be auto-attached */
> - opts.usdt_cookie = 0xcafedeadbeeffeed;
> + opts.usdt_cookie = expected_cookie;
> skel->links.usdt0 = bpf_program__attach_usdt(skel->progs.usdt0,
> 0 /*self*/, "/proc/self/exe",
> "test", "usdt0", &opts);
> if (!ASSERT_OK_PTR(skel->links.usdt0, "usdt0_link"))
> goto cleanup;
>
> +#if defined(__x86_64__) || defined(__i386__)
> + opts.usdt_cookie = expected_cookie;
> + skel->links.usdt_sib = bpf_program__attach_usdt(skel->progs.usdt_sib,
> + 0 /*self*/, "/proc/self/exe",
> + "test", "usdt_sib", &opts);
formate issue in the above?
> + if (!ASSERT_OK_PTR(skel->links.usdt_sib, "usdt_sib_link"))
> + goto cleanup;
> +#endif
> +
> trigger_func(1);
>
> ASSERT_EQ(bss->usdt0_called, 1, "usdt0_called");
> ASSERT_EQ(bss->usdt3_called, 1, "usdt3_called");
> ASSERT_EQ(bss->usdt12_called, 1, "usdt12_called");
>
> - ASSERT_EQ(bss->usdt0_cookie, 0xcafedeadbeeffeed, "usdt0_cookie");
> + ASSERT_EQ(bss->usdt0_cookie, expected_cookie, "usdt0_cookie");
> ASSERT_EQ(bss->usdt0_arg_cnt, 0, "usdt0_arg_cnt");
> ASSERT_EQ(bss->usdt0_arg_ret, -ENOENT, "usdt0_arg_ret");
> ASSERT_EQ(bss->usdt0_arg_size, -ENOENT, "usdt0_arg_size");
> @@ -156,6 +204,16 @@ static void subtest_basic_usdt(void)
> ASSERT_EQ(bss->usdt3_args[1], 42, "usdt3_arg2");
> ASSERT_EQ(bss->usdt3_args[2], (uintptr_t)&bla, "usdt3_arg3");
>
> +#if defined(__x86_64__) || defined(__i386__)
> + trigger_sib_spec();
> + ASSERT_EQ(bss->usdt_sib_called, 1, "usdt_sib_called");
> + ASSERT_EQ(bss->usdt_sib_cookie, expected_cookie, "usdt_sib_cookie");
> + ASSERT_EQ(bss->usdt_sib_arg_cnt, 1, "usdt_sib_arg_cnt");
> + ASSERT_EQ(bss->usdt_sib_arg, nums[0], "usdt_sib_arg");
> + ASSERT_EQ(bss->usdt_sib_arg_ret, 0, "usdt_sib_arg_ret");
> + ASSERT_EQ(bss->usdt_sib_arg_size, sizeof(nums[0]), "usdt_sib_arg_size");
> +#endif
> +
> cleanup:
> test_usdt__destroy(skel);
> }
> diff --git a/tools/testing/selftests/bpf/progs/test_usdt.c b/tools/testing/selftests/bpf/progs/test_usdt.c
> index 096488f47fbc..63db72253316 100644
> --- a/tools/testing/selftests/bpf/progs/test_usdt.c
> +++ b/tools/testing/selftests/bpf/progs/test_usdt.c
> @@ -107,4 +107,36 @@ int BPF_USDT(usdt12, int a1, int a2, long a3, long a4, unsigned a5,
> return 0;
> }
>
> +
> +int usdt_sib_called;
> +u64 usdt_sib_cookie;
> +int usdt_sib_arg_cnt;
> +int usdt_sib_arg_ret;
> +u64 usdt_sib_arg;
> +int usdt_sib_arg_size;
> +
> +/*
> + * usdt_sib is only tested on x86-related architectures, so it requires
> + * manual attach since auto-attach will panic tests under other architectures
> + */
> +SEC("usdt")
> +int usdt_sib(struct pt_regs *ctx)
> +{
> + long tmp;
> +
> + if (my_pid != (bpf_get_current_pid_tgid() >> 32))
> + return 0;
> +
> + __sync_fetch_and_add(&usdt_sib_called, 1);
> +
> + usdt_sib_cookie = bpf_usdt_cookie(ctx);
> + usdt_sib_arg_cnt = bpf_usdt_arg_cnt(ctx);
> +
> + usdt_sib_arg_ret = bpf_usdt_arg(ctx, 0, &tmp);
> + usdt_sib_arg = (short)tmp;
Maybe change usdt_sib_arg type as short?
> + usdt_sib_arg_size = bpf_usdt_arg_size(ctx, 0);
> +
> + return 0;
> +}
> +
> char _license[] SEC("license") = "GPL";
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