Re:Re: [PATCH v7 2/2] selftests/bpf: Force -O2 for USDT selftests to cover SIB handling logic

[赵佳炜 <phoenix500526@163.com>]

Hi Yonghong, 

I noticed that the USDT argument specification generated by GCC 14 is '8@array(,%rax,8)'. 
This pattern is currently not handled correctly. I'm exploring whether I can use DWARF information
to calculate the address of this variable. This approach seems to work. However, since I can't
reproduce the same issue on my machine, I plan to implement this approach for the PC-relative
issue in a separate patch. Would that affect the merging of this patch?


At 2025-08-07 02:17:34, "Yonghong Song" <yonghong.song@linux.dev> wrote:
>
>
>On 8/6/25 2:24 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@4+t1(%rip)".
>>
>> In this patch:
>> - add usdt_o2 test case to cover SIB addressing usdt argument spec
>>    handling logic
>>
>> Signed-off-by: Jiawei Zhao <phoenix500526@163.com>
>> ---
>>   tools/testing/selftests/bpf/Makefile          |  8 +++
>>   .../selftests/bpf/prog_tests/usdt_o2.c        | 71 +++++++++++++++++++
>>   .../selftests/bpf/progs/test_usdt_o2.c        | 37 ++++++++++
>>   3 files changed, 116 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 910d8d6402ef..68cf6a9cf05f 100644
>> --- a/tools/testing/selftests/bpf/Makefile
>> +++ b/tools/testing/selftests/bpf/Makefile
>> @@ -759,6 +759,14 @@ TRUNNER_BPF_BUILD_RULE := $$(error no BPF objects should be built)
>>   TRUNNER_BPF_CFLAGS :=
>>   $(eval $(call DEFINE_TEST_RUNNER,test_maps))
>>   
>> +# Use -O2 optimization to generate SIB addressing usdt argument spec
>> +# Only apply on x86 architecture where SIB addressing is relevant
>> +ifeq ($(ARCH), x86)
>> +$(OUTPUT)/usdt_o2.test.o: CFLAGS:=$(subst O0,O2,$(CFLAGS))
>> +$(OUTPUT)/cpuv4/usdt_o2.test.o: CFLAGS:=$(subst O0,O2,$(CFLAGS))
>> +$(OUTPUT)/no_alu32/usdt_o2.test.o: CFLAGS:=$(subst O0,O2,$(CFLAGS))
>> +endif
>
>I tried your selftest with gcc14 and llvm20 in my environment. See below:
>
>llvm20:
>Displaying notes found in: .note.stapsdt
>   Owner                Data size        Description
>   stapsdt              0x0000002f       NT_STAPSDT (SystemTap probe descriptors)
>     Provider: test
>     Name: usdt1
>     Location: 0x00000000000003ac, Base: 0x0000000000000000, Semaphore: 0x0000000000000000
>     Arguments: 8@-64(%rbp)
>
>gcc14:
>Displaying notes found in: .note.stapsdt
>   Owner                Data size        Description
>   stapsdt              0x00000034       NT_STAPSDT (SystemTap probe descriptors)
>     Provider: test
>     Name: usdt1
>     Location: 0x0000000000000334, Base: 0x0000000000000000, Semaphore: 0x0000000000000000
>     Arguments: 8@array(,%rax,8)
>
>llvm20 and gcc14 generate different usdt patterns. '8@-64(%rbp)' already supports so
>with SIB support, the test should pass CI, I think.
>
>> +
>>   # 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..f04b756b3640
>> --- /dev/null
>> +++ b/tools/testing/selftests/bpf/prog_tests/usdt_o2.c
>> @@ -0,0 +1,71 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/* Copyright (c) 2025 Jiawei Zhao <phoenix500526@163.com>. */
>> +#include <test_progs.h>
>> +
>> +#define _SDT_HAS_SEMAPHORES 1
>> +#include "../sdt.h"
>> +#include "test_usdt_o2.skel.h"
>> +
>> +int lets_test_this(int);
>> +
>> +#define test_value 0xFEDCBA9876543210ULL
>> +#define SEC(name) __attribute__((section(name), used))
>> +
>> +
>> +static volatile __u64 array[1] = {test_value};
>> +unsigned short test_usdt1_semaphore SEC(".probes");
>> +
>> +static __always_inline void trigger_func(void)
>> +{
>> +	/* Base address + offset + (index * scale) */
>> +	if (test_usdt1_semaphore) {
>> +		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";