linux-kernel - Re:Re: [PATCH v6 2/2] selftests/bpf: Force -O2 for USDT selftests to cover SIB handling logic

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Message-ID: <393dae7e.9b04.1987ec77091.Coremail.phoenix500526@163.com>
Date: Wed, 6 Aug 2025 17:47:36 +0800 (CST)
From: 赵佳炜 <phoenix500526@....com>
To: "Yonghong Song" <yonghong.song@...ux.dev>
Cc: ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
	bpf@...r.kernel.org, linux-kselftest@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re:Re: [PATCH v6 2/2] selftests/bpf: Force -O2 for USDT selftests
 to cover SIB handling logic



OK, I’ve already added an usdt_o2 test and passed it. 




At 2025-08-06 03:42:22, "Yonghong Song" <yonghong.song@...ux.dev> wrote:
>
>
>On 8/2/25 1:48 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)".
>>
>> In this patch:
>> - force -O2 optimization for usdt.test.o to generate SIB addressing usdt
>>    argument spec.
>> - change the global variable t1 to a local variable, to avoid compiler
>>    generating PC-relative addressing mode for it.
>>
>> Signed-off-by: Jiawei Zhao <phoenix500526@....com>
>> ---
>>   tools/testing/selftests/bpf/Makefile          |  8 ++++++++
>>   tools/testing/selftests/bpf/prog_tests/usdt.c | 18 ++++++++++++------
>>   2 files changed, 20 insertions(+), 6 deletions(-)
>>
>> diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
>> index 910d8d6402ef..4b77d06d5c42 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 $(call DEFINE_TEST_RUNNER,test_maps))
>>   
>> +# Force usdt.c to use -O2 optimization to generate SIB addressing
>> +# Only apply on x86 architecture where SIB addressing is relevant
>> +ifeq ($(ARCH), x86)
>> +$(OUTPUT)/usdt.test.o: CFLAGS:=$(subst O0,O2,$(CFLAGS))
>> +$(OUTPUT)/cpuv4/usdt.test.o: CFLAGS:=$(subst O0,O2,$(CFLAGS))
>> +$(OUTPUT)/no_alu32/usdt.test.o: CFLAGS:=$(subst O0,O2,$(CFLAGS))
>> +endif
>
>This is no good. You should not change from -O0 to -O2. The existing usdt.c
>test should be kept. I assume at -O0 level, the compiler probably
>won't generate SIB pattern.
>
>You could add another usdt test e.g. usdt_o2.c and force
>usdt_o2 is compiled with -O2 optimizations and in usdt_o2 focusing on
>SIB probe.
>
>> +
>>   # 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.c b/tools/testing/selftests/bpf/prog_tests/usdt.c
>> index 495d66414b57..86f354d25aef 100644
>> --- a/tools/testing/selftests/bpf/prog_tests/usdt.c
>> +++ b/tools/testing/selftests/bpf/prog_tests/usdt.c
>> @@ -14,10 +14,15 @@ static volatile int idx = 2;
>>   static volatile __u64 bla = 0xFEDCBA9876543210ULL;
>>   static volatile short nums[] = {-1, -2, -3, -4};
>>   
>> -static volatile struct {
>> -	int x;
>> -	signed char y;
>> -} t1 = { 1, -127 };
>> +/*
>> + * TODO:  At O2 optimization level, t1's USDT argument spec becomes -1@...1(%rip).
>> + * Since libbpf doesn't support RIP addressing mode yet, this causes "unrecognized register" errors.
>> + * This test will be re-enabled once libbpf supports RIP addressing mode.
>> + */
>> +// static volatile struct {
>> +//	int x;
>> +//	signed char y;
>> +// } t1 = { 1, -127 };
>>   
>
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