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

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Message-ID: <e7ba3f7f-38b8-4c06-8aff-ef1fb8d04d86@linux.dev>
Date: Thu, 7 Aug 2025 11:01:08 -0700
From: Yonghong Song <yonghong.song@...ux.dev>
To: 赵佳炜 <phoenix500526@....com>
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: [PATCH v7 2/2] selftests/bpf: Force -O2 for USDT selftests to
 cover SIB handling logic



On 8/6/25 7:57 PM, 赵佳炜 wrote:
>
>
>
> Hi Yonghong,
>
> I noticed that the USDT argument specification generated by GCC 14 is '8@...ay(,%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

I think 'array' should be in symbol table, so there is no need to check dwarf in my opinion.

> 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?

Let us handle this since '8@...ay(,%rax,8)' may appear in CI environment.

>
>
> At 2025-08-07 02:17:34, "Yonghong Song" <yonghong.song@...ux.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@...1(%rip)".
>>>
>>> In this patch:
>>> - add usdt_o2 test case to cover SIB addressing usdt argument spec
>>>     handling logic
>>>
>>> Signed-off-by: Jiawei Zhao <phoenix500526@....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@...ay(,%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.
>>
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