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

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Message-ID: <058d4d74-23ff-444b-a773-9d45204900f2@linux.dev>
Date: Tue, 5 Aug 2025 12:42:22 -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 v6 2/2] selftests/bpf: Force -O2 for USDT selftests to
 cover SIB handling logic



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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