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

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Message-ID: <20250802084803.108777-3-phoenix500526@163.com>
Date: Sat,  2 Aug 2025 08:48:03 +0000
From: Jiawei Zhao <phoenix500526@....com>
To: ast@...nel.org
Cc: daniel@...earbox.net,
	andrii@...nel.org,
	yonghong.song@...ux.dev,
	bpf@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v6 2/2] selftests/bpf: Force -O2 for USDT selftests to cover SIB handling logic

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
+
 # 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 };
 
 #define SEC(name) __attribute__((section(name), used))
 
@@ -27,6 +32,7 @@ unsigned short test_usdt12_semaphore SEC(".probes");
 
 static void __always_inline trigger_func(int x) {
 	long y = 42;
+	signed char t1 = -127;
 
 	if (test_usdt0_semaphore)
 		STAP_PROBE(test, usdt0);
@@ -36,7 +42,7 @@ static void __always_inline trigger_func(int x) {
 		STAP_PROBE12(test, usdt12,
 			     x, x + 1, y, x + y, 5,
 			     y / 7, bla, &bla, -9, nums[x],
-			     nums[idx], t1.y);
+			     nums[idx], t1);
 	}
 }
 
@@ -106,7 +112,7 @@ static void subtest_basic_usdt(void)
 	ASSERT_EQ(bss->usdt12_args[8], -9, "usdt12_arg9");
 	ASSERT_EQ(bss->usdt12_args[9], nums[1], "usdt12_arg10");
 	ASSERT_EQ(bss->usdt12_args[10], nums[idx], "usdt12_arg11");
-	ASSERT_EQ(bss->usdt12_args[11], t1.y, "usdt12_arg12");
+	ASSERT_EQ(bss->usdt12_args[11], -127, "usdt12_arg12");
 
 	int usdt12_expected_arg_sizes[12] = { 4, 4, 8, 8, 4, 8, 8, 8, 4, 2, 2, 1 };
 
-- 
2.43.0