On Mon, Aug 18, 2025 at 12:46 AM Jiawei Zhao phoenix500526@163.com wrote:
usdt_o1 is intended to exercise the SIB (Scale-Index-Base) argument handling in libbpf's USDT path. With GCC 13 this reliably produced a SIB-form argument (e.g. 8@(%rdx,%rax,8)), but with newer GCC (e.g. 15) the compiler frequently optimizes the probe argument into a plain register (e.g. 8@%rax) or a stack slot, so the test stops covering the SIB code path and becomes flaky across toolchains.
Force a SIB memory operand in the probe by:
- placing the base pointer into %rdx and the index into %rax using an empty inline asm with output constraints ("=d", "=a") and matching inputs
- immediately passing base[idx] to STAP_PROBE1.
- only enable on x86 platform.
This makes the compiler encode the operand as SIB (base + index8), which in .note.stapsdt shows up as 8@(%rdx,%rax,8) regardless of GCC version. A memory clobber and noinline prevent reordering/re-allocation around the probe site.
This change is x86_64-specific and does not alter program semantics; it only stabilizes the USDT argument shape so the test consistently validates SIB handling. Clang historically prefers stack temporaries for such operands, but the selftests build with GCC, and this keeps behavior stable across GCC versions without introducing a separate .S file.
Signed-off-by: Jiawei Zhao phoenix500526@163.com
.../testing/selftests/bpf/prog_tests/usdt_o1.c | 18 +++++++++++++++--- 1 file changed, 15 insertions(+), 3 deletions(-)
See the suggestion on the previous patch. sdt.h has STAP_PROBE_ASM() macro that allows to trigger USDTs from asm block. I have never used it, but this looks like a perfect opportunity to make use of it. Can you please give it a try?
diff --git a/tools/testing/selftests/bpf/prog_tests/usdt_o1.c b/tools/testing/selftests/bpf/prog_tests/usdt_o1.c index 706168e804cb..6c04519b3757 100644 --- a/tools/testing/selftests/bpf/prog_tests/usdt_o1.c +++ b/tools/testing/selftests/bpf/prog_tests/usdt_o1.c @@ -18,11 +18,19 @@ int lets_test_this(int); static volatile __u64 array[1] = {test_value};
-static __always_inline void trigger_func(void) +static noinline void trigger_func(void) { +#if defined(__x86_64__) || defined(__i386__) /* Base address + offset + (index * scale) */
for (volatile int i = 0; i <= 0; i++)STAP_PROBE1(test, usdt1, array[i]);
/* Force SIB addressing with inline assembly */const __u64 *base;__u32 idx;/* binding base to %rdx and idx to %rax */asm volatile("" : "=d"(base), "=a"(idx) : "0"(array), "1"((__u32)0) : "memory");STAP_PROBE1(test, usdt1, base[idx]);+#else
STAP_PROBE1(test, usdt1, array[0]);+#endif }
static void basic_sib_usdt(void) @@ -66,5 +74,9 @@ static void basic_sib_usdt(void)
void test_usdt_o1(void) { +#if !defined(__x86_64__) && !defined(__i386__)
test__skip();return;+#endif basic_sib_usdt(); } -- 2.43.0