On Thu, Aug 21, 2025 at 03:27:10PM +0000, 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@4+t1(%rip)".
The current USDT implementation in libbpf cannot parse these two formats, causing `bpf_program__attach_usdt()` to fail with -ENOENT (unrecognized register).
This patch series adds support for SIB addressing mode in USDT probes. The main changes include:
- add correct handling logic for SIB-addressed arguments in `parse_usdt_arg`.
- add an usdt_o2 test case to cover SIB addressing mode.
Testing shows that the SIB probe correctly generates 8@(%rcx,%rax,8) argument spec and passes all validation checks.
The modification history of this patch series: Change since v1:
- refactor the code to make it more readable
- modify the commit message to explain why and how
Change since v2:
- fix the `scale` uninitialized error
Change since v3:
- force -O2 optimization for usdt.test.o to generate SIB addressing usdt and pass all test cases.
Change since v4:
- split the patch into two parts, one for the fix and the other for the test
Change since v5:
- Only enable optimization for x86 architecture to generate SIB addressing usdt argument spec.
Change since v6:
- Add an usdt_o2 test case to cover SIB addressing mode.
- Reinstate the usdt.c test case.
Change since v7:
- Refactor modifications to __bpf_usdt_arg_spec to avoid increasing its size, achieving better compatibility
- Fix some minor code style issues
- Refactor the usdt_o2 test case, removing semaphore and adding GCC attribute to force -O2 optimization
Change since v8:
- Refactor the usdt_o2 test case, using assembly to force SIB addressing mode.
Change since v9:
- Only enable the usdt_o2 test case on x86_64 and i386 architectures since the SIB addressing mode is only supported on x86_64 and i386.
Change since v10:
- Replace `__attribute__((optimize("O2")))` with `#pragma GCC optimize("O1")` to fix the issue where the optimized compilation condition works improperly.
- Renamed test case usdt_o2 and relevant files name to usdt_o1 in that O1 level optimization is enough to generate SIB addressing usdt argument spec.
Change since v11:
- Replace `STAP_PROBE1` with `STAP_PROBE_ASM`
- Use bit fields instead of bit shifting operations
- Merge the usdt_o1 test case into the usdt test case
hi, I can see patchset v12 twice with different stats.. this one's the latter, but you might want to resend with new version
jirka
Jiawei Zhao (2): libbpf: fix USDT SIB argument handling causing unrecognized register error selftests/bpf: Enrich subtest_basic_usdt case in selftests to cover SIB handling logic
tools/lib/bpf/usdt.bpf.h | 47 ++++++++++++++- tools/lib/bpf/usdt.c | 58 +++++++++++++++++-- tools/testing/selftests/bpf/prog_tests/usdt.c | 44 +++++++++++++- tools/testing/selftests/bpf/progs/test_usdt.c | 30 ++++++++++ 4 files changed, 170 insertions(+), 9 deletions(-)
-- 2.43.0