Re: [PATCH bpf-next v12 0/2] libbpf: fix USDT SIB argument handling causing unrecognized register error

[Jiri Olsa <olsajiri@gmail.com>]

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
> 
>