On Tue, Apr 29, 2025 at 7:19 AM Anton Protopopov
<a.s.protopopov@xxxxxxxxx> wrote:
>
> With the latest LLVM bpf selftests build will fail with
> the following error message:
>
> progs/profiler.inc.h:710:31: error: default initialization of an object of type 'typeof ((parent_task)->real_cred->uid.val)' (aka 'const unsigned int') leaves the object uninitialized and is incompatible with C++ [-Werror,-Wdefault-const-init-unsafe]
this is BPF-side code, what does C++ have to do with this, I'm confused...
Also, why using __u8[] is suddenly ok, and using the actual type
isn't? Eventually it all is initialized by bpf_probe_read_kernel(), so
compiler is wrong or I am misunderstanding something... Can you please
help me understand this?
> 710 | proc_exec_data->parent_uid = BPF_CORE_READ(parent_task, real_cred, uid.val);
> | ^
> tools/testing/selftests/bpf/tools/include/bpf/bpf_core_read.h:520:35: note: expanded from macro 'BPF_CORE_READ'
> 520 | ___type((src), a, ##__VA_ARGS__) __r; \
> | ^
>
> Fix this by declaring __r to be an array of __u8 of a proper size.
>
> Fixes: 792001f4f7aa ("libbpf: Add user-space variants of BPF_CORE_READ() family of macros")
> Fixes: a4b09a9ef945 ("libbpf: Add non-CO-RE variants of BPF_CORE_READ() macro family")
> Signed-off-by: Anton Protopopov <a.s.protopopov@xxxxxxxxx>
> ---
> tools/lib/bpf/bpf_core_read.h | 16 ++++++++--------
> 1 file changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h
> index c0e13cdf9660..b7395b75658c 100644
> --- a/tools/lib/bpf/bpf_core_read.h
> +++ b/tools/lib/bpf/bpf_core_read.h
> @@ -517,9 +517,9 @@ extern void *bpf_rdonly_cast(const void *obj, __u32 btf_id) __ksym __weak;
> * than enough for any practical purpose.
> */
> #define BPF_CORE_READ(src, a, ...) ({ \
> - ___type((src), a, ##__VA_ARGS__) __r; \
> + __u8 __r[sizeof(___type((src), a, ##__VA_ARGS__))]; \
> BPF_CORE_READ_INTO(&__r, (src), a, ##__VA_ARGS__); \
> - __r; \
> + *(___type((src), a, ##__VA_ARGS__) *)__r; \
> })
>
> /*
> @@ -533,16 +533,16 @@ extern void *bpf_rdonly_cast(const void *obj, __u32 btf_id) __ksym __weak;
> * input argument.
> */
> #define BPF_CORE_READ_USER(src, a, ...) ({ \
> - ___type((src), a, ##__VA_ARGS__) __r; \
> + __u8 __r[sizeof(___type((src), a, ##__VA_ARGS__))]; \
> BPF_CORE_READ_USER_INTO(&__r, (src), a, ##__VA_ARGS__); \
> - __r; \
> + *(___type((src), a, ##__VA_ARGS__) *)__r; \
> })
>
> /* Non-CO-RE variant of BPF_CORE_READ() */
> #define BPF_PROBE_READ(src, a, ...) ({ \
> - ___type((src), a, ##__VA_ARGS__) __r; \
> + __u8 __r[sizeof(___type((src), a, ##__VA_ARGS__))]; \
> BPF_PROBE_READ_INTO(&__r, (src), a, ##__VA_ARGS__); \
> - __r; \
> + *(___type((src), a, ##__VA_ARGS__) *)__r; \
> })
>
> /*
> @@ -552,9 +552,9 @@ extern void *bpf_rdonly_cast(const void *obj, __u32 btf_id) __ksym __weak;
> * not restricted to kernel types only.
> */
> #define BPF_PROBE_READ_USER(src, a, ...) ({ \
> - ___type((src), a, ##__VA_ARGS__) __r; \
> + __u8 __r[sizeof(___type((src), a, ##__VA_ARGS__))]; \
> BPF_PROBE_READ_USER_INTO(&__r, (src), a, ##__VA_ARGS__); \
> - __r; \
> + *(___type((src), a, ##__VA_ARGS__) *)__r; \
> })
>
> #endif
> --
> 2.34.1
>
>
