[PATCH v4 bpf-next] bpf: fix uninitialized values in BPF_{CORE,PROBE}_READ

[PATCH v4 bpf-next] bpf: fix uninitialized values in BPF_{CORE,PROBE}_READ

[Anton Protopopov]

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]
      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;                               \
          |                                          ^

This happens because BPF_CORE_READ (and other macro) declare the
variable __r using the ___type macro which can inherit const modifier
from intermediate types.

Fix this by using __typeof_unqual__, when supported. (And when it
is not supported, the problem shouldn't appear, as older compilers
haven't complained.)

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@gmail.com>
---
 tools/lib/bpf/bpf_core_read.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h
index c0e13cdf9660..b997c68bd945 100644
--- a/tools/lib/bpf/bpf_core_read.h
+++ b/tools/lib/bpf/bpf_core_read.h
@@ -388,7 +388,13 @@ extern void *bpf_rdonly_cast(const void *obj, __u32 btf_id) __ksym __weak;
 #define ___arrow10(a, b, c, d, e, f, g, h, i, j) a->b->c->d->e->f->g->h->i->j
 #define ___arrow(...) ___apply(___arrow, ___narg(__VA_ARGS__))(__VA_ARGS__)
 
+#if defined(__clang__) && (__clang_major__ >= 19)
+#define ___type(...) __typeof_unqual__(___arrow(__VA_ARGS__))
+#elif defined(__GNUC__) && (__GNUC__ >= 14)
+#define ___type(...) __typeof_unqual__(___arrow(__VA_ARGS__))
+#else
 #define ___type(...) typeof(___arrow(__VA_ARGS__))
+#endif
 
 #define ___read(read_fn, dst, src_type, src, accessor)			    \
 	read_fn((void *)(dst), sizeof(*(dst)), &((src_type)(src))->accessor)
-- 
2.34.1

Re: [PATCH v4 bpf-next] bpf: fix uninitialized values in BPF_{CORE,PROBE}_READ

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Andrii Nakryiko <andrii@kernel.org>:

On Fri,  2 May 2025 19:30:31 +0000 you 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]
>       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;                               \
>           |                                          ^
> 
> [...]

Here is the summary with links:
  - [v4,bpf-next] bpf: fix uninitialized values in BPF_{CORE,PROBE}_READ
    https://git.kernel.org/bpf/bpf-next/c/41d4ce6df3f4

You are awesome, thank you!
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