libbpf: add bpf_core_cast() macro

Add bpf_core_cast() macro that wraps bpf_rdonly_cast() kfunc. It's more
ergonomic than kfunc, as it automatically extracts btf_id with
bpf_core_type_id_kernel(), and works with type names. It also casts result
to (T *) pointer. See the definition of the macro, it's self-explanatory.

libbpf declares bpf_rdonly_cast() extern as __weak __ksym and should be
safe to not conflict with other possible declarations in user code.

But we do have a conflict with current BPF selftests that declare their
externs with first argument as `void *obj`, while libbpf opts into more
permissive `const void *obj`. This causes conflict, so we fix up BPF
selftests uses in the same patch.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240130212023.183765-2-andrii@kernel.org
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/tools/testing/selftests/bpf/bpf_kfuncs.h b/tools/testing/selftests/bpf/bpf_kfuncs.h
index 23c24f852f4f..0cd4111f954c 100644
--- a/tools/testing/selftests/bpf/bpf_kfuncs.h
+++ b/tools/testing/selftests/bpf/bpf_kfuncs.h
@@ -63,7 +63,7 @@ extern int bpf_sk_assign_tcp_reqsk(struct __sk_buff *skb, struct sock *sk,
 
 void *bpf_cast_to_kern_ctx(void *) __ksym;
 
-void *bpf_rdonly_cast(void *obj, __u32 btf_id) __ksym;
+extern void *bpf_rdonly_cast(const void *obj, __u32 btf_id) __ksym __weak;
 
 extern int bpf_get_file_xattr(struct file *file, const char *name,
 			      struct bpf_dynptr *value_ptr) __ksym;