libbpf: Introduce bpf_ksym_exists() macro.

Introduce bpf_ksym_exists() macro that can be used by BPF programs
to detect at load time whether particular ksym (either variable or kfunc)
is present in the kernel.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230317201920.62030-4-alexei.starovoitov@gmail.com

diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
index 7d12d3e..e7e1a8a 100644 (file)
--- a/tools/lib/bpf/bpf_helpers.h
+++ b/tools/lib/bpf/bpf_helpers.h
@@ -177,6 +177,11 @@ enum libbpf_tristate {
 #define __kptr_untrusted __attribute__((btf_type_tag("kptr_untrusted")))
 #define __kptr __attribute__((btf_type_tag("kptr")))
 
+#define bpf_ksym_exists(sym) ({                                                                        \
+       _Static_assert(!__builtin_constant_p(!!sym), #sym " should be marked as __weak");       \
+       !!sym;                                                                                  \
+})
+
 #ifndef ___bpf_concat
 #define ___bpf_concat(a, b) a ## b
 #endif