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 <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: Alexei Starovoitov

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