bpf: add bpf_features enum

This commit adds a kernel side enum for use in conjucntion with BTF
CO-RE bpf_core_enum_value_exists. The goal of the enum is to assist
with available BPF features detection.

Support for bpf_rdonly_cast to void* is the first feature listed in
the enum. Here is an example usage:

  if (bpf_core_enum_value_exists(enum bpf_features,
                                 BPF_FEAT_RDONLY_CAST_TO_VOID))
     ... bpf_rdonly_cast(..., 0) ...

Signed-off-by: Eduard Zingerman <[email protected]>

Author: eddyz87

kernel/bpf/verifier.c

@@ -44,6 +44,11 @@ static const struct bpf_verifier_ops * const bpf_verifier_ops[] = {
 #undef BPF_LINK_TYPE
 };
 
+enum bpf_features {
+	BPF_FEAT_RDONLY_CAST_TO_VOID = 0,
+	BPF_FEAT_TOTAL,
+};
+
 struct bpf_mem_alloc bpf_global_percpu_ma;
 static bool bpf_global_percpu_ma_set;
 
@@ -24436,6 +24441,8 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr, bpfptr_t uattr, __u3
 	u32 log_true_size;
 	bool is_priv;
 
+	BTF_TYPE_EMIT(enum bpf_features);
+
 	/* no program is valid */
 	if (ARRAY_SIZE(bpf_verifier_ops) == 0)
 		return -EINVAL;