bpf: Teach the verifier to recognize rdonly_mem as not null.

Teach the verifier to recognize PTR_TO_MEM | MEM_RDONLY as not NULL
otherwise if (!bpf_ksym_exists(known_kfunc)) doesn't go through
dead code elimination.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/20230321203854.3035-3-alexei.starovoitov@gmail.com

kernel/bpf/verifier.c

@@ -486,8 +486,17 @@ static bool type_is_sk_pointer(enum bpf_reg_type type)
 		type == PTR_TO_XDP_SOCK;
 }
 
+static bool type_may_be_null(u32 type)
+{
+	return type & PTR_MAYBE_NULL;
+}
+
 static bool reg_type_not_null(enum bpf_reg_type type)
 {
+	if (type_may_be_null(type))
+		return false;
+
+	type = base_type(type);
 	return type == PTR_TO_SOCKET ||
 		type == PTR_TO_TCP_SOCK ||
 		type == PTR_TO_MAP_VALUE ||
@@ -531,11 +540,6 @@ static bool type_is_rdonly_mem(u32 type)
 	return type & MEM_RDONLY;
 }
 
-static bool type_may_be_null(u32 type)
-{
-	return type & PTR_MAYBE_NULL;
-}
-
 static bool is_acquire_function(enum bpf_func_id func_id,
 				const struct bpf_map *map)
 {