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 <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: David Vernet <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

kernel/bpf/verifier.c

···
 		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 ||
···
 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,