bpf: Allow helpers access trusted PTR_TO_BTF_ID.

The verifier rejects the code:
  bpf_strncmp(task->comm, 16, "my_task");
with the message:
  16: (85) call bpf_strncmp#182
  R1 type=trusted_ptr_ expected=fp, pkt, pkt_meta, map_key, map_value, mem, ringbuf_mem, buf

Teach the verifier that such access pattern is safe.
Do not allow untrusted and legacy ptr_to_btf_id to be passed into helpers.
Reported-by: David Vernet <void@manifault.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230313235845.61029-3-alexei.starovoitov@gmail.comSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

kernel/bpf/verifier.c

@@ -6303,6 +6303,9 @@ static int check_helper_mem_access(struct bpf_verifier_env *env, int regno,
 				env,
 				regno, reg->off, access_size,
 				zero_size_allowed, ACCESS_HELPER, meta);
+	case PTR_TO_BTF_ID:
+		return check_ptr_to_btf_access(env, regs, regno, reg->off,
+					       access_size, BPF_READ, -1);
 	case PTR_TO_CTX:
 		/* in case the function doesn't know how to access the context,
 		 * (because we are in a program of type SYSCALL for example), we
@@ -7014,6 +7017,7 @@ static const struct bpf_reg_types mem_types = {
 		PTR_TO_MEM,
 		PTR_TO_MEM | MEM_RINGBUF,
 		PTR_TO_BUF,
+		PTR_TO_BTF_ID | PTR_TRUSTED,
 	},
 };
 
@@ -7145,6 +7149,17 @@ static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
 	if (base_type(reg->type) != PTR_TO_BTF_ID)
 		return 0;
 
+	if (compatible == &mem_types) {
+		if (!(arg_type & MEM_RDONLY)) {
+			verbose(env,
+				"%s() may write into memory pointed by R%d type=%s\n",
+				func_id_name(meta->func_id),
+				regno, reg_type_str(env, reg->type));
+			return -EACCES;
+		}
+		return 0;
+	}
+
 	switch ((int)reg->type) {
 	case PTR_TO_BTF_ID:
 	case PTR_TO_BTF_ID | PTR_TRUSTED: