bpf: reuse btf_prepare_func_args() check for main program BTF validation

Instead of btf_check_subprog_arg_match(), use btf_prepare_func_args()
logic to validate "trustworthiness" of main BPF program's BTF information,
if it is present.

We ignored results of original BTF check anyway, often times producing
confusing and ominously-sounding "reg type unsupported for arg#0
function" message, which has no apparent effect on program correctness
and verification process.

All the -EFAULT returning sanity checks are already performed in
check_btf_info_early(), so there is zero reason to have this duplication
of logic between btf_check_subprog_call() and btf_check_subprog_arg_match().
Dropping btf_check_subprog_arg_match() simplifies
btf_check_func_arg_match() further removing `bool processing_call` flag.

One subtle bit that was done by btf_check_subprog_arg_match() was
potentially marking main program's BTF as unreliable. We do this
explicitly now with a dedicated simple check, preserving the original
behavior, but now based on well factored btf_prepare_func_args() logic.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231215011334.2307144-3-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 12 insertions, 13 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 6555785b9f63..c26e9ab5226c 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -19904,6 +19904,7 @@ static void free_states(struct bpf_verifier_env *env)
 static int do_check_common(struct bpf_verifier_env *env, int subprog)
 {
 	bool pop_log = !(env->log.level & BPF_LOG_LEVEL2);
+	struct bpf_subprog_info *sub = subprog_info(env, subprog);
 	struct bpf_verifier_state *state;
 	struct bpf_reg_state *regs;
 	int ret, i;
@@ -19930,9 +19931,9 @@ static int do_check_common(struct bpf_verifier_env *env, int subprog)
 	state->first_insn_idx = env->subprog_info[subprog].start;
 	state->last_insn_idx = -1;
 
+
 	regs = state->frame[state->curframe]->regs;
 	if (subprog || env->prog->type == BPF_PROG_TYPE_EXT) {
-		struct bpf_subprog_info *sub = subprog_info(env, subprog);
 		const char *sub_name = subprog_name(env, subprog);
 		struct bpf_subprog_arg_info *arg;
 		struct bpf_reg_state *reg;
@@ -19979,21 +19980,19 @@ static int do_check_common(struct bpf_verifier_env *env, int subprog)
 			}
 		}
 	} else {
+		/* if main BPF program has associated BTF info, validate that
+		 * it's matching expected signature, and otherwise mark BTF
+		 * info for main program as unreliable
+		 */
+		if (env->prog->aux->func_info_aux) {
+			ret = btf_prepare_func_args(env, 0);
+			if (ret || sub->arg_cnt != 1 || sub->args[0].arg_type != ARG_PTR_TO_CTX)
+				env->prog->aux->func_info_aux[0].unreliable = true;
+		}
+
 		/* 1st arg to a function */
 		regs[BPF_REG_1].type = PTR_TO_CTX;
 		mark_reg_known_zero(env, regs, BPF_REG_1);
-		ret = btf_check_subprog_arg_match(env, subprog, regs);
-		if (ret == -EFAULT)
-			/* unlikely verifier bug. abort.
-			 * ret == 0 and ret < 0 are sadly acceptable for
-			 * main() function due to backward compatibility.
-			 * Like socket filter program may be written as:
-			 * int bpf_prog(struct pt_regs *ctx)
-			 * and never dereference that ctx in the program.
-			 * 'struct pt_regs' is a type mismatch for socket
-			 * filter that should be using 'struct __sk_buff'.
-			 */
-			goto out;
 	}
 
 	ret = do_check(env);