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 <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

include/linux/bpf.h

···
 			   struct btf_func_model *m);
 
 struct bpf_reg_state;
-int btf_check_subprog_arg_match(struct bpf_verifier_env *env, int subprog,
-				struct bpf_reg_state *regs);
 int btf_check_subprog_call(struct bpf_verifier_env *env, int subprog,
 			   struct bpf_reg_state *regs);
 int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog);

kernel/bpf/btf.c

···
 static int btf_check_func_arg_match(struct bpf_verifier_env *env,
 				    const struct btf *btf, u32 func_id,
 				    struct bpf_reg_state *regs,
-				    bool ptr_to_mem_ok,
-				    bool processing_call)
+				    bool ptr_to_mem_ok)
 {
 	enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
 	struct bpf_verifier_log *log = &env->log;
···
 					i, btf_type_str(t));
 				return -EINVAL;
 			}
-		} else if (ptr_to_mem_ok && processing_call) {
+		} else if (ptr_to_mem_ok) {
 			const struct btf_type *resolve_ret;
 			u32 type_size;
 
···
 	return 0;
 }
 
-/* Compare BTF of a function declaration with given bpf_reg_state.
- * Returns:
- * EFAULT - there is a verifier bug. Abort verification.
- * EINVAL - there is a type mismatch or BTF is not available.
- * 0 - BTF matches with what bpf_reg_state expects.
- * Only PTR_TO_CTX and SCALAR_VALUE states are recognized.
- */
-int btf_check_subprog_arg_match(struct bpf_verifier_env *env, int subprog,
-				struct bpf_reg_state *regs)
-{
-	struct bpf_prog *prog = env->prog;
-	struct btf *btf = prog->aux->btf;
-	bool is_global;
-	u32 btf_id;
-	int err;
-
-	if (!prog->aux->func_info)
-		return -EINVAL;
-
-	btf_id = prog->aux->func_info[subprog].type_id;
-	if (!btf_id)
-		return -EFAULT;
-
-	if (prog->aux->func_info_aux[subprog].unreliable)
-		return -EINVAL;
-
-	is_global = prog->aux->func_info_aux[subprog].linkage == BTF_FUNC_GLOBAL;
-	err = btf_check_func_arg_match(env, btf, btf_id, regs, is_global, false);
-
-	/* Compiler optimizations can remove arguments from static functions
-	 * or mismatched type can be passed into a global function.
-	 * In such cases mark the function as unreliable from BTF point of view.
-	 */
-	if (err)
-		prog->aux->func_info_aux[subprog].unreliable = true;
-	return err;
-}
-
 /* Compare BTF of a function call with given bpf_reg_state.
  * Returns:
  * EFAULT - there is a verifier bug. Abort verification.
  * EINVAL - there is a type mismatch or BTF is not available.
  * 0 - BTF matches with what bpf_reg_state expects.
  * Only PTR_TO_CTX and SCALAR_VALUE states are recognized.
- *
- * NOTE: the code is duplicated from btf_check_subprog_arg_match()
- * because btf_check_func_arg_match() is still doing both. Once that
- * function is split in 2, we can call from here btf_check_subprog_arg_match()
- * first, and then treat the calling part in a new code path.
  */
 int btf_check_subprog_call(struct bpf_verifier_env *env, int subprog,
 			   struct bpf_reg_state *regs)
···
 		return -EINVAL;
 
 	is_global = prog->aux->func_info_aux[subprog].linkage == BTF_FUNC_GLOBAL;
-	err = btf_check_func_arg_match(env, btf, btf_id, regs, is_global, true);
+	err = btf_check_func_arg_match(env, btf, btf_id, regs, is_global);
 
 	/* Compiler optimizations can remove arguments from static functions
 	 * or mismatched type can be passed into a global function.

kernel/bpf/verifier.c

···
 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;
···
 	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;
···
 			}
 		}
 	} 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);

tools/testing/selftests/bpf/prog_tests/log_fixup.c

···
 	if (test__start_subtest("bad_core_relo_trunc_none"))
 		bad_core_relo(0, TRUNC_NONE /* full buf */);
 	if (test__start_subtest("bad_core_relo_trunc_partial"))
-		bad_core_relo(300, TRUNC_PARTIAL /* truncate original log a bit */);
+		bad_core_relo(280, TRUNC_PARTIAL /* truncate original log a bit */);
 	if (test__start_subtest("bad_core_relo_trunc_full"))
-		bad_core_relo(210, TRUNC_FULL  /* truncate also libbpf's message patch */);
+		bad_core_relo(220, TRUNC_FULL  /* truncate also libbpf's message patch */);
 	if (test__start_subtest("bad_core_relo_subprog"))
 		bad_core_relo_subprog();
 	if (test__start_subtest("missing_map"))

tools/testing/selftests/bpf/progs/cgrp_kfunc_failure.c

···
 }
 
 SEC("kretprobe/cgroup_destroy_locked")
-__failure __msg("reg type unsupported for arg#0 function")
+__failure __msg("calling kernel function bpf_cgroup_acquire is not allowed")
 int BPF_PROG(cgrp_kfunc_acquire_unsafe_kretprobe, struct cgroup *cgrp)
 {
 	struct cgroup *acquired;

tools/testing/selftests/bpf/progs/task_kfunc_failure.c

···
 }
 
 SEC("lsm/task_free")
-__failure __msg("reg type unsupported for arg#0 function")
+__failure __msg("R1 must be a rcu pointer")
 int BPF_PROG(task_kfunc_from_lsm_task_free, struct task_struct *task)
 {
 	struct task_struct *acquired;