bpf: tidy up exception callback management a bit

Use the fact that we are passing subprog index around and have
a corresponding struct bpf_subprog_info in bpf_verifier_env for each
subprogram. We don't need to separately pass around a flag whether
subprog is exception callback or not, each relevant verifier function
can determine this using provided subprog index if we maintain
bpf_subprog_info properly.

Also move out exception callback-specific logic from
btf_prepare_func_args(), keeping it generic. We can enforce all these
restriction right before exception callback verification pass. We add
out parameter, arg_cnt, for now, but this will be unnecessary with
subsequent refactoring and will be removed.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20231204233931.49758-4-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf.h

@@ -2494,7 +2494,7 @@ int btf_check_subprog_arg_match(struct bpf_verifier_env *env, int subprog,
 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,
-			  struct bpf_reg_state *reg, bool is_ex_cb);
+			  struct bpf_reg_state *reg, u32 *nargs);
 int btf_check_type_match(struct bpf_verifier_log *log, const struct bpf_prog *prog,
 			 struct btf *btf, const struct btf_type *t);
 const char *btf_find_decl_tag_value(const struct btf *btf, const struct btf_type *pt,

kernel/bpf/btf.c

@@ -6956,7 +6956,7 @@ int btf_check_subprog_call(struct bpf_verifier_env *env, int subprog,
  * (either PTR_TO_CTX or SCALAR_VALUE).
  */
 int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog,
-			  struct bpf_reg_state *regs, bool is_ex_cb)
+			  struct bpf_reg_state *regs, u32 *arg_cnt)
 {
 	struct bpf_verifier_log *log = &env->log;
 	struct bpf_prog *prog = env->prog;
@@ -7013,6 +7013,7 @@ int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog,
 			tname, nargs, MAX_BPF_FUNC_REG_ARGS);
 		return -EINVAL;
 	}
+	*arg_cnt = nargs;
 	/* check that function returns int, exception cb also requires this */
 	t = btf_type_by_id(btf, t->type);
 	while (btf_type_is_modifier(t))
@@ -7062,14 +7063,6 @@ int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog,
 			i, btf_type_str(t), tname);
 		return -EINVAL;
 	}
-	/* We have already ensured that the callback returns an integer, just
-	 * like all global subprogs. We need to determine it only has a single
-	 * scalar argument.
-	 */
-	if (is_ex_cb && (nargs != 1 || regs[BPF_REG_1].type != SCALAR_VALUE)) {
-		bpf_log(log, "exception cb only supports single integer argument\n");
-		return -EINVAL;
-	}
 	return 0;
 }
 

kernel/bpf/verifier.c

@@ -442,6 +442,25 @@ static struct bpf_func_info_aux *subprog_aux(const struct bpf_verifier_env *env,
 	return &env->prog->aux->func_info_aux[subprog];
 }
 
+static struct bpf_subprog_info *subprog_info(struct bpf_verifier_env *env, int subprog)
+{
+	return &env->subprog_info[subprog];
+}
+
+static void mark_subprog_exc_cb(struct bpf_verifier_env *env, int subprog)
+{
+	struct bpf_subprog_info *info = subprog_info(env, subprog);
+
+	info->is_cb = true;
+	info->is_async_cb = true;
+	info->is_exception_cb = true;
+}
+
+static bool subprog_is_exc_cb(struct bpf_verifier_env *env, int subprog)
+{
+	return subprog_info(env, subprog)->is_exception_cb;
+}
+
 static bool reg_may_point_to_spin_lock(const struct bpf_reg_state *reg)
 {
 	return btf_record_has_field(reg_btf_record(reg), BPF_SPIN_LOCK);
@@ -2892,6 +2911,7 @@ static int add_subprog_and_kfunc(struct bpf_verifier_env *env)
 			if (env->subprog_info[i].start != ex_cb_insn)
 				continue;
 			env->exception_callback_subprog = i;
+			mark_subprog_exc_cb(env, i);
 			break;
 		}
 	}
@@ -19166,9 +19186,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
 
 		env->exception_callback_subprog = env->subprog_cnt - 1;
 		/* Don't update insn_cnt, as add_hidden_subprog always appends insns */
-		env->subprog_info[env->exception_callback_subprog].is_cb = true;
+		mark_subprog_exc_cb(env, env->exception_callback_subprog);
-		env->subprog_info[env->exception_callback_subprog].is_async_cb = true;
-		env->subprog_info[env->exception_callback_subprog].is_exception_cb = true;
 	}
 
 	for (i = 0; i < insn_cnt; i++, insn++) {
@@ -19868,7 +19886,7 @@ static void free_states(struct bpf_verifier_env *env)
 	}
 }
 
-static int do_check_common(struct bpf_verifier_env *env, int subprog, bool is_ex_cb)
+static int do_check_common(struct bpf_verifier_env *env, int subprog)
 {
 	bool pop_log = !(env->log.level & BPF_LOG_LEVEL2);
 	struct bpf_verifier_state *state;
@@ -19899,9 +19917,23 @@ static int do_check_common(struct bpf_verifier_env *env, int subprog, bool is_ex
 
 	regs = state->frame[state->curframe]->regs;
 	if (subprog || env->prog->type == BPF_PROG_TYPE_EXT) {
-		ret = btf_prepare_func_args(env, subprog, regs, is_ex_cb);
+		u32 nargs;
+
+		ret = btf_prepare_func_args(env, subprog, regs, &nargs);
 		if (ret)
 			goto out;
+		if (subprog_is_exc_cb(env, subprog)) {
+			state->frame[0]->in_exception_callback_fn = true;
+			/* We have already ensured that the callback returns an integer, just
+			 * like all global subprogs. We need to determine it only has a single
+			 * scalar argument.
+			 */
+			if (nargs != 1 || regs[BPF_REG_1].type != SCALAR_VALUE) {
+				verbose(env, "exception cb only supports single integer argument\n");
+				ret = -EINVAL;
+				goto out;
+			}
+		}
 		for (i = BPF_REG_1; i <= BPF_REG_5; i++) {
 			if (regs[i].type == PTR_TO_CTX)
 				mark_reg_known_zero(env, regs, i);
@@ -19915,12 +19947,6 @@ static int do_check_common(struct bpf_verifier_env *env, int subprog, bool is_ex
 				regs[i].id = ++env->id_gen;
 			}
 		}
-		if (is_ex_cb) {
-			state->frame[0]->in_exception_callback_fn = true;
-			env->subprog_info[subprog].is_cb = true;
-			env->subprog_info[subprog].is_async_cb = true;
-			env->subprog_info[subprog].is_exception_cb = true;
-		}
 	} else {
 		/* 1st arg to a function */
 		regs[BPF_REG_1].type = PTR_TO_CTX;
@@ -20000,7 +20026,7 @@ again:
 
 		env->insn_idx = env->subprog_info[i].start;
 		WARN_ON_ONCE(env->insn_idx == 0);
-		ret = do_check_common(env, i, env->exception_callback_subprog == i);
+		ret = do_check_common(env, i);
 		if (ret) {
 			return ret;
 		} else if (env->log.level & BPF_LOG_LEVEL) {
@@ -20030,7 +20056,7 @@ static int do_check_main(struct bpf_verifier_env *env)
 	int ret;
 
 	env->insn_idx = 0;
-	ret = do_check_common(env, 0, false);
+	ret = do_check_common(env, 0);
 	if (!ret)
 		env->prog->aux->stack_depth = env->subprog_info[0].stack_depth;
 	return ret;