bpf: extract generic helper from process_timer_func()

Refactor the verifier by pulling the common logic from
process_timer_func() into a dedicated helper. This allows reusing
process_async_func() helper for verifying bpf_task_work struct in the
next patch.

Signed-off-by: Mykyta Yatsenko <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>

Author: mykyta5

kernel/bpf/verifier.c

@@ -8520,8 +8520,10 @@ static int process_spin_lock(struct bpf_verifier_env *env, int regno, int flags)
 	return 0;
 }
 
-static int process_timer_func(struct bpf_verifier_env *env, int regno,
-			      struct bpf_call_arg_meta *meta)
+/* Check if @regno is a pointer to a specific field in a map value */
+static int check_map_field_pointer(struct bpf_verifier_env *env, u32 regno,
+				   enum btf_field_type field_type, u32 field_off,
+				   const char *struct_name)
 {
 	struct bpf_reg_state *regs = cur_regs(env), *reg = &regs[regno];
 	bool is_const = tnum_is_const(reg->var_off);
@@ -8530,26 +8532,40 @@ static int process_timer_func(struct bpf_verifier_env *env, int regno,
 
 	if (!is_const) {
 		verbose(env,
-			"R%d doesn't have constant offset. bpf_timer has to be at the constant offset\n",
-			regno);
+			"R%d doesn't have constant offset. %s has to be at the constant offset\n",
+			regno, struct_name);
 		return -EINVAL;
 	}
 	if (!map->btf) {
-		verbose(env, "map '%s' has to have BTF in order to use bpf_timer\n",
-			map->name);
+		verbose(env, "map '%s' has to have BTF in order to use %s\n", map->name,
+			struct_name);
 		return -EINVAL;
 	}
-	if (!btf_record_has_field(map->record, BPF_TIMER)) {
-		verbose(env, "map '%s' has no valid bpf_timer\n", map->name);
+	if (!btf_record_has_field(map->record, field_type)) {
+		verbose(env, "map '%s' has no valid %s\n", map->name, struct_name);
 		return -EINVAL;
 	}
-	if (map->record->timer_off != val + reg->off) {
-		verbose(env, "off %lld doesn't point to 'struct bpf_timer' that is at %d\n",
-			val + reg->off, map->record->timer_off);
+	if (field_off != val + reg->off) {
+		verbose(env, "off %lld doesn't point to 'struct %s' that is at %d\n",
+			val + reg->off, struct_name, field_off);
 		return -EINVAL;
 	}
+	return 0;
+}
+
+static int process_timer_func(struct bpf_verifier_env *env, int regno,
+			      struct bpf_call_arg_meta *meta)
+{
+	struct bpf_reg_state *regs = cur_regs(env), *reg = &regs[regno];
+	struct bpf_map *map = reg->map_ptr;
+	int err;
+
+	err = check_map_field_pointer(env, regno, BPF_TIMER, map->record->timer_off, "bpf_timer");
+	if (err)
+		return err;
+
 	if (meta->map_ptr) {
-		verifier_bug(env, "Two map pointers in a timer helper");
+		verifier_bug(env, "Two map pointers in a bpf_timer helper");
 		return -EFAULT;
 	}
 	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {