bpf: extract iterator argument type and name validation logic

Verifier enforces that all iterator structs are named `bpf_iter_<name>`
and that whenever iterator is passed to a kfunc it's passed as a valid PTR ->
STRUCT chain (with potentially const modifiers in between).

We'll need this check for upcoming changes, so instead of duplicating
the logic, extract it into a helper function.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240808232230.2848712-2-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/btf.h

@@ -580,6 +580,7 @@ bool btf_is_prog_ctx_type(struct bpf_verifier_log *log, const struct btf *btf,
 int get_kern_ctx_btf_id(struct bpf_verifier_log *log, enum bpf_prog_type prog_type);
 bool btf_types_are_same(const struct btf *btf1, u32 id1,
 			const struct btf *btf2, u32 id2);
+int btf_check_iter_arg(struct btf *btf, const struct btf_type *func, int arg_idx);
 #else
 static inline const struct btf_type *btf_type_by_id(const struct btf *btf,
 						    u32 type_id)
@@ -654,6 +655,10 @@ static inline bool btf_types_are_same(const struct btf *btf1, u32 id1,
 {
 	return false;
 }
+static inline int btf_check_iter_arg(struct btf *btf, const struct btf_type *func, int arg_idx)
+{
+	return -EOPNOTSUPP;
+}
 #endif
 
 static inline bool btf_type_is_struct_ptr(struct btf *btf, const struct btf_type *t)

kernel/bpf/btf.c

@@ -8047,15 +8047,44 @@ BTF_ID_LIST_GLOBAL(btf_tracing_ids, MAX_BTF_TRACING_TYPE)
 BTF_TRACING_TYPE_xxx
 #undef BTF_TRACING_TYPE
 
+/* Validate well-formedness of iter argument type.
+ * On success, return positive BTF ID of iter state's STRUCT type.
+ * On error, negative error is returned.
+ */
+int btf_check_iter_arg(struct btf *btf, const struct btf_type *func, int arg_idx)
+{
+	const struct btf_param *arg;
+	const struct btf_type *t;
+	const char *name;
+	int btf_id;
+
+	if (btf_type_vlen(func) <= arg_idx)
+		return -EINVAL;
+
+	arg = &btf_params(func)[arg_idx];
+	t = btf_type_skip_modifiers(btf, arg->type, NULL);
+	if (!t || !btf_type_is_ptr(t))
+		return -EINVAL;
+	t = btf_type_skip_modifiers(btf, t->type, &btf_id);
+	if (!t || !__btf_type_is_struct(t))
+		return -EINVAL;
+
+	name = btf_name_by_offset(btf, t->name_off);
+	if (!name || strncmp(name, ITER_PREFIX, sizeof(ITER_PREFIX) - 1))
+		return -EINVAL;
+
+	return btf_id;
+}
+
 static int btf_check_iter_kfuncs(struct btf *btf, const char *func_name,
 				 const struct btf_type *func, u32 func_flags)
 {
 	u32 flags = func_flags & (KF_ITER_NEW | KF_ITER_NEXT | KF_ITER_DESTROY);
-	const char *name, *sfx, *iter_name;
-	const struct btf_param *arg;
+	const char *sfx, *iter_name;
 	const struct btf_type *t;
 	char exp_name[128];
 	u32 nr_args;
+	int btf_id;
 
 	/* exactly one of KF_ITER_{NEW,NEXT,DESTROY} can be set */
 	if (!flags || (flags & (flags - 1)))
@@ -8066,28 +8095,21 @@ static int btf_check_iter_kfuncs(struct btf *btf, const char *func_name,
 	if (nr_args < 1)
 		return -EINVAL;
 
-	arg = &btf_params(func)[0];
-	t = btf_type_skip_modifiers(btf, arg->type, NULL);
-	if (!t || !btf_type_is_ptr(t))
-		return -EINVAL;
-	t = btf_type_skip_modifiers(btf, t->type, NULL);
-	if (!t || !__btf_type_is_struct(t))
-		return -EINVAL;
-
-	name = btf_name_by_offset(btf, t->name_off);
-	if (!name || strncmp(name, ITER_PREFIX, sizeof(ITER_PREFIX) - 1))
-		return -EINVAL;
+	btf_id = btf_check_iter_arg(btf, func, 0);
+	if (btf_id < 0)
+		return btf_id;
 
 	/* sizeof(struct bpf_iter_<type>) should be a multiple of 8 to
 	 * fit nicely in stack slots
 	 */
+	t = btf_type_by_id(btf, btf_id);
 	if (t->size == 0 || (t->size % 8))
 		return -EINVAL;
 
 	/* validate bpf_iter_<type>_{new,next,destroy}(struct bpf_iter_<type> *)
 	 * naming pattern
 	 */
-	iter_name = name + sizeof(ITER_PREFIX) - 1;
+	iter_name = btf_name_by_offset(btf, t->name_off) + sizeof(ITER_PREFIX) - 1;
 	if (flags & KF_ITER_NEW)
 		sfx = "new";
 	else if (flags & KF_ITER_NEXT)