[PATCH bpf-next v4 04/12] bpf: Let the by-value struct walk take the kinds of member it accepts

[Yonghong Song <yonghong.song@linux.dev>]

btf_struct_member_walk() hardcodes that every member of a struct returned
by value must be a scalar. A later patch needs the same walk to also
accept arena pointers, so give it a mask of the member kinds it allows.

The mask reaches callers through btf_struct_is_composed_of(), which
replaces btf_type_is_scalar_struct() as the entry point exported to
btf.c. btf_type_is_scalar_struct() stays as a verifier-local wrapper for
the call sites that only ever ask about scalars.

There is no functional change: btf_member_kind_allowed() is exactly
btf_type_is_scalar() when the mask is BTF_MEMBER_SCALAR alone, and the
shared check is the last statement of the loop body, so dropping the array
branch's continue neither skips a member nor checks one twice.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 include/linux/bpf_verifier.h | 11 +++++++++--
 kernel/bpf/btf.c             |  2 +-
 kernel/bpf/verifier.c        | 38 ++++++++++++++++++++++++++----------
 3 files changed, 38 insertions(+), 13 deletions(-)

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 3eb61edc8c5e..ae9f606539f4 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -1488,8 +1488,15 @@ int bpf_jmp_offset(struct bpf_insn *insn);
 struct bpf_iarray *bpf_insn_successors(struct bpf_verifier_env *env, u32 idx);
 void bpf_fmt_stack_mask(char *buf, ssize_t buf_sz, u64 stack_mask);
 bool bpf_subprog_is_global(const struct bpf_verifier_env *env, int subprog);
-bool btf_type_is_scalar_struct(struct bpf_verifier_env *env, const struct btf *btf,
-			       const struct btf_type *t);
+
+/* Kinds of member a by-value struct or union may be composed of. */
+enum btf_member_kind {
+	BTF_MEMBER_SCALAR	= BIT(0), /* an int or an enum */
+	BTF_MEMBER_ARENA_PTR	= BIT(1), /* a pointer carrying the "arena" type tag */
+};
+
+bool btf_struct_is_composed_of(struct bpf_verifier_env *env, const struct btf *btf,
+			       const struct btf_type *t, u32 member_kinds);
 
 int bpf_find_subprog(struct bpf_verifier_env *env, int off);
 bool bpf_is_throw_kfunc(struct bpf_insn *insn);
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 280530d25886..b1f4ef614d4c 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -7979,7 +7979,7 @@ static int btf_validate_return_type(struct bpf_verifier_env *env, struct btf *bt
 		 */
 		bool local_func = subprog && !is_global;
 
-		if (local_func || btf_type_is_scalar_struct(env, btf, t))
+		if (local_func || btf_struct_is_composed_of(env, btf, t, BTF_MEMBER_SCALAR))
 			return 0;
 	}
 
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index ed077a5422af..90139f1b78d1 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -11646,9 +11646,23 @@ static bool is_kfunc_arg_implicit(const struct bpf_call_arg_meta *meta, u32 arg_
 	return argn <= arg_idx;
 }
 
-/* Returns true if struct is composed of scalars, 4 levels of nesting allowed */
+static bool btf_member_kind_allowed(const struct btf *btf, const struct btf_type *t,
+				    u32 member_kinds)
+{
+	if ((member_kinds & BTF_MEMBER_SCALAR) && btf_type_is_scalar(t))
+		return true;
+	if ((member_kinds & BTF_MEMBER_ARENA_PTR) && btf_type_is_arena_ptr(btf, t))
+		return true;
+	return false;
+}
+
+/*
+ * Returns true if every member of struct @t is of a kind listed in
+ * @member_kinds, 4 levels of nesting allowed. An array member counts as its
+ * element type.
+ */
 static bool btf_struct_member_walk(struct bpf_verifier_env *env, const struct btf *btf,
-				   const struct btf_type *t, int rec)
+				   const struct btf_type *t, u32 member_kinds, int rec)
 {
 	const struct btf_type *member_type;
 	const struct btf_member *member;
@@ -11666,7 +11680,7 @@ static bool btf_struct_member_walk(struct bpf_verifier_env *env, const struct bt
 				verbose(env, "max struct nesting depth exceeded\n");
 				return false;
 			}
-			if (!btf_struct_member_walk(env, btf, member_type, rec + 1))
+			if (!btf_struct_member_walk(env, btf, member_type, member_kinds, rec + 1))
 				return false;
 			continue;
 		}
@@ -11675,21 +11689,25 @@ static bool btf_struct_member_walk(struct bpf_verifier_env *env, const struct bt
 			if (!array->nelems)
 				return false;
 			member_type = btf_type_skip_modifiers(btf, array->type, NULL);
-			if (!btf_type_is_scalar(member_type))
-				return false;
-			continue;
 		}
-		if (!btf_type_is_scalar(member_type))
+		if (!btf_member_kind_allowed(btf, member_type, member_kinds))
 			return false;
 	}
 	return true;
 }
 
-bool btf_type_is_scalar_struct(struct bpf_verifier_env *env,
+bool btf_struct_is_composed_of(struct bpf_verifier_env *env,
 			       const struct btf *btf,
-			       const struct btf_type *t)
+			       const struct btf_type *t, u32 member_kinds)
+{
+	return btf_struct_member_walk(env, btf, t, member_kinds, 0);
+}
+
+static bool btf_type_is_scalar_struct(struct bpf_verifier_env *env,
+				      const struct btf *btf,
+				      const struct btf_type *t)
 {
-	return btf_struct_member_walk(env, btf, t, 0);
+	return btf_struct_is_composed_of(env, btf, t, BTF_MEMBER_SCALAR);
 }
 
 enum kfunc_ptr_arg_type {
-- 
2.53.0-Meta