bpf: Let the by-value struct walk take the kinds of member it accepts

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>
Link: https://lore.kernel.org/r/20260829061534.1693789-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 9 insertions, 2 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);