bpf: Make BTF type match stricter for release arguments

The current of behavior of btf_struct_ids_match for release arguments is
that when type match fails, it retries with first member type again
(recursively). Since the offset is already 0, this is akin to just
casting the pointer in normal C, since if type matches it was just
embedded inside parent sturct as an object. However, we want to reject
cases for release function type matching, be it kfunc or BPF helpers.

An example is the following:

struct foo {
	struct bar b;
};

struct foo *v = acq_foo();
rel_bar(&v->b); // btf_struct_ids_match fails btf_types_are_same, then
		// retries with first member type and succeeds, while
		// it should fail.

Hence, don't walk the struct and only rely on btf_types_are_same for
strict mode. All users of strict mode must be dealing with zero offset
anyway, since otherwise they would want the struct to be walked.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220424214901.2743946-10-memxor@gmail.com

1 files changed, 2 insertions, 1 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 6141564c76c8..0af5793ba417 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1748,7 +1748,8 @@ int btf_struct_access(struct bpf_verifier_log *log, const struct btf *btf,
 		      u32 *next_btf_id, enum bpf_type_flag *flag);
 bool btf_struct_ids_match(struct bpf_verifier_log *log,
 			  const struct btf *btf, u32 id, int off,
-			  const struct btf *need_btf, u32 need_type_id);
+			  const struct btf *need_btf, u32 need_type_id,
+			  bool strict);
 
 int btf_distill_func_proto(struct bpf_verifier_log *log,
 			   struct btf *btf,