bpf: Support struct btf_struct_meta via KF_IMPLICIT_ARGS

The following kfuncs currently accept void *meta__ign argument:
  * bpf_obj_new_impl
  * bpf_obj_drop_impl
  * bpf_percpu_obj_new_impl
  * bpf_percpu_obj_drop_impl
  * bpf_refcount_acquire_impl
  * bpf_list_push_back_impl
  * bpf_list_push_front_impl
  * bpf_rbtree_add_impl

The __ign suffix is an indicator for the verifier to skip the argument
in check_kfunc_args(). Then, in fixup_kfunc_call() the verifier may
set the value of this argument to struct btf_struct_meta *
kptr_struct_meta from insn_aux_data.

BPF programs must pass a dummy NULL value when calling these kfuncs.

Additionally, the list and rbtree _impl kfuncs also accept an implicit
u64 argument, which doesn't require __ign suffix because it's a
scalar, and BPF programs explicitly pass 0.

Add new kfuncs with KF_IMPLICIT_ARGS [1], that correspond to each
_impl kfunc accepting meta__ign. The existing _impl kfuncs remain
unchanged for backwards compatibility.

To support this, add "btf_struct_meta" to the list of recognized
implicit argument types in resolve_btfids.

Implement is_kfunc_arg_implicit() in the verifier, that determines
implicit args by inspecting both a non-_impl BTF prototype of the
kfunc.

Update the special_kfunc_list in the verifier and relevant checks to
support both the old _impl and the new KF_IMPLICIT_ARGS variants of
btf_struct_meta users.

[1] https://lore.kernel.org/bpf/20260120222638.3976562-1-ihor.solodrai@linux.dev/

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20260327203241.3365046-1-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

2 files changed, 3 insertions, 2 deletions

diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
index 5208f650080f..f8a91fa7584f 100644
--- a/tools/bpf/resolve_btfids/main.c
+++ b/tools/bpf/resolve_btfids/main.c
@@ -1065,6 +1065,7 @@ static bool is_kf_implicit_arg(const struct btf *btf, const struct btf_param *p)
 {
 	static const char *const kf_implicit_arg_types[] = {
 		"bpf_prog_aux",
+		"btf_struct_meta",
 	};
 	const struct btf_type *t;
 	const char *name;
diff --git a/tools/testing/selftests/bpf/progs/percpu_alloc_fail.c b/tools/testing/selftests/bpf/progs/percpu_alloc_fail.c
index f2b8eb2ff76f..81813c724fa9 100644
--- a/tools/testing/selftests/bpf/progs/percpu_alloc_fail.c
+++ b/tools/testing/selftests/bpf/progs/percpu_alloc_fail.c
@@ -110,7 +110,7 @@ int BPF_PROG(test_array_map_3)
 }
 
 SEC("?fentry.s/bpf_fentry_test1")
-__failure __msg("arg#0 expected for bpf_percpu_obj_drop_impl()")
+__failure __msg("arg#0 expected for bpf_percpu_obj_drop()")
 int BPF_PROG(test_array_map_4)
 {
 	struct val_t __percpu_kptr *p;
@@ -124,7 +124,7 @@ int BPF_PROG(test_array_map_4)
 }
 
 SEC("?fentry.s/bpf_fentry_test1")
-__failure __msg("arg#0 expected for bpf_obj_drop_impl()")
+__failure __msg("arg#0 expected for bpf_obj_drop()")
 int BPF_PROG(test_array_map_5)
 {
 	struct val_t *p;