[PATCH bpf-next v9 7/9] bpf: allow bpf_list_front/back result as the prev argument of bpf_list_add_impl

[Chengkaitao <pilgrimtao@gmail.com>]

From: Kaitao Cheng <chengkaitao@kylinos.cn>

KF_ARG_PTR_TO_LIST_NODE normally requires an owning reference
(PTR_TO_BTF_ID | MEM_ALLOC and ref_obj_id). For bpf_list_add_impl's
third argument (prev), allow a non-owning reference with ref_obj_id==0
so that the result of bpf_list_front() or bpf_list_back() can be passed
as the insertion point. When prev is such a non-owning ref, skip the
MEM_ALLOC/ref_obj_id checks and jump to the shared list-node processing.
Owning refs (e.g. from pop + refcount_acquire) still pass the existing
checks and reach the same label.

Add BTF suffix __nonown_allowed (is_kfunc_arg_nonown_allowed) and
document it under kfuncs.rst.

Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn>
---
 Documentation/bpf/kfuncs.rst | 20 +++++++++++++++++++-
 kernel/bpf/helpers.c         |  4 ++--
 kernel/bpf/verifier.c        | 13 +++++++++++++
 3 files changed, 34 insertions(+), 3 deletions(-)

diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
index 75e6c078e0e7..6760c547dd32 100644
--- a/Documentation/bpf/kfuncs.rst
+++ b/Documentation/bpf/kfuncs.rst
@@ -207,7 +207,25 @@ Here, the buffer may be NULL. If the buffer is not NULL, it must be at least
 buffer__szk bytes in size. The kfunc is responsible for checking if the buffer
 is NULL before using it.
 
-2.3.5 __str Annotation
+2.3.5 __nonown_allowed Annotation
+----------------------------------
+
+This annotation is used to indicate that the parameter may be a non-owning reference.
+
+An example is given below::
+
+        __bpf_kfunc int bpf_list_add_impl(..., struct bpf_list_node
+                                          *prev__nonown_allowed, ...)
+        {
+                ...
+        }
+
+For the ``prev__nonown_allowed`` parameter (resolved as ``KF_ARG_PTR_TO_LIST_NODE``),
+suffix ``__nonown_allowed`` retains the usual owning-pointer rules and also
+permits a non-owning reference with no ref_obj_id (e.g. the return value of
+bpf_list_front() / bpf_list_back()).
+
+2.3.6 __str Annotation
 ----------------------------
 This annotation is used to indicate that the argument is a constant string.
 
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 68c83a009275..743341aae5c0 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2449,10 +2449,10 @@ __bpf_kfunc int bpf_list_push_back_impl(struct bpf_list_head *head,
 
 __bpf_kfunc int bpf_list_add_impl(struct bpf_list_head *head,
 				  struct bpf_list_node *new,
-				  struct bpf_list_node *prev,
+				  struct bpf_list_node *prev__nonown_allowed,
 				  void *meta__ign, u64 off)
 {
-	struct bpf_list_node_kern *n = (void *)new, *p = (void *)prev;
+	struct bpf_list_node_kern *n = (void *)new, *p = (void *)prev__nonown_allowed;
 	struct btf_struct_meta *meta = meta__ign;
 	struct list_head *prev_ptr = &p->list_head;
 
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 69dcf0105973..514a0aab93b8 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -12265,6 +12265,11 @@ static bool is_kfunc_arg_nullable(const struct btf *btf, const struct btf_param
 	return btf_param_match_suffix(btf, arg, "__nullable");
 }
 
+static bool is_kfunc_arg_nonown_allowed(const struct btf *btf, const struct btf_param *arg)
+{
+	return btf_param_match_suffix(btf, arg, "__nonown_allowed");
+}
+
 static bool is_kfunc_arg_const_str(const struct btf *btf, const struct btf_param *arg)
 {
 	return btf_param_match_suffix(btf, arg, "__str");
@@ -13736,6 +13741,13 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 				return ret;
 			break;
 		case KF_ARG_PTR_TO_LIST_NODE:
+			if (is_kfunc_arg_nonown_allowed(btf, &args[i]) &&
+			    type_is_non_owning_ref(reg->type) && !reg->ref_obj_id) {
+				/* Allow bpf_list_front/back return value as
+				 * list_add_impl's third arg (R3).
+				 */
+				goto check_ok;
+			}
 			if (reg->type != (PTR_TO_BTF_ID | MEM_ALLOC)) {
 				verbose(env, "arg#%d expected pointer to allocated object\n", i);
 				return -EINVAL;
@@ -13744,6 +13756,7 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 				verbose(env, "allocated object must be referenced\n");
 				return -EINVAL;
 			}
+check_ok:
 			ret = process_kf_arg_ptr_to_list_node(env, reg, regno, meta);
 			if (ret < 0)
 				return ret;
-- 
2.50.1 (Apple Git-155)