bpf: Add bpf_list_{front,back} kfunc

In the kernel fq qdisc implementation, it only needs to look at
the fields of the first node in a list but does not always
need to remove it from the list. It is more convenient to have
a peek kfunc for the list. It works similar to the bpf_rbtree_first().

This patch adds bpf_list_{front,back} kfunc. The verifier is changed
such that the kfunc returning "struct bpf_list_node *" will be
marked as non-owning. The exception is the KF_ACQUIRE kfunc. The
net effect is only the new bpf_list_{front,back} kfuncs will
have its return pointer marked as non-owning.

Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20250506015857.817950-8-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 36150d340c168a61cba732001c6e4d5609fa9c4a..78cefb41266a1d50d06d6e15fb4569fd76fafa0a 100644 (file)
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2293,6 +2293,26 @@ __bpf_kfunc struct bpf_list_node *bpf_list_pop_back(struct bpf_list_head *head)
        return __bpf_list_del(head, true);
 }
 
+__bpf_kfunc struct bpf_list_node *bpf_list_front(struct bpf_list_head *head)
+{
+       struct list_head *h = (struct list_head *)head;
+
+       if (list_empty(h) || unlikely(!h->next))
+               return NULL;
+
+       return (struct bpf_list_node *)h->next;
+}
+
+__bpf_kfunc struct bpf_list_node *bpf_list_back(struct bpf_list_head *head)
+{
+       struct list_head *h = (struct list_head *)head;
+
+       if (list_empty(h) || unlikely(!h->next))
+               return NULL;
+
+       return (struct bpf_list_node *)h->prev;
+}
+
 __bpf_kfunc struct bpf_rb_node *bpf_rbtree_remove(struct bpf_rb_root *root,
                                                  struct bpf_rb_node *node)
 {
@@ -3236,6 +3256,8 @@ BTF_ID_FLAGS(func, bpf_list_push_front_impl)
 BTF_ID_FLAGS(func, bpf_list_push_back_impl)
 BTF_ID_FLAGS(func, bpf_list_pop_front, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_list_pop_back, KF_ACQUIRE | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_list_front, KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_list_back, KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_task_acquire, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_task_release, KF_RELEASE)
 BTF_ID_FLAGS(func, bpf_rbtree_remove, KF_ACQUIRE | KF_RET_NULL)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index acb2f44316cccb510b0e841e2e46ddd20a311fa1..99aa2c890e7bd4d8cde57d9d5944d35798d69369 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -12079,6 +12079,8 @@ enum special_kfunc_type {
        KF_bpf_list_push_back_impl,
        KF_bpf_list_pop_front,
        KF_bpf_list_pop_back,
+       KF_bpf_list_front,
+       KF_bpf_list_back,
        KF_bpf_cast_to_kern_ctx,
        KF_bpf_rdonly_cast,
        KF_bpf_rcu_read_lock,
@@ -12124,6 +12126,8 @@ BTF_ID(func, bpf_list_push_front_impl)
 BTF_ID(func, bpf_list_push_back_impl)
 BTF_ID(func, bpf_list_pop_front)
 BTF_ID(func, bpf_list_pop_back)
+BTF_ID(func, bpf_list_front)
+BTF_ID(func, bpf_list_back)
 BTF_ID(func, bpf_cast_to_kern_ctx)
 BTF_ID(func, bpf_rdonly_cast)
 BTF_ID(func, bpf_rbtree_remove)
@@ -12160,6 +12164,8 @@ BTF_ID(func, bpf_list_push_front_impl)
 BTF_ID(func, bpf_list_push_back_impl)
 BTF_ID(func, bpf_list_pop_front)
 BTF_ID(func, bpf_list_pop_back)
+BTF_ID(func, bpf_list_front)
+BTF_ID(func, bpf_list_back)
 BTF_ID(func, bpf_cast_to_kern_ctx)
 BTF_ID(func, bpf_rdonly_cast)
 BTF_ID(func, bpf_rcu_read_lock)
@@ -12598,7 +12604,9 @@ static bool is_bpf_list_api_kfunc(u32 btf_id)
        return btf_id == special_kfunc_list[KF_bpf_list_push_front_impl] ||
               btf_id == special_kfunc_list[KF_bpf_list_push_back_impl] ||
               btf_id == special_kfunc_list[KF_bpf_list_pop_front] ||
-              btf_id == special_kfunc_list[KF_bpf_list_pop_back];
+              btf_id == special_kfunc_list[KF_bpf_list_pop_back] ||
+              btf_id == special_kfunc_list[KF_bpf_list_front] ||
+              btf_id == special_kfunc_list[KF_bpf_list_back];
 }
 
 static bool is_bpf_rbtree_api_kfunc(u32 btf_id)
@@ -13903,7 +13911,7 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
                        if (is_kfunc_ret_null(&meta))
                                regs[BPF_REG_0].id = id;
                        regs[BPF_REG_0].ref_obj_id = id;
-               } else if (is_rbtree_node_type(ptr_type)) {
+               } else if (is_rbtree_node_type(ptr_type) || is_list_node_type(ptr_type)) {
                        ref_set_non_owning(env, &regs[BPF_REG_0]);
                }