Subject: [PATCH 5/8] bpf/arena: Add bpf_arena_map_kern_vm_start() and bpf_prog_arena()

From: Tejun Heo <[email-masked]>
Date: Wed, 20 May 2026 13:50:49 -1000

struct bpf_arena is opaque to callers outside arena.c. Add two helpers
for struct_ops subsystems that need to reach into an arena:

  bpf_arena_map_kern_vm_start(struct bpf_map *map)
    returns @map's kern_vm_start. A sched_ext follow-up needs this
    to translate kern_va <-> uaddr.

  bpf_prog_arena(struct bpf_prog *prog)
    returns the bpf_map of the arena referenced by @prog (NULL if
    @prog references no arena). The verifier enforces at most one
    arena per program. Used by struct_ops callers that auto-discover
    an arena from a member prog and need to take a map reference.

Suggested-by: Kumar Kartikeya Dwivedi <[email-masked]>
Signed-off-by: Tejun Heo <[email-masked]>
---
 include/linux/bpf.h |  2 ++
 kernel/bpf/arena.c  | 26 ++++++++++++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 5b99d786e98c..e1ba57c10aaa 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -618,6 +618,8 @@ void bpf_rb_root_free(const struct btf_field *field, void *rb_root,
 		      struct bpf_spin_lock *spin_lock);
 u64 bpf_arena_get_kern_vm_start(struct bpf_arena *arena);
 u64 bpf_arena_get_user_vm_start(struct bpf_arena *arena);
+u64 bpf_arena_map_kern_vm_start(struct bpf_map *map);
+struct bpf_map *bpf_prog_arena(struct bpf_prog *prog);
 int bpf_obj_name_cpy(char *dst, const char *src, unsigned int size);
 
 struct bpf_offload_dev;
diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
index a811cf6170fa..51b9ae36feb6 100644
--- a/kernel/bpf/arena.c
+++ b/kernel/bpf/arena.c
@@ -84,6 +84,32 @@ u64 bpf_arena_get_user_vm_start(struct bpf_arena *arena)
 	return arena ? arena->user_vm_start : 0;
 }
 
+/**
+ * bpf_arena_map_kern_vm_start - kern_vm_start lookup by struct bpf_map *
+ * @map: a BPF_MAP_TYPE_ARENA map
+ *
+ * Return @map's kern_vm_start.
+ */
+u64 bpf_arena_map_kern_vm_start(struct bpf_map *map)
+{
+	return bpf_arena_get_kern_vm_start(container_of(map, struct bpf_arena, map));
+}
+
+/**
+ * bpf_prog_arena - return the bpf_map of the arena referenced by @prog
+ * @prog: a loaded BPF program
+ *
+ * The verifier enforces at most one arena per program and stores it in
+ * prog->aux->arena. Return that arena's underlying bpf_map, or NULL if
+ * @prog does not reference an arena.
+ */
+struct bpf_map *bpf_prog_arena(struct bpf_prog *prog)
+{
+	struct bpf_arena *arena = prog->aux->arena;
+
+	return arena ? &arena->map : NULL;
+}
+
 static long arena_map_peek_elem(struct bpf_map *map, void *value)
 {
 	return -EOPNOTSUPP;
-- 
2.54.0