[RFC PATCH 6/9] bpf/arena: Add bpf_arena_map_kern_vm_start()

Tejun Heo
2026-04-27T10:51:18Z

bpf_arena_get_kern_vm_start() takes a struct bpf_arena *. The struct
itself isn't exposed, so callers outside arena.c that hold only a
struct bpf_map * (e.g. struct_ops subsystems handling an arena map fd)
can't reach it.

Add bpf_arena_map_kern_vm_start() which takes struct bpf_map * and
container_of()s to the arena, with a type check. A sched_ext follow-up
needs this to translate kern_va <-> uaddr for arena pages it claims.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
include/linux/bpf.h | 1 +
kernel/bpf/arena.c | 13 +++++++++++++
2 files changed, 14 insertions(+)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 587e5ff387bf..1614af65a2d1 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -617,6 +617,7 @@ 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);
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 73e43617761c..d67a6000221d 100644
--- a/kernel/bpf/arena.c
+++ b/kernel/bpf/arena.c
@@ -85,6 +85,19 @@ 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, or 0 (with WARN) if @map isn't an arena.
+ */
+u64 bpf_arena_map_kern_vm_start(struct bpf_map *map)
+{
+ if (WARN_ON_ONCE(!map || map->map_type != BPF_MAP_TYPE_ARENA))
+ return 0;
+ return bpf_arena_get_kern_vm_start(container_of(map, struct bpf_arena, map));
+}
+
static long arena_map_peek_elem(struct bpf_map *map, void *value)
{
return -EOPNOTSUPP;
--
2.53.0