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

[Emil Tsalapatis]

On Wed May 20, 2026 at 7:50 PM EDT, Tejun Heo wrote:
> 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 <memxor at gmail.com>
> Signed-off-by: Tejun Heo <tj at kernel.org>

Reviewed-by: Emil Tsalapatis <emil at etsalapatis.com>

> ---
>  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;