Re: [PATCH bpf-next 5/8] bpf: add poke dependency tracking for prog array maps

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Mon, Nov 18, 2019 at 5:38 PM Daniel Borkmann <daniel@iogearbox.net> wrote:
>
> This work adds program tracking to prog array maps. This is needed such
> that upon prog array updates/deletions we can fix up all programs which
> make use of this tail call map. We add ops->map_poke_{un,}track() helpers
> to maps to maintain the list of programs and ops->map_poke_run() for
> triggering the actual update. bpf_array_aux is extended to contain the
> list head and poke_mutex in order to serialize program patching during
> updates/deletions. bpf_free_used_maps() will untrack the program shortly
> before dropping the reference to the map.
>
> The prog_array_map_poke_run() is triggered during updates/deletions and
> walks the maintained prog list. It checks in their poke_tabs whether the
> map and key is matching and runs the actual bpf_arch_text_poke() for
> patching in the nop or new jmp location. Depending on the type of update,
> we use one of BPF_MOD_{NOP_TO_JUMP,JUMP_TO_NOP,JUMP_TO_JUMP}.
>
> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
> ---
>  include/linux/bpf.h   |  36 ++++++++++
>  kernel/bpf/arraymap.c | 151 +++++++++++++++++++++++++++++++++++++++++-
>  kernel/bpf/core.c     |   9 ++-
>  3 files changed, 193 insertions(+), 3 deletions(-)
>

[...]

>  #endif /* _LINUX_BPF_H */
> diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
> index 5be12db129cc..d2b559c6659e 100644
> --- a/kernel/bpf/arraymap.c
> +++ b/kernel/bpf/arraymap.c
> @@ -586,10 +586,14 @@ int bpf_fd_array_map_update_elem(struct bpf_map *map, struct file *map_file,
>         if (IS_ERR(new_ptr))
>                 return PTR_ERR(new_ptr);
>
> +       bpf_map_poke_lock(map);
>         old_ptr = xchg(array->ptrs + index, new_ptr);
> +       if (map->ops->map_poke_run)
> +               map->ops->map_poke_run(map, index, old_ptr, new_ptr);
> +       bpf_map_poke_unlock(map);

so this is a bit subtle, if I understand correctly. I was originally
going to suggest that if no map->ops->map_poke_run is set, then
bpf_map_pole_{lock,unlock} shouldn't be called at all. But then I
realized that this creates a race, where xchg can happen in different
order than map_poke_runs. Am I right?

If yes, I wonder if it will be better to express this logic more
explicitly as below, to avoid someone else "optimizing" the code
later:

if (map->ops->map_poke_run) {
    bpf_map_poke_lock(map);
    old_ptr = xchg(array->ptrs + index, new_ptr);
    bpf_map_poke_unlock(map);
} else {
    old_ptr = xchg(array->ptrs + index, new_ptr);
}

This will make it more apparent that something different is happing
when poke tracking is supported by a map.

Am I overthinking this?

> +
>         if (old_ptr)
>                 map->ops->map_fd_put_ptr(old_ptr);
> -
>         return 0;
>  }
>

[...]

> +static void prog_array_map_poke_untrack(struct bpf_map *map,
> +                                       struct bpf_prog_aux *prog_aux)
> +{
> +       struct prog_poke_elem *elem, *tmp;
> +       struct bpf_array_aux *aux;
> +
> +       aux = container_of(map, struct bpf_array, map)->aux;
> +       mutex_lock(&aux->poke_mutex);
> +       list_for_each_entry_safe(elem, tmp, &aux->poke_progs, list) {
> +               if (elem->aux == prog_aux) {
> +                       list_del_init(&elem->list);
> +                       kfree(elem);

break; ?

> +               }
> +       }
> +       mutex_unlock(&aux->poke_mutex);
> +}
> +

[...]

> +
> +                       ret = bpf_arch_text_poke(poke->ip, type,
> +                                                old ? (u8 *)old->bpf_func +
> +                                                poke->adj_off : NULL,
> +                                                new ? (u8 *)new->bpf_func +
> +                                                poke->adj_off : NULL);

nit: extract old/new address calculation, so it's not multi-line
wrapped? It's a bit hard to follow.

> +                       BUG_ON(ret < 0 && ret != -EINVAL);
> +               }
> +       }
> +}
> +

[...]