Re: [RFC PATCH bpf-next 07/16] bpf: add bpf_map target

Andrii Nakryiko
2020-04-13T22:18:48Z

On Wed, Apr 8, 2020 at 4:26 PM Yonghong Song <yhs@fb.com> wrote:
>
> This patch added bpf_map target. Traversing all bpf_maps
> through map_idr. A reference is held for the map during
> the show() to ensure safety and correctness for field accesses.
>
> Signed-off-by: Yonghong Song <yhs@fb.com>
> ---
> kernel/bpf/syscall.c | 104 +++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 104 insertions(+)
>
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index b5e4f18cc633..62a872a406ca 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -3797,3 +3797,107 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, siz
>
> return err;
> }
> +
> +struct bpfdump_seq_map_info {
> + struct bpf_map *map;
> + u32 id;
> +};
> +
> +static struct bpf_map *bpf_map_seq_get_next(u32 *id)
> +{
> + struct bpf_map *map;
> +
> + spin_lock_bh(&map_idr_lock);
> + map = idr_get_next(&map_idr, id);
> + if (map)
> + map = __bpf_map_inc_not_zero(map, false);
> + spin_unlock_bh(&map_idr_lock);
> +
> + return map;
> +}
> +
> +static void *bpf_map_seq_start(struct seq_file *seq, loff_t *pos)
> +{
> + struct bpfdump_seq_map_info *info = seq->private;
> + struct bpf_map *map;
> + u32 id = info->id + 1;

shouldn't it always start from id=0? This seems buggy and should break
on seq_file restart.

> +
> + map = bpf_map_seq_get_next(&id);
> + if (!map)

bpf_map_seq_get_next will return error code, not NULL, if bpf_map
refcount couldn't be incremented. So this must be IS_ERR(map).

> + return NULL;
> +
> + ++*pos;
> + info->map = map;
> + info->id = id;
> + return map;
> +}
> +
> +static void *bpf_map_seq_next(struct seq_file *seq, void *v, loff_t *pos)
> +{
> + struct bpfdump_seq_map_info *info = seq->private;
> + struct bpf_map *map;
> + u32 id = info->id + 1;
> +
> + ++*pos;
> + map = bpf_map_seq_get_next(&id);
> + if (!map)

same here, IS_ERR(map)

> + return NULL;
> +
> + __bpf_map_put(info->map, true);
> + info->map = map;
> + info->id = id;
> + return map;
> +}
> + …