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

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Date:   Mon, 13 Apr 2020 15:18:32 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [RFC PATCH bpf-next 07/16] bpf: add bpf_map target

On Wed, Apr 8, 2020 at 4:26 PM Yonghong Song <yhs@...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@...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;
> +}
> +

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