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Date:   Mon, 13 Apr 2020 15:47:34 -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 Mon, Apr 13, 2020 at 3:18 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> 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.

Actually never mind this, from reading fs/seq_file.c code I've been
under impression that start is only called for full restarts, but
that's not true.


>
> > +
> > +       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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