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Date:   Thu, 20 Aug 2020 23:31:34 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next v2 1/3] bpf: implement link_query for bpf iterators

On Thu, Aug 20, 2020 at 3:50 PM Yonghong Song <yhs@...com> wrote:
>
> This patch implemented bpf_link callback functions
> show_fdinfo and fill_link_info to support link_query
> interface.
>
> The general interface for show_fdinfo and fill_link_info
> will print/fill the target_name. Each targets can
> register show_fdinfo and fill_link_info callbacks
> to print/fill more target specific information.
>
> For example, the below is a fdinfo result for a bpf
> task iterator.
>   $ cat /proc/1749/fdinfo/7
>   pos:    0
>   flags:  02000000
>   mnt_id: 14
>   link_type:      iter
>   link_id:        11
>   prog_tag:       990e1f8152f7e54f
>   prog_id:        59
>   target_name:    task
>
> Signed-off-by: Yonghong Song <yhs@...com>
> ---
>  include/linux/bpf.h            |  6 ++++
>  include/uapi/linux/bpf.h       |  7 ++++
>  kernel/bpf/bpf_iter.c          | 58 ++++++++++++++++++++++++++++++++++
>  tools/include/uapi/linux/bpf.h |  7 ++++
>  4 files changed, 78 insertions(+)
>

[...]

> +
> +static int bpf_iter_link_fill_link_info(const struct bpf_link *link,
> +                                       struct bpf_link_info *info)
> +{
> +       struct bpf_iter_link *iter_link =
> +               container_of(link, struct bpf_iter_link, link);
> +       char __user *ubuf = u64_to_user_ptr(info->iter.target_name);
> +       bpf_iter_fill_link_info_t fill_link_info;
> +       u32 ulen = info->iter.target_name_len;
> +       const char *target_name;
> +       u32 target_len;
> +
> +       if (ulen && !ubuf)
> +               return -EINVAL;
> +
> +       target_name = iter_link->tinfo->reg_info->target;
> +       target_len =  strlen(target_name);
> +       info->iter.target_name_len = target_len + 1;
> +       if (!ubuf)
> +               return 0;

this might return prematurely before fill_link_info() below gets a
chance to fill in some extra info?

> +
> +       if (ulen >= target_len + 1) {
> +               if (copy_to_user(ubuf, target_name, target_len + 1))
> +                       return -EFAULT;
> +       } else {
> +               char zero = '\0';
> +
> +               if (copy_to_user(ubuf, target_name, ulen - 1))
> +                       return -EFAULT;
> +               if (put_user(zero, ubuf + ulen - 1))
> +                       return -EFAULT;
> +               return -ENOSPC;
> +       }
> +
> +       fill_link_info = iter_link->tinfo->reg_info->fill_link_info;
> +       if (fill_link_info)
> +               return fill_link_info(&iter_link->aux, info);
> +
> +       return 0;
> +}
> +

[...]

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