netdev - Re: [PATCH bpf-next 4/7] bpf: implement BPF XDP link-specific introspection APIs

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Message-ID: <7a68d9cc-f8dc-a11f-f1d4-7307519be866@gmail.com>
Date:   Mon, 13 Jul 2020 08:32:30 -0600
From:   David Ahern <dsahern@...il.com>
To:     Andrii Nakryiko <andriin@...com>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, ast@...com, daniel@...earbox.net
Cc:     andrii.nakryiko@...il.com, kernel-team@...com,
        Jakub Kicinski <kicinski@...com>, Andrey Ignatov <rdna@...com>,
        Takshak Chahande <ctakshak@...com>,
        Toke Høiland-Jørgensen 
        <toke@...hat.com>
Subject: Re: [PATCH bpf-next 4/7] bpf: implement BPF XDP link-specific
 introspection APIs

On 7/10/20 4:49 PM, Andrii Nakryiko wrote:
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 025687120442..a9c634be8dd7 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -8973,6 +8973,35 @@ static void bpf_xdp_link_dealloc(struct bpf_link *link)
>  	kfree(xdp_link);
>  }
>  
> +static void bpf_xdp_link_show_fdinfo(const struct bpf_link *link,
> +				     struct seq_file *seq)
> +{
> +	struct bpf_xdp_link *xdp_link = container_of(link, struct bpf_xdp_link, link);
> +	u32 ifindex = 0;
> +
> +	rtnl_lock();
> +	if (xdp_link->dev)
> +		ifindex = xdp_link->dev->ifindex;
> +	rtnl_unlock();

Patch 2 you set dev but don't hold a refcnt on it which is why you need
the locking here. How do you know that the dev pointer is even valid here?

If xdp_link is going to have dev reference you need to take the refcnt
and you need to handle NETDEV notifications to cleanup the bpf_link when
the device goes away.