netdev - Re: [PATCH bpf v2] xsk: fix crash in poll when device does not support ndo_xsk

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Date:   Thu, 3 Oct 2019 17:00:54 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Magnus Karlsson <magnus.karlsson@...el.com>
Cc:     bjorn.topel@...el.com, ast@...nel.org, netdev@...r.kernel.org,
        jonathan.lemon@...il.com, bpf@...r.kernel.org
Subject: Re: [PATCH bpf v2] xsk: fix crash in poll when device does not
 support ndo_xsk_wakeup

On Wed, Oct 02, 2019 at 08:31:59AM +0200, Magnus Karlsson wrote:
> Fixes a crash in poll() when an AF_XDP socket is opened in copy mode
> and the bound device does not have ndo_xsk_wakeup defined. Avoid
> trying to call the non-existing ndo and instead call the internal xsk
> sendmsg function to send packets in the same way (from the
> application's point of view) as calling sendmsg() in any mode or
> poll() in zero-copy mode would have done. The application should
> behave in the same way independent on if zero-copy mode or copy mode
> is used.
> 
> Fixes: 77cd0d7b3f25 ("xsk: add support for need_wakeup flag in AF_XDP rings")
> Reported-by: syzbot+a5765ed8cdb1cca4d249@...kaller.appspotmail.com
> Signed-off-by: Magnus Karlsson <magnus.karlsson@...el.com>

Applied, thanks!

[...]
> +static int xsk_generic_xmit(struct sock *sk)
>  {
> -	u32 max_batch = TX_BATCH_SIZE;
>  	struct xdp_sock *xs = xdp_sk(sk);
> +	u32 max_batch = TX_BATCH_SIZE;
>  	bool sent_frame = false;
>  	struct xdp_desc desc;
>  	struct sk_buff *skb;
> @@ -394,6 +392,18 @@ static int xsk_generic_xmit(struct sock *sk, struct msghdr *m,
>  	return err;
>  }
>  
> +static int __xsk_sendmsg(struct sock *sk)

Bit unclear why you don't just pass xs directly in here from both call
sites ...

> +{
> +	struct xdp_sock *xs = xdp_sk(sk);
> +
> +	if (unlikely(!(xs->dev->flags & IFF_UP)))
> +		return -ENETDOWN;
> +	if (unlikely(!xs->tx))
> +		return -ENOBUFS;
> +
> +	return xs->zc ? xsk_zc_xmit(xs) : xsk_generic_xmit(sk);

... and for the xsk_generic_xmit() pass in &xs->sk. Presumably generated
code should be the same, but maybe small cleanup for next batch of AF_XDP
patches.

> +}
> +
>  static int xsk_sendmsg(struct socket *sock, struct msghdr *m, size_t total_len)
>  {
>  	bool need_wait = !(m->msg_flags & MSG_DONTWAIT);
> @@ -402,21 +412,18 @@ static int xsk_sendmsg(struct socket *sock, struct msghdr *m, size_t total_len)
>  
>  	if (unlikely(!xsk_is_bound(xs)))
>  		return -ENXIO;
> -	if (unlikely(!(xs->dev->flags & IFF_UP)))
> -		return -ENETDOWN;
> -	if (unlikely(!xs->tx))
> -		return -ENOBUFS;
> -	if (need_wait)
> +	if (unlikely(need_wait))
>  		return -EOPNOTSUPP;
>  
> -	return (xs->zc) ? xsk_zc_xmit(sk) : xsk_generic_xmit(sk, m, total_len);
> +	return __xsk_sendmsg(sk);
>  }

Thanks,
Daniel