linux-kernel - Re: [PATCH bpf] xdp: fix race on generic receive path

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Date:   Tue, 2 Jul 2019 17:40:14 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Ilya Maximets <i.maximets@...sung.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org, xdp-newbies@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Björn Töpel <bjorn.topel@...el.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH bpf] xdp: fix race on generic receive path

On Tue,  2 Jul 2019 17:36:34 +0300, Ilya Maximets wrote:
> Unlike driver mode, generic xdp receive could be triggered
> by different threads on different CPU cores at the same time
> leading to the fill and rx queue breakage. For example, this
> could happen while sending packets from two processes to the
> first interface of veth pair while the second part of it is
> open with AF_XDP socket.
> 
> Need to take a lock for each generic receive to avoid race.
> 
> Fixes: c497176cb2e4 ("xsk: add Rx receive functions and poll support")
> Signed-off-by: Ilya Maximets <i.maximets@...sung.com>

> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index a14e8864e4fa..19f41d2b670c 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -119,17 +119,22 @@ int xsk_generic_rcv(struct xdp_sock *xs, struct xdp_buff *xdp)
>  {
>  	u32 metalen = xdp->data - xdp->data_meta;
>  	u32 len = xdp->data_end - xdp->data;
> +	unsigned long flags;
>  	void *buffer;
>  	u64 addr;
>  	int err;
>  
> -	if (xs->dev != xdp->rxq->dev || xs->queue_id != xdp->rxq->queue_index)
> -		return -EINVAL;
> +	spin_lock_irqsave(&xs->rx_lock, flags);

Why _irqsave, rather than _bh?

> +	if (xs->dev != xdp->rxq->dev || xs->queue_id != xdp->rxq->queue_index) {
> +		err = -EINVAL;
> +		goto out_unlock;
> +	}
>  
>  	if (!xskq_peek_addr(xs->umem->fq, &addr) ||
>  	    len > xs->umem->chunk_size_nohr - XDP_PACKET_HEADROOM) {
> -		xs->rx_dropped++;
> -		return -ENOSPC;
> +		err = -ENOSPC;
> +		goto out_drop;
>  	}
>  
>  	addr += xs->umem->headroom;