linux-kernel - Re: [PATCH] amd-xgbe: set skb to NULL after freeing it

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Message-Id: <20181128.104931.1898736297454841540.davem@davemloft.net>
Date:   Wed, 28 Nov 2018 10:49:31 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     bianpan2016@....com
Cc:     thomas.lendacky@....com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] amd-xgbe: set skb to NULL after freeing it

From: Pan Bian <bianpan2016@....com>
Date: Wed, 28 Nov 2018 16:09:45 +0800

> @@ -2754,6 +2754,7 @@ static int xgbe_rx_poll(struct xgbe_channel *channel, int budget)
>  				netif_err(pdata, rx_err, netdev,
>  					  "error in received packet\n");
>  			dev_kfree_skb(skb);
> +			skb = NULL;
>  			goto next_packet;
>  		}
>  
> @@ -2806,6 +2807,7 @@ static int xgbe_rx_poll(struct xgbe_channel *channel, int budget)
>  			netif_err(pdata, rx_err, netdev,
>  				  "packet length exceeds configured MTU\n");
>  			dev_kfree_skb(skb);
> +			skb = NULL;
>  			goto next_packet;
>  		}
>  
> -- 
> 2.7.4

If either of these two cases are executed, it means that it must be
the case that:

	!last || context_next

is FALSE, therefore the conditional in the tail code of this function:

	/* Check if we need to save state before leaving */
	if (received && (!last || context_next)) {
		rdata = XGBE_GET_DESC_DATA(ring, ring->cur);
		rdata->state_saved = 1;
		rdata->state.skb = skb;
		rdata->state.len = len;
		rdata->state.error = error;
	}

will not trigger, and therefore we will not record the free'd SKB.

And if we iterate the loop again, it is guaranteed that 'skb' will
be set to a new packet or NULL.

So there is no problem here.