netdev - Re: [PATCH] Fix tcp splice length

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Message-ID: <497EA55A.2050905@cosmosbay.com>
Date:	Tue, 27 Jan 2009 07:10:34 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Dimitris Michailidis <dm@...lsio.com>
CC:	davem@...emloft.net, netdev@...r.kernel.org,
	Willy Tarreau <w@....eu>
Subject: Re: [PATCH] Fix tcp splice length

CCed Willy Tarreau

Dimitris Michailidis a écrit :
> commit 6c242233648471868b44ea091d461f2db6a93f10
> Author: Dimitris Michailidis <dm@...lsio.com>
> Date:   Mon Jan 26 20:46:56 2009 -0800
> 
>     Fix length tcp_splice_data_recv passes to skb_splice_bits.
>     
>     tcp_splice_data_recv has two lengths to consider: the len parameter it
>     gets from tcp_read_sock, which specifies the amount of data in the skb,
>     and rd_desc->count, which is the amount of data the splice caller still
>     wants.  Currently it passes just the latter to skb_splice_bits, which then
>     splices min(rd_desc->count, skb->len - offset) bytes.
>     
>     Most of the time this is fine, except when the skb contains urgent data.
>     In that case len goes only up to the urgent byte and is less than
>     skb->len - offset.  By ignoring len tcp_splice_data_recv may a) splice
>     data tcp_read_sock told it not to, b) return to tcp_read_sock a value > len.
>     
>     Now, tcp_read_sock doesn't handle used > len and leaves the socket in a
>     bad state (both sk_receive_queue and copied_seq are bad at that point)
>     resulting in duplicated data and corruption.
>     
>     Fix by passing min(rd_desc->count, len) to skb_splice_bits.
>     
>     Signed-off-by: Dimitris Michailidis <dm@...lsio.com>
> 
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 0cd71b8..76b148b 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -524,7 +524,8 @@ static int tcp_splice_data_recv(read_descriptor_t *rd_desc, struct sk_buff *skb,
>  	struct tcp_splice_state *tss = rd_desc->arg.data;
>  	int ret;
>  
> -	ret = skb_splice_bits(skb, offset, tss->pipe, rd_desc->count, tss->flags);
> +	ret = skb_splice_bits(skb, offset, tss->pipe, min(rd_desc->count, len),
> +			      tss->flags);
>  	if (ret > 0)
>  		rd_desc->count -= ret;
>  	return ret;

Nice spot Dimitris !

Acked-by: Eric Dumazet <dada1@...mosbay.com>

This fixes a bug present in previous linux versions (before commit 
33966dd0e2f68f26943cd9ee93ec6abbc6547a8e tcp: splice as many packets
 as possible at once)
 
It should be backported as well, changing tss->len by min(tss->len, len) ?



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