Re: [bpf PATCH 1/3] bpf: sockmap, fix scatterlist update on error path in send with apply

[David Miller <davem@davemloft.net>]

From: John Fastabend <john.fastabend@gmail.com>
Date: Wed, 02 May 2018 10:47:27 -0700

> When the call to do_tcp_sendpage() fails to send the complete block
> requested we either retry if only a partial send was completed or
> abort if we receive a error less than or equal to zero. Before
> returning though we must update the scatterlist length/offset to
> account for any partial send completed.
> 
> Before this patch we did this at the end of the retry loop, but
> this was buggy when used while applying a verdict to fewer bytes
> than in the scatterlist. When the scatterlist length was being set
> we forgot to account for the apply logic reducing the size variable.
> So the result was we chopped off some bytes in the scatterlist without
> doing proper cleanup on them. This results in a WARNING when the
> sock is tore down because the bytes have previously been charged to
> the socket but are never uncharged.
> 
> The simple fix is to simply do the accounting inside the retry loop
> subtracting from the absolute scatterlist values rather than trying
> to accumulate the totals and subtract at the end.
> 
> Reported-by: Alexei Starovoitov <ast@kernel.org>
> Signed-off-by: John Fastabend <john.fastabend@gmail.com>

Acked-by: David S. Miller <davem@davemloft.net>