On Fri, Jan 09, 2009 at 09:51:17PM +0100, Eric Dumazet wrote:

(...)

quoted text
> > Also, in your second mail, you're saying that your change

> > might return more data than requested by the user. I can't

> > find why, could you please explain to me, as I'm still quite

> > ignorant in this area ?

>

> Well, I just tested various user programs and indeed got this

> strange result :

>

> Here I call splice() with len=1000 (0x3e8), and you can see

> it gives a result of 1460 at the second call.

huh, not nice indeed!
While looking at the code to see how this could be possible, I

came across this minor thing (unrelated IMHO) :
	if (__skb_splice_bits(skb, &offset, &tlen, &spd))

		goto done;

quoted text
>>>>>>	else if (!tlen)    <<<<<<

		goto done;

	/*

	 * now see if we have a frag_list to map

	 */

	if (skb_shinfo(skb)->frag_list) {

		struct sk_buff *list = skb_shinfo(skb)->frag_list;
		for (; list && tlen; list = list->next) {

			if (__skb_splice_bits(list, &offset, &tlen, &spd))

				break;

		}

	}
    done:
Above on the enlighted line, we'd better remove the else and leave a plain

"if (!tlen)". Otherwise, when the first call to __skb_splice_bits() zeroes

tlen, we still enter the if and evaluate the for condition for nothing. But

let's leave that for later.
> I suspect a bug in splice code, that my patch just exposed.
I've checked in skb_splice_bits() and below and can't see how we can move

more than the requested len.
However, with your change, I don't clearly see how we break out of

the loop in tcp_read_sock(). Maybe we first read 1000 then loop again

and read remaining data ? I suspect that we should at least exit when

((struct tcp_splice_state *)desc->arg.data)->len = 0.
At least that's something easy to add just before or after !desc->count

for a test.
Regards,

Willy
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