From: Vlad Yasevich <[EMAIL PROTECTED]>
Date: Fri, 29 Sep 2006 14:16:57 -0400
> I've attached the patch, in case people want to look at the code.
>
> However, we question if this is a good idea or if this is going to break
> things...
Modification of skb->truesize is very dangerous and is only legal
in a very limited set of circumstances.
The core problem is that if any other reference exists to the skb
you could corrupt existing socket accounting by changing skb->truesize.
Let's say that the packet went into a network tap like tcpdump and
the packet has been charged to an AF_PACKET socket via skb->truesize.
If you modify skb->truesize then when the AF_PACKET socket releases
it's reference it will subtract the wrong amount of skb->truesize
from the socket receive buffer.
If, on the other hand, you know you have exclusive access to the
skb and there are no other references, setting skb->truesize can
be OK. However setting it to sizeof(struct sk_buff) doesn't make
sense since there is at least:
sizeof(struct sk_buff) + sizeof(struct skb_shared_info)
memory assosciated with any SKB whatsoever in the kernel. That is,
even for a "zero length" skb->data buffer, we still always allocate
the skb_shared_info area which must be accounted for.
BTW, I think the whole chunking mechanism in the SCTP code is the
largest source of problems and maintainability issues in that stack.
Any time I want to make major modifications to SKBs to make them
smaller or whatever, the most difficult piece of code to adjust is
this code.
-
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to [EMAIL PROTECTED]
More majordomo info at http://vger.kernel.org/majordomo-info.html
