Subject: Re: [PATCH net v2] mld, igmp: Fix reserved tailroom calculation

From: Hannes Frederic Sowa <[email-masked]>
Date: Tue, 1 Mar 2016 11:09:53 +0100

On 01.03.2016 00:03, Benjamin Poirier wrote:
> The current reserved_tailroom calculation fails to take hlen and tlen into
> account.
>
> skb:
> [__hlen__|__data____________|__tlen___|__extra__]
> ^                                               ^
> head                                            skb_end_offset
>
> In this representation, hlen + data + tlen is the size passed to alloc_skb.
> "extra" is the extra space made available in __alloc_skb because of
> rounding up by kmalloc. We can reorder the representation like so:
>
> [__hlen__|__data____________|__extra__|__tlen___]
> ^                                               ^
> head                                            skb_end_offset
>
> The maximum space available for ip headers and payload without
> fragmentation is min(mtu, data + extra). Therefore,
> reserved_tailroom
> = data + extra + tlen - min(mtu, data + extra)
> = skb_end_offset - hlen - min(mtu, skb_end_offset - hlen - tlen)
> = skb_tailroom - min(mtu, skb_tailroom - tlen) ; after skb_reserve(hlen)
>
> Compare the second line to the current expression:
> reserved_tailroom = skb_end_offset - min(mtu, skb_end_offset)
> and we can see that hlen and tlen are not taken into account.
>
> The min() in the third line can be expanded into:
> if mtu < skb_tailroom - tlen:
> 	reserved_tailroom = skb_tailroom - mtu
> else:
> 	reserved_tailroom = tlen
>
> Depending on hlen, tlen, mtu and the number of multicast address records,
> the current code may output skbs that have less tailroom than
> dev->needed_tailroom or it may output more skbs than needed because not all
> space available is used.
>
> Fixes: 4c672e4b ("ipv6: mld: fix add_grhead skb_over_panic for devs with large MTUs")
> Signed-off-by: Benjamin Poirier <[email-masked]>

I like it, thanks a lot!

Acked-by: Hannes Frederic Sowa <[email-masked]>