Re: [PATCH 1/3] skbuff: Update truesize in pskb_expand_head

[Eric Dumazet <eric.dumazet@gmail.com>]

On Wed, 2013-06-12 at 19:05 +1000, Dave Wiltshire wrote:
> Some call sites to pskb_expand_head subsequently update the skb truesize
> and others don't (even with non-zero arguments). This is likely a memory
> audit leak. Fixed this up by moving the memory accounting to the
> skbuff.c file and removing it from the calling sites.
> 
> Signed-off-by: Dave Wiltshire <david.wiltshire@gmx.com>
> ---
>  drivers/net/wireless/mwl8k.c |    1 -
>  kernel/audit.c               |    2 --
>  net/core/skbuff.c            |    1 +
>  net/netlink/af_netlink.c     |    3 +--
>  net/wireless/util.c          |    2 --
>  5 files changed, 2 insertions(+), 7 deletions(-)

Ouch.

Sorry, you cannot do that.

skb->truesize is really complex, because there is a strong relation
between skb->truesize and memory accounting on sockets.

So pskb_expand_head() should not touch skb->truesize.

Only callers can do that when needed, and if possible.

An example of very careful truesize manipulation can be found in
tcp_tso_segment()