Re: [PATCH NET-NEXT] ipv6: skb_expand_head() adjust skb->truesize incorrectly

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

On 8/27/21 8:23 AM, Vasily Averin wrote:

> I asked Alexey Kuznetsov to look at this problem. Below is his answer:
> "I think the current scheme is obsolete. It was created
> when we had only two kinds of skb accounting (rmem & wmem)
> and with more kinds of accounting it just does not work.
> Even there we had ignored problems with adjusting accounting.
> 
> Logically the best solution would be replacing ->destructor,
> set_owner* etc with skb_ops. Something like:
> 
> struct skb_ops
> {
>         void init(struct sk_buff * skb, struct skb_ops * ops, struct
> sock * owner);
>         void fini(struct sk_buff * skb);
>         void update(struct sk_buff * skb, int adjust);
>         void inherit(struct sk_buff * skb2, struct sk_buff * skb);
> };
> 
> init - is replacement for skb_set_owner_r|w
> fini - is replacement for skb_orphan
> update - is new operation to be used in places where skb->truesize changes,
>        instead of awful constructions like:
> 
>        if (!skb->sk || skb->destructor == sock_edemux)
>             skb->truesize += size - osize;
> 
>        Now it will look like:
> 
>        if (skb->ops)
>             skb->ops->update(skb, size - osize);
> 
> inherit - is replacement for also awful constructs like:
> 
>       if (skb->sk)
>             skb_set_owner_w(skb2, skb->sk);
> 
>       Now it will be:
> 
>       if (skb->ops)
>             skb->ops->inherit(skb2, skb);
> 
> The implementation looks mostly obvious.
> Some troubles can be only with new functionality:
> update of accounting was never done before.
> 
> 
> More efficient, functionally equivalent, but uglier and less flexible
> alternative would be removal of ->destructor, replaced with
> a small numeric indicator of ownership:
> 
> enum
> {
>         SKB_OWNER_NONE,  /* aka destructor == NULL */
>         SKB_OWNER_WMEM,  /* aka destructor == sk_wfree */
>         SKB_OWNER_RMEM,  /* aka destructor == sk_rfree */
>         SKB_OWNER_SK,    /* aka destructor == sk_edemux */
>         SKB_OWNER_TCP,   /* aka destructor == tcp_wfree */
> }
> 
> And the same init,fini,inherit,update become functions
> w/o any inidirect calls. Not sure it is really more efficient though."
> 

Well, this does not look as stable material, and would add a bunch
of indirect calls which are quite expensive these days (CONFIG_RETPOLINE=y)

I suggest we work on a fix, using existing infra, then eventually later
try to refactor if this is really bringing improvements.

A fix could simply be a revert of 0c9f227bee119 ("ipv6: use skb_expand_head in ip6_xmit")
since only IPv6 has the problem (because of arbitrary headers size)