Re: [RFC PATCH v3 09/12] net: add support for skbs with unreadable frags

[David Ahern]

On 11/5/23 7:44 PM, Mina Almasry wrote:
> diff --git a/net/core/datagram.c b/net/core/datagram.c
> index 176eb5834746..cdd4fb129968 100644
> --- a/net/core/datagram.c
> +++ b/net/core/datagram.c
> @@ -425,6 +425,9 @@ static int __skb_datagram_iter(const struct sk_buff *skb, int offset,
>  			return 0;
>  	}
>  
> +	if (skb_frags_not_readable(skb))
> +		goto short_copy;
> +
>  	/* Copy paged appendix. Hmm... why does this look so complicated? */
>  	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
>  		int end;
> @@ -616,6 +619,9 @@ int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk,
>  {
>  	int frag;
>  
> +	if (skb_frags_not_readable(skb))
> +		return -EFAULT;

This check ....
> +
>  	if (msg && msg->msg_ubuf && msg->sg_from_iter)
>  		return msg->sg_from_iter(sk, skb, from, length);


... should go here. That allows custome sg_from_iter to have access to
the skb. What matters is not expecting struct page (e.g., refcounting);
if the custom iter does not do that then all is well. io_uring's iter
does not look at the pages, so all good.

>  
> diff --git a/net/core/gro.c b/net/core/gro.c
> index 42d7f6755f32..56046d65386a 100644
> --- a/net/core/gro.c
> +++ b/net/core/gro.c
> @@ -390,6 +390,9 @@ static void gro_pull_from_frag0(struct sk_buff *skb, int grow)
>  {
>  	struct skb_shared_info *pinfo = skb_shinfo(skb);
>  
> +	if (WARN_ON_ONCE(skb_frags_not_readable(skb)))
> +		return;
> +
>  	BUG_ON(skb->end - skb->tail < grow);
>  
>  	memcpy(skb_tail_pointer(skb), NAPI_GRO_CB(skb)->frag0, grow);
> @@ -411,7 +414,7 @@ static void gro_try_pull_from_frag0(struct sk_buff *skb)
>  {
>  	int grow = skb_gro_offset(skb) - skb_headlen(skb);
>  
> -	if (grow > 0)
> +	if (grow > 0 && !skb_frags_not_readable(skb))
>  		gro_pull_from_frag0(skb, grow);
>  }
>  
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 13eca4fd25e1..f01673ed2eff 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -1230,6 +1230,14 @@ void skb_dump(const char *level, const struct sk_buff *skb, bool full_pkt)
>  		struct page *p;
>  		u8 *vaddr;
>  
> +		if (skb_frag_is_page_pool_iov(frag)) {

Why skb_frag_is_page_pool_iov here vs skb_frags_not_readable?