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

[Mina Almasry]

On Mon, Nov 6, 2023 at 4:16 PM David Ahern <dsahern@xxxxxxxxxx> wrote:
>
> 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?

Seems like a silly choice on my end. I should probably check
skb_frags_not_readable() and not kmap any frags in that case. Will do.

-- 
Thanks,
Mina