Re: [PATCH v4 net] net: udp: fix Fast/frag0 UDP GRO

[Willem de Bruijn]

On Mon, Nov 9, 2020 at 7:29 PM Alexander Lobakin <alobakin@xxxxx> wrote:
>
> From: Alexander Lobakin <alobakin@xxxxx>
> Date: Tue, 10 Nov 2020 00:17:18 +0000
>
> > While testing UDP GSO fraglists forwarding through driver that uses
> > Fast GRO (via napi_gro_frags()), I was observing lots of out-of-order
> > iperf packets:
> >
> > [ ID] Interval           Transfer     Bitrate         Jitter
> > [SUM]  0.0-40.0 sec  12106 datagrams received out-of-order
> >
> > Simple switch to napi_gro_receive() or any other method without frag0
> > shortcut completely resolved them.
> >
> > I've found that UDP GRO uses udp_hdr(skb) in its .gro_receive()
> > callback. While it's probably OK for non-frag0 paths (when all
> > headers or even the entire frame are already in skb->data), this
> > inline points to junk when using Fast GRO (napi_gro_frags() or
> > napi_gro_receive() with only Ethernet header in skb->data and all
> > the rest in shinfo->frags) and breaks GRO packet compilation and
> > the packet flow itself.
> > To support both modes, skb_gro_header_fast() + skb_gro_header_slow()
> > are typically used. UDP even has an inline helper that makes use of
> > them, udp_gro_udphdr(). Use that instead of troublemaking udp_hdr()
> > to get rid of the out-of-order delivers.
> >
> > Present since the introduction of plain UDP GRO in 5.0-rc1.
> >
> > Since v3 [1]:
> >  - restore the original {,__}udp{4,6}_lib_lookup_skb() and use
> >    private versions of them inside GRO code (Willem).
>
> Note: this doesn't cover a support for nested tunnels as it's out of
> the subject and requires more invasive changes. It will be handled
> separately in net-next series.

Thanks for looking into that.

In that case, should the p->data + off change be deferred to that,
too? It adds some risk unrelated to the bug fix.

> > Since v2 [2]:
> >  - dropped redundant check introduced in v2 as it's performed right
> >    before (thanks to Eric);
> >  - udp_hdr() switched to data + off for skbs from list (also Eric);
> >  - fixed possible malfunction of {,__}udp{4,6}_lib_lookup_skb() with
> >    Fast/frag0 due to ip{,v6}_hdr() usage (Willem).
> >
> > Since v1 [3]:
> >  - added a NULL pointer check for "uh" as suggested by Willem.
> >
> > [1] https://lore.kernel.org/netdev/MgZce9htmEtCtHg7pmWxXXfdhmQ6AHrnltXC41zOoo@xxxxxxxxxxxxxxxxxxxx
> > [2] https://lore.kernel.org/netdev/0eaG8xtbtKY1dEKCTKUBubGiC9QawGgB3tVZtNqVdY@xxxxxxxxxxxxxxxxxxxx
> > [3] https://lore.kernel.org/netdev/YazU6GEzBdpyZMDMwJirxDX7B4sualpDG68ADZYvJI@xxxxxxxxxxxxxxxxxxxx
> >
> > Fixes: e20cf8d3f1f7 ("udp: implement GRO for plain UDP sockets.")
> > Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> > Cc: Willem de Bruijn <willemb@xxxxxxxxxx>
> > Signed-off-by: Alexander Lobakin <alobakin@xxxxx>
> > ---
> >  net/ipv4/udp_offload.c | 23 +++++++++++++++++++----
> >  net/ipv6/udp_offload.c | 14 +++++++++++++-
> >  2 files changed, 32 insertions(+), 5 deletions(-)
> >
> > diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> > index e67a66fbf27b..6064efe17cdb 100644
> > --- a/net/ipv4/udp_offload.c
> > +++ b/net/ipv4/udp_offload.c
> > @@ -366,11 +366,11 @@ static struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb,
> >  static struct sk_buff *udp_gro_receive_segment(struct list_head *head,
> >                                              struct sk_buff *skb)
> >  {
> > -     struct udphdr *uh = udp_hdr(skb);
> > +     struct udphdr *uh = udp_gro_udphdr(skb);
> >       struct sk_buff *pp = NULL;
> >       struct udphdr *uh2;
> >       struct sk_buff *p;
> > -     unsigned int ulen;
> > +     u32 ulen, off;

a specific reason for changing type here?

> >       int ret = 0;
> >
> >       /* requires non zero csum, for symmetry with GSO */
> > @@ -385,6 +385,9 @@ static struct sk_buff *udp_gro_receive_segment(struct list_head *head,
> >               NAPI_GRO_CB(skb)->flush = 1;
> >               return NULL;
> >       }
> > +
> > +     off = skb_gro_offset(skb);
> > +
> >       /* pull encapsulating udp header */
> >       skb_gro_pull(skb, sizeof(struct udphdr));
> >
> > @@ -392,7 +395,7 @@ static struct sk_buff *udp_gro_receive_segment(struct list_head *head,
> >               if (!NAPI_GRO_CB(p)->same_flow)
> >                       continue;
> >
> > -             uh2 = udp_hdr(p);
> > +             uh2 = (void *)p->data + off;
> >
> >               /* Match ports only, as csum is always non zero */
> >               if ((*(u32 *)&uh->source != *(u32 *)&uh2->source)) {
> > @@ -500,6 +503,16 @@ struct sk_buff *udp_gro_receive(struct list_head *head, struct sk_buff *skb,
> >  }
> >  EXPORT_SYMBOL(udp_gro_receive);
> >
> > +static struct sock *udp4_gro_lookup_skb(struct sk_buff *skb, __be16 sport,
> > +                                     __be16 dport)
> > +{
> > +     const struct iphdr *iph = skb_gro_network_header(skb);
> > +
> > +     return __udp4_lib_lookup(dev_net(skb->dev), iph->saddr, sport,
> > +                              iph->daddr, dport, inet_iif(skb),
> > +                              inet_sdif(skb), &udp_table, NULL);
> > +}
> > +
> >  INDIRECT_CALLABLE_SCOPE
> >  struct sk_buff *udp4_gro_receive(struct list_head *head, struct sk_buff *skb)
> >  {
> > @@ -523,7 +536,9 @@ struct sk_buff *udp4_gro_receive(struct list_head *head, struct sk_buff *skb)
> >  skip:
> >       NAPI_GRO_CB(skb)->is_ipv6 = 0;
> >       rcu_read_lock();
> > -     sk = static_branch_unlikely(&udp_encap_needed_key) ? udp4_lib_lookup_skb(skb, uh->source, uh->dest) : NULL;
> > +     sk = static_branch_unlikely(&udp_encap_needed_key) ?
> > +          udp4_gro_lookup_skb(skb, uh->source, uh->dest) :
> > +          NULL;

Does this indentation pass checkpatch?

Else, the line limit is no longer strict,a and this only shortens the
line, so a single line is fine.