Re: [PATCH v2 net] net: udp: fix Fast/frag0 UDP GRO

From: Alexander Lobakin
Date: Mon Nov 09 2020 - 14:56:40 EST

From: Eric Dumazet <eric.dumazet@xxxxxxxxx>
Date: Mon, 9 Nov 2020 20:29:03 +0100

> On 11/9/20 7:26 PM, Alexander Lobakin wrote:
>> From: Eric Dumazet <eric.dumazet@xxxxxxxxx>
>> Date: Mon, 9 Nov 2020 18:37:36 +0100
>>
>>> On 11/9/20 5:56 PM, Alexander Lobakin wrote:
>>>> 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:
>>>>
>.
>>>>
>>>> Since v1 [1]:
>>>> - added a NULL pointer check for "uh" as suggested by Willem.
>>>>
>>>> [1] https://lore.kernel.org/netdev/YazU6GEzBdpyZMDMwJirxDX7B4sualpDG68ADZYvJI@xxxxxxxxxxxxxxxxxxxx
>>>>
>>>> Fixes: e20cf8d3f1f7 ("udp: implement GRO for plain UDP sockets.")
>>>> Signed-off-by: Alexander Lobakin <alobakin@xxxxx>
>>>> ---
>>>> net/ipv4/udp_offload.c | 7 ++++++-
>>>> 1 file changed, 6 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
>>>> index e67a66fbf27b..7f6bd221880a 100644
>>>> --- a/net/ipv4/udp_offload.c
>>>> +++ b/net/ipv4/udp_offload.c
>>>> @@ -366,13 +366,18 @@ 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;
>>>> int ret = 0;
>>>>
>>>> + if (unlikely(!uh)) {
>>>
>>> How uh could be NULL here ?
>>>
>>> My understanding is that udp_gro_receive() is called
>>> only after udp4_gro_receive() or udp6_gro_receive()
>>> validated that udp_gro_udphdr(skb) was not NULL.
>>
>> Right, but only after udp{4,6}_lib_lookup_skb() in certain cases.
>> I don't know for sure if their logic can actually edit skb->data,
>> so it's better to check from my point of view.
>
> Not really. This would send a wrong signal to readers of this code.
>
> I do not think these functions can mess with GRO internals.
>
> This would be a nightmare, GRO is already way too complex.
>
> In fact these functions should use a const qualifier
> for their " struct sk_buff *skb" argument to prevent future bugs.

Agree, you're right. Lack of const qualifiers gave me a doubt that
these functions can't edit skbs. They really should use it if they
really can't.
I'll omit the check in v3.

> I will test and submit this patch :

That would be a very nice one, thanks.

> diff --git a/include/net/ip.h b/include/net/ip.h
> index 2d6b985d11ccaa75827b3a15ac3f898d7a193242..e20874059f826eb0f9e899aed556bfbc9c9d71e8 100644
> --- a/include/net/ip.h
> +++ b/include/net/ip.h
> @@ -99,7 +99,7 @@ static inline void ipcm_init_sk(struct ipcm_cookie *ipcm,
> #define PKTINFO_SKB_CB(skb) ((struct in_pktinfo *)((skb)->cb))
>
> /* return enslaved device index if relevant */
> -static inline int inet_sdif(struct sk_buff *skb)
> +static inline int inet_sdif(const struct sk_buff *skb)
> {
> #if IS_ENABLED(CONFIG_NET_L3_MASTER_DEV)
> if (skb && ipv4_l3mdev_skb(IPCB(skb)->flags))
> diff --git a/include/net/udp.h b/include/net/udp.h
> index 295d52a73598277dc5071536f777d1a87e7df1d1..877832bed4713a011a514a2f6f522728c8c89e20 100644
> --- a/include/net/udp.h
> +++ b/include/net/udp.h
> @@ -164,7 +164,7 @@ static inline void udp_csum_pull_header(struct sk_buff *skb)
> UDP_SKB_CB(skb)->cscov -= sizeof(struct udphdr);
> }
>
> -typedef struct sock *(*udp_lookup_t)(struct sk_buff *skb, __be16 sport,
> +typedef struct sock *(*udp_lookup_t)(const struct sk_buff *skb, __be16 sport,
> __be16 dport);
>
> INDIRECT_CALLABLE_DECLARE(struct sk_buff *udp4_gro_receive(struct list_head *,
> @@ -313,7 +313,7 @@ struct sock *udp4_lib_lookup(struct net *net, __be32 saddr, __be16 sport,
> struct sock *__udp4_lib_lookup(struct net *net, __be32 saddr, __be16 sport,
> __be32 daddr, __be16 dport, int dif, int sdif,
> struct udp_table *tbl, struct sk_buff *skb);
> -struct sock *udp4_lib_lookup_skb(struct sk_buff *skb,
> +struct sock *udp4_lib_lookup_skb(const struct sk_buff *skb,
> __be16 sport, __be16 dport);
> struct sock *udp6_lib_lookup(struct net *net,
> const struct in6_addr *saddr, __be16 sport,
> @@ -324,7 +324,7 @@ struct sock *__udp6_lib_lookup(struct net *net,
> const struct in6_addr *daddr, __be16 dport,
> int dif, int sdif, struct udp_table *tbl,
> struct sk_buff *skb);
> -struct sock *udp6_lib_lookup_skb(struct sk_buff *skb,
> +struct sock *udp6_lib_lookup_skb(const struct sk_buff *skb,
> __be16 sport, __be16 dport);
>
> /* UDP uses skb->dev_scratch to cache as much information as possible and avoid
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index 09f0a23d1a01741d335ce45f25fe70a4e00698c7..8b8dadfea6c9854e6bfaa0fabcb774db39976da3 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -541,7 +541,7 @@ static inline struct sock *__udp4_lib_lookup_skb(struct sk_buff *skb,
> inet_sdif(skb), udptable, skb);
> }
>
> -struct sock *udp4_lib_lookup_skb(struct sk_buff *skb,
> +struct sock *udp4_lib_lookup_skb(const struct sk_buff *skb,
> __be16 sport, __be16 dport)
> {
> const struct iphdr *iph = ip_hdr(skb);
> diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
> index 29d9691359b9c49ccb56a11f79e3658b1a76700d..adfe9ca6f516612b5aad6d6c654c7da1dd56a50e 100644
> --- a/net/ipv6/udp.c
> +++ b/net/ipv6/udp.c
> @@ -276,7 +276,7 @@ static struct sock *__udp6_lib_lookup_skb(struct sk_buff *skb,
> inet6_sdif(skb), udptable, skb);
> }
>
> -struct sock *udp6_lib_lookup_skb(struct sk_buff *skb,
> +struct sock *udp6_lib_lookup_skb(const struct sk_buff *skb,
> __be16 sport, __be16 dport)
> {
> const struct ipv6hdr *iph = ipv6_hdr(skb);
>

Al