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

[Eric Dumazet]

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.

I will test and submit this patch :

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);