ip: Add offset parameter to ip_cmsg_recv

Add ip_cmsg_recv_offset function which takes an offset argument
that indicates the starting offset in skb where data is being received
from. This will be useful in the case of UDP and provided checksum
to user space.

ip_cmsg_recv is an inline call to ip_cmsg_recv_offset with offset of
zero.

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 41 insertions, 2 deletions

diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index 513d506ffebb..a317797b3cd0 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -37,6 +37,7 @@
 #include <net/route.h>
 #include <net/xfrm.h>
 #include <net/compat.h>
+#include <net/checksum.h>
 #if IS_ENABLED(CONFIG_IPV6)
 #include <net/transp_v6.h>
 #endif
@@ -96,6 +97,20 @@ static void ip_cmsg_recv_retopts(struct msghdr *msg, struct sk_buff *skb)
         put_cmsg(msg, SOL_IP, IP_RETOPTS, opt->optlen, opt->__data);
 }
 
+static void ip_cmsg_recv_checksum(struct msghdr *msg, struct sk_buff *skb,
+                                  int offset)
+{
+        __wsum csum = skb->csum;
+
+        if (skb->ip_summed != CHECKSUM_COMPLETE)
+                return;
+
+        if (offset != 0)
+                csum = csum_sub(csum, csum_partial(skb->data, offset, 0));
+
+        put_cmsg(msg, SOL_IP, IP_CHECKSUM, sizeof(__wsum), &csum);
+}
+
 static void ip_cmsg_recv_security(struct msghdr *msg, struct sk_buff *skb)
 {
         char *secdata;
@@ -191,9 +206,16 @@ void ip_cmsg_recv_offset(struct msghdr *msg, struct sk_buff *skb,
                         return;
         }
 
-        if (flags & IP_CMSG_ORIGDSTADDR)
+        if (flags & IP_CMSG_ORIGDSTADDR) {
                 ip_cmsg_recv_dstaddr(msg, skb);
 
+                flags &= ~IP_CMSG_ORIGDSTADDR;
+                if (!flags)
+                        return;
+        }
+
+        if (flags & IP_CMSG_CHECKSUM)
+                ip_cmsg_recv_checksum(msg, skb, offset);
 }
 EXPORT_SYMBOL(ip_cmsg_recv_offset);
 
@@ -533,6 +555,7 @@ static int do_ip_setsockopt(struct sock *sk, int level,
         case IP_MULTICAST_ALL:
         case IP_MULTICAST_LOOP:
         case IP_RECVORIGDSTADDR:
+        case IP_CHECKSUM:
                 if (optlen >= sizeof(int)) {
                         if (get_user(val, (int __user *) optval))
                                 return -EFAULT;
@@ -630,6 +653,19 @@ static int do_ip_setsockopt(struct sock *sk, int level,
                 else
                         inet->cmsg_flags &= ~IP_CMSG_ORIGDSTADDR;
                 break;
+        case IP_CHECKSUM:
+                if (val) {
+                        if (!(inet->cmsg_flags & IP_CMSG_CHECKSUM)) {
+                                inet_inc_convert_csum(sk);
+                                inet->cmsg_flags |= IP_CMSG_CHECKSUM;
+                        }
+                } else {
+                        if (inet->cmsg_flags & IP_CMSG_CHECKSUM) {
+                                inet_dec_convert_csum(sk);
+                                inet->cmsg_flags &= ~IP_CMSG_CHECKSUM;
+                        }
+                }
+                break;
         case IP_TOS:    /* This sets both TOS and Precedence */
                 if (sk->sk_type == SOCK_STREAM) {
                         val &= ~INET_ECN_MASK;
@@ -1233,6 +1269,9 @@ static int do_ip_getsockopt(struct sock *sk, int level, int optname,
         case IP_RECVORIGDSTADDR:
                 val = (inet->cmsg_flags & IP_CMSG_ORIGDSTADDR) != 0;
                 break;
+        case IP_CHECKSUM:
+                val = (inet->cmsg_flags & IP_CMSG_CHECKSUM) != 0;
+                break;
         case IP_TOS:
                 val = inet->tos;
                 break;
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 53358d88f110..97ef1f8b7be8 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1329,7 +1329,7 @@ try_again:
                 *addr_len = sizeof(*sin);
         }
         if (inet->cmsg_flags)
-                ip_cmsg_recv(msg, skb);
+                ip_cmsg_recv_offset(msg, skb, sizeof(struct udphdr));
 
         err = copied;
         if (flags & MSG_TRUNC)