[PATCH kvmtool] net: uip: Fix GCC 10 warning about checksum calculation

[Andre Przywara <andre.przywara@xxxxxxx>]

GCC 10.1 generates a warning in net/ip/csum.c about exceeding a buffer
limit in a memcpy operation:
------------------
In function ‘memcpy’,
    inlined from ‘uip_csum_udp’ at net/uip/csum.c:58:3:
/usr/include/aarch64-linux-gnu/bits/string_fortified.h:34:10: error: writing 1 byte into a region of size 0 [-Werror=stringop-overflow=]
   34 |   return __builtin___memcpy_chk (__dest, __src, __len, __bos0 (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from net/uip/csum.c:1:
net/uip/csum.c: In function ‘uip_csum_udp’:
include/kvm/uip.h:132:6: note: at offset 0 to object ‘sport’ with size 2 declared here
  132 |  u16 sport;
------------------

This warning originates from the code taking the address of the "sport"
member, then using that with some pointer arithmetic in a memcpy call.
GCC now sees that the object is only a u16, so copying 12 bytes into it
cannot be any good.
It's somewhat debatable whether this is a legitimate warning, as there
is enough storage at that place, and we knowingly use the struct and
its variabled-sized member at the end.

However we can also rewrite the code, to not abuse the "&" operation of
some *member*, but take the address of the struct itself.
This makes the code less dodgy, and indeed appeases GCC 10.

Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
Reported-by: Alexandru Elisei <alexandru.elisei@xxxxxxx>
---
 net/uip/csum.c | 26 ++++++++++++--------------
 1 file changed, 12 insertions(+), 14 deletions(-)

diff --git a/net/uip/csum.c b/net/uip/csum.c
index 7ca8bada..607c9f1c 100644
--- a/net/uip/csum.c
+++ b/net/uip/csum.c
@@ -37,7 +37,7 @@ u16 uip_csum_udp(struct uip_udp *udp)
 	struct uip_pseudo_hdr hdr;
 	struct uip_ip *ip;
 	int udp_len;
-	u8 *pad;
+	u8 *udp_hdr = (u8*)udp + offsetof(struct uip_udp, sport);
 
 	ip	  = &udp->ip;
 
@@ -50,13 +50,12 @@ u16 uip_csum_udp(struct uip_udp *udp)
 	udp_len	  = uip_udp_len(udp);
 
 	if (udp_len % 2) {
-		pad = (u8 *)&udp->sport + udp_len;
-		*pad = 0;
-		memcpy((u8 *)&udp->sport + udp_len + 1, &hdr, sizeof(hdr));
-		return uip_csum(0, (u8 *)&udp->sport, udp_len + 1 + sizeof(hdr));
+		udp_hdr[udp_len] = 0;		/* zero padding */
+		memcpy(udp_hdr + udp_len + 1, &hdr, sizeof(hdr));
+		return uip_csum(0, udp_hdr, udp_len + 1 + sizeof(hdr));
 	} else {
-		memcpy((u8 *)&udp->sport + udp_len, &hdr, sizeof(hdr));
-		return uip_csum(0, (u8 *)&udp->sport, udp_len + sizeof(hdr));
+		memcpy(udp_hdr + udp_len, &hdr, sizeof(hdr));
+		return uip_csum(0, udp_hdr, udp_len + sizeof(hdr));
 	}
 
 }
@@ -66,7 +65,7 @@ u16 uip_csum_tcp(struct uip_tcp *tcp)
 	struct uip_pseudo_hdr hdr;
 	struct uip_ip *ip;
 	u16 tcp_len;
-	u8 *pad;
+	u8 *tcp_hdr = (u8*)tcp + offsetof(struct uip_tcp, sport);
 
 	ip	  = &tcp->ip;
 	tcp_len   = ntohs(ip->len) - uip_ip_hdrlen(ip);
@@ -81,12 +80,11 @@ u16 uip_csum_tcp(struct uip_tcp *tcp)
 		pr_warning("tcp_len(%d) is too large", tcp_len);
 
 	if (tcp_len % 2) {
-		pad = (u8 *)&tcp->sport + tcp_len;
-		*pad = 0;
-		memcpy((u8 *)&tcp->sport + tcp_len + 1, &hdr, sizeof(hdr));
-		return uip_csum(0, (u8 *)&tcp->sport, tcp_len + 1 + sizeof(hdr));
+		tcp_hdr[tcp_len] = 0;		/* zero padding */
+		memcpy(tcp_hdr + tcp_len + 1, &hdr, sizeof(hdr));
+		return uip_csum(0, tcp_hdr, tcp_len + 1 + sizeof(hdr));
 	} else {
-		memcpy((u8 *)&tcp->sport + tcp_len, &hdr, sizeof(hdr));
-		return uip_csum(0, (u8 *)&tcp->sport, tcp_len + sizeof(hdr));
+		memcpy(tcp_hdr + tcp_len, &hdr, sizeof(hdr));
+		return uip_csum(0, tcp_hdr, tcp_len + sizeof(hdr));
 	}
 }
-- 
2.17.1