[PATCH] libnetlink: fix alignment of netlink messages

[Hauke Mehrtens]

An padding to align a message should not only be added between
different attributes of a netlink message, but also at the end of the
message to pad it to the correct size.

Without this patch the following command does not work and returns an
error code:
ip link add type nlmon

Without this ip from busybox sends this:
sendmsg(3, {msg_name={sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, msg_namelen=12, msg_iov=[{iov_base={{len=45, type=0x10 /* NLMSG_??? */, flags=NLM_F_REQUEST|NLM_F_ACK|0x600, seq=1474057401, pid=0}, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\20\0\22\0\t\0\1nlmon"}, iov_len=45}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 45
return value: 2

The normal ip utile from iproute2 sends this:
sendmsg(3, {msg_name={sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, msg_namelen=12, msg_iov=[{iov_base={{len=48, type=0x10 /* NLMSG_??? */, flags=NLM_F_REQUEST|NLM_F_ACK|0x600, seq=1473716938, pid=0}, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\20\0\22\0\t\0\1nlmon\0\0\0"}, iov_len=48}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 48
return value: 0

With this patch ip from busybox sends this:
sendmsg(3, {msg_name={sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, msg_namelen=12, msg_iov=[{iov_base={{len=48, type=0x10 /* NLMSG_??? */, flags=NLM_F_REQUEST|NLM_F_ACK|0x600, seq=1473716908, pid=0}, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\20\0\22\0\t\0\1nlmon\0\0\0"}, iov_len=48}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 48
return value: 0

Signed-off-by: Hauke Mehrtens <hauke at hauke-m.de>
---
 networking/libiproute/libnetlink.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/networking/libiproute/libnetlink.c b/networking/libiproute/libnetlink.c
index 9d5c641..7e0ff1b 100644
--- a/networking/libiproute/libnetlink.c
+++ b/networking/libiproute/libnetlink.c
@@ -340,14 +340,14 @@ int FAST_FUNC addattr32(struct nlmsghdr *n, int maxlen, int type, uint32_t data)
 	int len = RTA_LENGTH(4);
 	struct rtattr *rta;
 
-	if ((int)(NLMSG_ALIGN(n->nlmsg_len) + len) > maxlen) {
+	if ((int)(NLMSG_ALIGN(n->nlmsg_len + len)) > maxlen) {
 		return -1;
 	}
 	rta = (struct rtattr*)(((char*)n) + NLMSG_ALIGN(n->nlmsg_len));
 	rta->rta_type = type;
 	rta->rta_len = len;
 	move_to_unaligned32(RTA_DATA(rta), data);
-	n->nlmsg_len = NLMSG_ALIGN(n->nlmsg_len) + len;
+	n->nlmsg_len = NLMSG_ALIGN(n->nlmsg_len + len);
 	return 0;
 }
 
@@ -356,14 +356,14 @@ int FAST_FUNC addattr_l(struct nlmsghdr *n, int maxlen, int type, void *data, in
 	int len = RTA_LENGTH(alen);
 	struct rtattr *rta;
 
-	if ((int)(NLMSG_ALIGN(n->nlmsg_len) + len) > maxlen) {
+	if ((int)(NLMSG_ALIGN(n->nlmsg_len + len)) > maxlen) {
 		return -1;
 	}
 	rta = (struct rtattr*)(((char*)n) + NLMSG_ALIGN(n->nlmsg_len));
 	rta->rta_type = type;
 	rta->rta_len = len;
 	memcpy(RTA_DATA(rta), data, alen);
-	n->nlmsg_len = NLMSG_ALIGN(n->nlmsg_len) + len;
+	n->nlmsg_len = NLMSG_ALIGN(n->nlmsg_len + len);
 	return 0;
 }
 
@@ -372,14 +372,14 @@ int FAST_FUNC rta_addattr32(struct rtattr *rta, int maxlen, int type, uint32_t d
 	int len = RTA_LENGTH(4);
 	struct rtattr *subrta;
 
-	if (RTA_ALIGN(rta->rta_len) + len > maxlen) {
+	if (RTA_ALIGN(rta->rta_len + len) > maxlen) {
 		return -1;
 	}
 	subrta = (struct rtattr*)(((char*)rta) + RTA_ALIGN(rta->rta_len));
 	subrta->rta_type = type;
 	subrta->rta_len = len;
 	move_to_unaligned32(RTA_DATA(subrta), data);
-	rta->rta_len = NLMSG_ALIGN(rta->rta_len) + len;
+	rta->rta_len = NLMSG_ALIGN(rta->rta_len + len);
 	return 0;
 }
 
@@ -388,14 +388,14 @@ int FAST_FUNC rta_addattr_l(struct rtattr *rta, int maxlen, int type, void *data
 	struct rtattr *subrta;
 	int len = RTA_LENGTH(alen);
 
-	if (RTA_ALIGN(rta->rta_len) + len > maxlen) {
+	if (RTA_ALIGN(rta->rta_len + len) > maxlen) {
 		return -1;
 	}
 	subrta = (struct rtattr*)(((char*)rta) + RTA_ALIGN(rta->rta_len));
 	subrta->rta_type = type;
 	subrta->rta_len = len;
 	memcpy(RTA_DATA(subrta), data, alen);
-	rta->rta_len = NLMSG_ALIGN(rta->rta_len) + len;
+	rta->rta_len = NLMSG_ALIGN(rta->rta_len + len);
 	return 0;
 }
 
-- 
2.9.3