netlink: Change rtnl_dump_done to always show error

The original code which became rtnl_dump_done only shows netlink errors if the protocol is NETLINK_SOCK_DIAG, but netlink dumps always appends the length which contains any error encountered during the dump. Update rtnl_dump_done to always show the error if there is one.

As an *example* without this patch, dumping a route object that exceeds the internal buffer size terminates with no message to the user -- the dump just ends because the NLMSG_DONE attribute was received. With this patch the user at least gets a message that the dump was aborted.

$ ip ro ls
default via 10.0.2.2 dev eth0
10.0.2.0/24 dev eth0 proto kernel scope link src 10.0.2.15
10.10.0.0/16 dev veth1 proto kernel scope link src 10.10.0.1
172.16.1.0/24 dev br0.11 proto kernel scope link src 172.16.1.1
Error: Buffer too small for object
Dump terminated

The point of this patch is to notify the user of a failure versus silently exiting on a partial dump. Because the NLMSG_DONE attribute was received, the entire dump needs to be restarted to use a larger buffer for EMSGSIZE errors. That could be done automatically but it has other user impacts (e.g., duplicate output if the dump is restarted) and should be the subject of a different patch.

---
 lib/libnetlink.c | 28 +++++++++++++++++-----------
 1 file changed, 17 insertions(+), 11 deletions(-)

index 5b75b2db4e0b..d4b831f67ea2 100644
@@ -266,21 +266,27 @@ static int rtnl_dump_done(const struct rtnl_handle *rth,
 {
 	int len = *(int *)NLMSG_DATA(h);
 
-	if (rth->proto == NETLINK_SOCK_DIAG) {
-		if (h->nlmsg_len < NLMSG_LENGTH(sizeof(int))) {
-			fprintf(stderr, "DONE truncated\n");
-			return -1;
-		}
-
+	if (h->nlmsg_len < NLMSG_LENGTH(sizeof(int))) {
+		fprintf(stderr, "DONE truncated\n");
+		return -1;
+	}
 
-		if (len < 0) {
-			errno = -len;
-			if (errno == ENOENT || errno == EOPNOTSUPP)
-				return -1;
+	if (len < 0) {
+		errno = -len;
+		switch (errno) {
+		case ENOENT:
+		case EOPNOTSUPP:
+			return -1;
+		case EMSGSIZE:
+			fprintf(stderr,
+				"Error: Buffer too small for object.\n");
+			break;
+		default:
 			perror("RTNETLINK answers");
-			return len;
 		}
+		return len;
 	}
+
 	return 0;
 }
 

2.11.0 (Apple Git-81)

Applied.

It is easier to seen iproute2 patches with:

Subject: [PATCH iproute2] netlink: Change rtnl_dump_done to always show error