[PATCH iproute2 5/5] libnetlink: do not read past datagram when recording dump

[Stephen Hemminger <stephen@networkplumber.org>]

With a dump file (ss -D), rtnl_dump_filter_l() wrote NLMSG_ALIGN(status)
bytes, reading up to 3 bytes past the received data, and past the
buffer when the datagram is larger than 32K.

Write the datagram, then explicit zero padding, since rtnl_from_file()
expects padded messages.

Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
---
 lib/libnetlink.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/lib/libnetlink.c b/lib/libnetlink.c
index de0681ac..f67f2b48 100644
--- a/lib/libnetlink.c
+++ b/lib/libnetlink.c
@@ -928,8 +928,13 @@ static int rtnl_dump_filter_l(struct rtnl_handle *rth,
 		if (status < 0)
 			return status;
 
-		if (rth->dump_fp)
-			fwrite(buf, 1, NLMSG_ALIGN(status), rth->dump_fp);
+		if (rth->dump_fp) {
+			static const char pad[NLMSG_ALIGNTO];
+
+			/* rtnl_from_file() expects padded messages */
+			fwrite(buf, 1, status, rth->dump_fp);
+			fwrite(pad, 1, NLMSG_ALIGN(status) - status, rth->dump_fp);
+		}
 
 		for (a = arg; a->filter; a++) {
 			struct nlmsghdr *h = (struct nlmsghdr *)buf;
-- 
2.53.0