libmnl: zero attribute padding

Sergei Trofimovich reports 'uninitialized bytes' warnings from nftables:

Syscall param sendmsg(msg.msg_iov[0]) points to uninitialised byte(s)
   at 0x55B9EFB: sendmsg (in /lib64/libc-2.25.so)
   by 0x43E658: mnl_nft_socket_sendmsg (mnl.c:239)
   by 0x43E658: mnl_batch_talk (mnl.c:254)
   by 0x407898: nft_netlink (libnftables.c:58)
   by 0x407898: nft_run (libnftables.c:96)
   by 0x407CD5: nft_run_cmd_from_buffer (libnftables.c:291)
   by 0x406EDE: main (main.c:274)

This is harmless, the uninitialized memory is the padding
that sometimes needs to be inserted between end of an attribute
and the beginning of the new attribute.

Zero it to silence memory sanitizer output.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/src/attr.c b/src/attr.c
index 4f13187..0359ba9 100644
--- a/src/attr.c
+++ b/src/attr.c
@@ -412,10 +412,15 @@ void mnl_attr_put(struct nlmsghdr *nlh, uint16_t type, size_t len,
 {
 	struct nlattr *attr = mnl_nlmsg_get_payload_tail(nlh);
 	uint16_t payload_len = MNL_ALIGN(sizeof(struct nlattr)) + len;
+	int pad;
 
 	attr->nla_type = type;
 	attr->nla_len = payload_len;
 	memcpy(mnl_attr_get_payload(attr), data, len);
+	pad = MNL_ALIGN(len) - len;
+	if (pad > 0)
+		memset(mnl_attr_get_payload(attr) + len, 0, pad);
+
 	nlh->nlmsg_len += MNL_ALIGN(payload_len);
 }