[PATCH net-next] netlink: remove the flex array from struct nlmsghdr

[Jakub Kicinski <kuba@kernel.org>]

I've added a flex array to struct nlmsghdr in
commit 738136a0e375 ("netlink: split up copies in the ack construction")
to allow accessing the data easily. It leads to warnings with clang,
if user space wraps this structure into another struct and the flex
array is not at the end of the container.

Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/20221114023927.GA685@u2004-local/
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
CC: nathan@kernel.org
CC: ndesaulniers@google.com
CC: trix@redhat.com
CC: llvm@lists.linux.dev
---
 include/uapi/linux/netlink.h | 2 --
 net/netlink/af_netlink.c     | 2 +-
 2 files changed, 1 insertion(+), 3 deletions(-)

diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
index 5da0da59bf01..e2ae82e3f9f7 100644
--- a/include/uapi/linux/netlink.h
+++ b/include/uapi/linux/netlink.h
@@ -48,7 +48,6 @@ struct sockaddr_nl {
  * @nlmsg_flags: Additional flags
  * @nlmsg_seq:   Sequence number
  * @nlmsg_pid:   Sending process port ID
- * @nlmsg_data:  Message payload
  */
 struct nlmsghdr {
 	__u32		nlmsg_len;
@@ -56,7 +55,6 @@ struct nlmsghdr {
 	__u16		nlmsg_flags;
 	__u32		nlmsg_seq;
 	__u32		nlmsg_pid;
-	__u8		nlmsg_data[];
 };
 
 /* Flags values */
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 9ebdf3262015..d73091f6bb0f 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2514,7 +2514,7 @@ void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,
 		if (!nlmsg_append(skb, nlmsg_len(nlh)))
 			goto err_bad_put;
 
-		memcpy(errmsg->msg.nlmsg_data, nlh->nlmsg_data,
+		memcpy(nlmsg_data(&errmsg->msg), nlmsg_data(nlh),
 		       nlmsg_len(nlh));
 	}
 
-- 
2.38.1