ipv4/inet_sock.h: Avoid thousands of -Wflex-array-member-not-at-end warnings

Use DEFINE_RAW_FLEX() to avoid thousands of -Wflex-array-member-not-at-end
warnings.

Remove struct ip_options_data, and adjust the rest of the code so that
flexible-array member struct ip_options_rcu::opt.__data[] ends last
in struct icmp_bxm.

Compensate for this by using the DEFINE_RAW_FLEX() helper to define each
on-stack struct instance that contained struct ip_options_data as a member,
and to define struct ip_options_rcu with a fixed on-stack size for its
nested flexible-array member opt.__data[].

Also, add a couple of code comments to prevent people from adding members
to a struct after another member that contains a flexible array.

With these changes, fix 2600 warnings of the following type:

include/net/inet_sock.h:65:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://patch.msgid.link/aVteBadWA6AbTp7X@kspp
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 7 insertions, 6 deletions

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index ff11d3a85a36..75fcb58795bb 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -1606,7 +1606,8 @@ void ip_send_unicast_reply(struct sock *sk, const struct sock *orig_sk,
 			   const struct ip_reply_arg *arg,
 			   unsigned int len, u64 transmit_time, u32 txhash)
 {
-	struct ip_options_data replyopts;
+	DEFINE_RAW_FLEX(struct ip_options_rcu, replyopts, opt.__data,
+			IP_OPTIONS_DATA_FIXED_SIZE);
 	struct ipcm_cookie ipc;
 	struct flowi4 fl4;
 	struct rtable *rt = skb_rtable(skb);
@@ -1615,18 +1616,18 @@ void ip_send_unicast_reply(struct sock *sk, const struct sock *orig_sk,
 	int err;
 	int oif;
 
-	if (__ip_options_echo(net, &replyopts.opt.opt, skb, sopt))
+	if (__ip_options_echo(net, &replyopts->opt, skb, sopt))
 		return;
 
 	ipcm_init(&ipc);
 	ipc.addr = daddr;
 	ipc.sockc.transmit_time = transmit_time;
 
-	if (replyopts.opt.opt.optlen) {
-		ipc.opt = &replyopts.opt;
+	if (replyopts->opt.optlen) {
+		ipc.opt = replyopts;
 
-		if (replyopts.opt.opt.srr)
-			daddr = replyopts.opt.opt.faddr;
+		if (replyopts->opt.srr)
+			daddr = replyopts->opt.faddr;
 	}
 
 	oif = arg->bound_dev_if;