[PATCH net] net: flow_dissector: fix tcp flags dissection on big-endian

[Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>]

    net/netfilter/nf_flow_table_offload.c: In function 'nf_flow_rule_match':
    net/netfilter/nf_flow_table_offload.c:80:21: warning: unsigned conversion from 'int' to '__be16' {aka 'short unsigned int'} changes value from '327680' to '0' [-Woverflow]
       80 |   mask->tcp.flags = TCP_FLAG_RST | TCP_FLAG_FIN;
          |                     ^~~~~~~~~~~~

Fixes: ac4bb5de2701 ("net: flow_dissector: add support for dissection of tcp flags")
Fixes: c29f74e0df7a ("netfilter: nf_flow_table: hardware offload support")
Reported-by: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
Signed-off-by: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
---
@Geert: I have removed the pad field and included the nitpick fix on the
        comment, given I have slightly updated this patch, I would prefer
        if you can provide a fresh Reviewed-by tag. Thanks.

 include/net/flow_dissector.h          | 8 ++++++--
 net/core/flow_dissector.c             | 2 +-
 net/netfilter/nf_flow_table_offload.c | 4 ++--
 3 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/include/net/flow_dissector.h b/include/net/flow_dissector.h
index b8c20e9f343e..9ff8dac9d5ec 100644
--- a/include/net/flow_dissector.h
+++ b/include/net/flow_dissector.h
@@ -189,10 +189,14 @@ struct flow_dissector_key_eth_addrs {
 
 /**
  * struct flow_dissector_key_tcp:
- * @flags: flags
+ * @flags: TCP flags, including the initial Data offset field bits (16-bits)
+ * @flag_word: Data offset + reserved bits + TCP flags + window (32-bits)
  */
 struct flow_dissector_key_tcp {
-	__be16 flags;
+	union {
+		__be16 flags;
+		__be32 flag_word;
+	};
 };
 
 /**
diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index ca871657a4c4..83af4633f306 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -756,7 +756,7 @@ __skb_flow_dissect_tcp(const struct sk_buff *skb,
 	key_tcp = skb_flow_dissector_target(flow_dissector,
 					    FLOW_DISSECTOR_KEY_TCP,
 					    target_container);
-	key_tcp->flags = (*(__be16 *) &tcp_flag_word(th) & htons(0x0FFF));
+	key_tcp->flag_word = tcp_flag_word(th);
 }
 
 static void
diff --git a/net/netfilter/nf_flow_table_offload.c b/net/netfilter/nf_flow_table_offload.c
index c94ebad78c5c..30205d57226d 100644
--- a/net/netfilter/nf_flow_table_offload.c
+++ b/net/netfilter/nf_flow_table_offload.c
@@ -87,8 +87,8 @@ static int nf_flow_rule_match(struct nf_flow_match *match,
 
 	switch (tuple->l4proto) {
 	case IPPROTO_TCP:
-		key->tcp.flags = 0;
-		mask->tcp.flags = TCP_FLAG_RST | TCP_FLAG_FIN;
+		key->tcp.flag_word = 0;
+		mask->tcp.flag_word = TCP_FLAG_RST | TCP_FLAG_FIN;
 		match->dissector.used_keys |= BIT(FLOW_DISSECTOR_KEY_TCP);
 		break;
 	case IPPROTO_UDP:
-- 
2.11.0