netdev - [PATCH v10 nf-next 1/3] net: pppoe: avoid zero-length arrays in struct pppoe

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Message-ID: <20250315195910.17659-2-ericwouds@gmail.com>
Date: Sat, 15 Mar 2025 20:59:08 +0100
From: Eric Woudstra <ericwouds@...il.com>
To: Michal Ostrowski <mostrows@...thlink.net>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Pablo Neira Ayuso <pablo@...filter.org>,
	Jozsef Kadlecsik <kadlec@...filter.org>,
	Simon Horman <horms@...nel.org>
Cc: netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org,
	linux-hardening@...r.kernel.org,
	Eric Woudstra <ericwouds@...il.com>,
	Nikolay Aleksandrov <razor@...ckwall.org>
Subject: [PATCH v10 nf-next 1/3] net: pppoe: avoid zero-length arrays in struct pppoe_hdr

Jakub Kicinski suggested following patch:

W=1 C=1 GCC build gives us:

net/bridge/netfilter/nf_conntrack_bridge.c: note: in included file (through
../include/linux/if_pppox.h, ../include/uapi/linux/netfilter_bridge.h,
../include/linux/netfilter_bridge.h): include/uapi/linux/if_pppox.h:
153:29: warning: array of flexible structures

It doesn't like that hdr has a zero-length array which overlaps proto.
The kernel code doesn't currently need those arrays.

PPPoE connection is functional after applying this patch.

Reviewed-by: Nikolay Aleksandrov <razor@...ckwall.org>

---

Split from patch-set: bridge-fastpath and related improvements v9

Signed-off-by: Eric Woudstra <ericwouds@...il.com>
---
 drivers/net/ppp/pppoe.c       | 2 +-
 include/uapi/linux/if_pppox.h | 4 ++++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ppp/pppoe.c b/drivers/net/ppp/pppoe.c
index 68e631718ab0..17946af6a8cf 100644
--- a/drivers/net/ppp/pppoe.c
+++ b/drivers/net/ppp/pppoe.c
@@ -882,7 +882,7 @@ static int pppoe_sendmsg(struct socket *sock, struct msghdr *m,
 	skb->protocol = cpu_to_be16(ETH_P_PPP_SES);
 
 	ph = skb_put(skb, total_len + sizeof(struct pppoe_hdr));
-	start = (char *)&ph->tag[0];
+	start = (char *)ph + sizeof(*ph);
 
 	error = memcpy_from_msg(start, m, total_len);
 	if (error < 0) {
diff --git a/include/uapi/linux/if_pppox.h b/include/uapi/linux/if_pppox.h
index 9abd80dcc46f..29b804aa7474 100644
--- a/include/uapi/linux/if_pppox.h
+++ b/include/uapi/linux/if_pppox.h
@@ -122,7 +122,9 @@ struct sockaddr_pppol2tpv3in6 {
 struct pppoe_tag {
 	__be16 tag_type;
 	__be16 tag_len;
+#ifndef __KERNEL__
 	char tag_data[];
+#endif
 } __attribute__ ((packed));
 
 /* Tag identifiers */
@@ -150,7 +152,9 @@ struct pppoe_hdr {
 	__u8 code;
 	__be16 sid;
 	__be16 length;
+#ifndef __KERNEL__
 	struct pppoe_tag tag[];
+#endif
 } __packed;
 
 /* Length of entire PPPoE + PPP header */
-- 
2.47.1