[PATCH v20 net] net: pppoe: avoid zero-length arrays in struct pppoe_hdr

[PATCH v20 net] net: pppoe: avoid zero-length arrays in struct pppoe_hdr

[Eric Woudstra]

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@blackwall.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Eric Woudstra <ericwouds@gmail.com>

---

This version was last send in:

[PATCH v19 nf-next 1/5] net: pppoe: avoid zero-length arrays in
struct pppoe_hdr

Although it is needed to prevent warnings caused by my patches send to
nf-next, it was asked to send this patch separately to netdev.

 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 4275b393a454..7900cc3212a5 100644
--- a/drivers/net/ppp/pppoe.c
+++ b/drivers/net/ppp/pppoe.c
@@ -885,7 +885,7 @@ 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 */
-- 
2.53.0

Re: [PATCH v20 net] net: pppoe: avoid zero-length arrays in struct pppoe_hdr

[Florian Westphal]

Eric Woudstra <ericwouds@gmail.com> wrote:
> Although it is needed to prevent warnings caused by my patches send to
> nf-next, it was asked to send this patch separately to netdev.

Yes, thanks for doing that.  While I occasionally do send non-nf patches
in pull requests, thats usually restricted to very small functional
dependencies, e.g. a new EXPORT_SYMBOL().

Dave/Eric D/Paolo/Jakub - this change can be applied to either net or
net-next; it avoids W=1 spew with patches under discussion on nf-next.

Re: [PATCH v20 net] net: pppoe: avoid zero-length arrays in struct pppoe_hdr

[Jakub Kicinski]

On Tue, 24 Feb 2026 23:09:06 +0100 Florian Westphal wrote:
> Eric Woudstra <ericwouds@gmail.com> wrote:
> > Although it is needed to prevent warnings caused by my patches send to
> > nf-next, it was asked to send this patch separately to netdev.  
> 
> Yes, thanks for doing that.  While I occasionally do send non-nf patches
> in pull requests, thats usually restricted to very small functional
> dependencies, e.g. a new EXPORT_SYMBOL().

Yup

> Dave/Eric D/Paolo/Jakub - this change can be applied to either net or
> net-next; it avoids W=1 spew with patches under discussion on nf-next.

I went with net-next.

Re: [PATCH v20 net] net: pppoe: avoid zero-length arrays in struct pppoe_hdr

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Tue, 24 Feb 2026 16:50:30 +0100 you wrote:
> 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
> 
> [...]

Here is the summary with links:
  - [v20,net] net: pppoe: avoid zero-length arrays in struct pppoe_hdr
    https://git.kernel.org/netdev/net-next/c/7717fbb14028

You are awesome, thank you!
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