[PATCH net] uapi: linux: restore IPPROTO_MAX to 256

[PATCH net] uapi: linux: restore IPPROTO_MAX to 256

[Pablo Neira Ayuso]

IPPROTO_MAX used to be 256, but with the introduction of IPPROTO_MPTCP
definition, IPPROTO_MAX was bumped to 263.

IPPROTO_MPTCP definition is used for the socket interface from
userspace. It is never used in the layer 4 protocol field of
IP headers.

IPPROTO_* definitions are used anywhere in the kernel as well as in
userspace to set the layer 4 protocol field in IP headers.

At least in Netfilter, there is code in userspace that relies on
IPPROTO_MAX (not inclusive) to check for the maximum layer 4 protocol.

This patch restores IPPROTO_MAX to 256.

Fixes: faf391c3826c ("tcp: Define IPPROTO_MPTCP")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
Alternatively, I can also define an internal __IPPROTO_MAX to 256 in
userspace.  I understand an update on uapi at this stage might be
complicated. Another possibility is to add a new definition
IPPROTO_FIELD_MAX to uapi and set it to 256 that userspace could start
using.

 include/uapi/linux/in.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/uapi/linux/in.h b/include/uapi/linux/in.h
index 07a4cb149305..0600b03b49ee 100644
--- a/include/uapi/linux/in.h
+++ b/include/uapi/linux/in.h
@@ -80,10 +80,10 @@ enum {
   IPPROTO_ETHERNET = 143,	/* Ethernet-within-IPv6 Encapsulation	*/
 #define IPPROTO_ETHERNET	IPPROTO_ETHERNET
   IPPROTO_RAW = 255,		/* Raw IP packets			*/
+  IPPROTO_MAX = 256,
 #define IPPROTO_RAW		IPPROTO_RAW
   IPPROTO_MPTCP = 262,		/* Multipath TCP connection		*/
 #define IPPROTO_MPTCP		IPPROTO_MPTCP
-  IPPROTO_MAX
 };
 #endif
 
-- 
2.30.2

Re: [PATCH net] uapi: linux: restore IPPROTO_MAX to 256

[Pablo Neira Ayuso]

On Wed, Jan 11, 2023 at 10:47:19PM +0100, Pablo Neira Ayuso wrote:
> IPPROTO_MAX used to be 256, but with the introduction of IPPROTO_MPTCP
> definition, IPPROTO_MAX was bumped to 263.
> 
> IPPROTO_MPTCP definition is used for the socket interface from
> userspace. It is never used in the layer 4 protocol field of
> IP headers.
> 
> IPPROTO_* definitions are used anywhere in the kernel as well as in
> userspace to set the layer 4 protocol field in IP headers.
> 
> At least in Netfilter, there is code in userspace that relies on
> IPPROTO_MAX (not inclusive) to check for the maximum layer 4 protocol.
> 
> This patch restores IPPROTO_MAX to 256.
> 
> Fixes: faf391c3826c ("tcp: Define IPPROTO_MPTCP")
> Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
> ---
> Alternatively, I can also define an internal __IPPROTO_MAX to 256 in
> userspace.  I understand an update on uapi at this stage might be
> complicated. Another possibility is to add a new definition
> IPPROTO_FIELD_MAX to uapi and set it to 256 that userspace could start
> using.

Scratch this.

This breaks inet_create() and inet6_create() which is going to break
MP-TCP with socket().

I'll post a v2 adding a new IPPROTO_FIELD_MAX definition 256.