[PATCH v2 net-next 2/2] net: socket: change MSG_CMSG_COMPAT to BIT(21)

From: Menglong Dong <redacted>

Currently, MSG_CMSG_COMPAT is defined as '1 << 31'. However, 'msg_flags'
is defined with type of 'int' somewhere, such as 'packet_recvmsg' and
other recvmsg functions:

static int packet_recvmsg(struct socket *sock, struct msghdr *msg,
			  size_t len,
			  int flags)

If MSG_CMSG_COMPAT is set in 'flags', it's value will be negative.
Once it perform bit operations with MSG_*, the upper 32 bits of
the result will be set, just like what Guenter Roeck explained
here:

https://lore.kernel.org/netdev/20210317013758.GA134033@roeck-us.net (local)

As David Laight suggested, fix this by change MSG_CMSG_COMPAT to
some value else. MSG_CMSG_COMPAT is an internal value, which is't
used in userspace, so this change works.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Menglong Dong <redacted>
---
v2:
- add comment to stop people from trying to use BIT(31)
---
 include/linux/socket.h | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/include/linux/socket.h b/include/linux/socket.h
index d5ebfe30d96b..61b2694d68dd 100644
--- a/include/linux/socket.h
+++ b/include/linux/socket.h

@@ -312,17 +312,21 @@ struct ucred {
 					 * plain text and require encryption
 					 */
 
+#if defined(CONFIG_COMPAT)
+#define MSG_CMSG_COMPAT		BIT(21)	/* This message needs 32 bit fixups */
+#else
+#define MSG_CMSG_COMPAT		0	/* We never have 32 bit fixups */
+#endif
+
 #define MSG_ZEROCOPY		BIT(26)	/* Use user data in kernel path */
 #define MSG_FASTOPEN		BIT(29)	/* Send data in TCP SYN */
 #define MSG_CMSG_CLOEXEC	BIT(30)	/* Set close_on_exec for file
 					 * descriptor received through
 					 * SCM_RIGHTS
 					 */
-#if defined(CONFIG_COMPAT)
-#define MSG_CMSG_COMPAT		BIT(31)	/* This message needs 32 bit fixups */
-#else
-#define MSG_CMSG_COMPAT		0	/* We never have 32 bit fixups */
-#endif
+/* Attention: Don't use BIT(31) for MSG_*, as 'msg_flags' is defined
+ * as 'int' somewhere and BIT(31) will make it become negative.
+ */
 
 
 /* Setsockoptions(2) level. Thanks to BSD these must match IPPROTO_xxx */

-- 
2.31.0