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

From: menglong8 . dong
Date: Sun Mar 21 2021 - 08:43:54 EST

From: Menglong Dong <dong.menglong@xxxxxxxxxx>

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


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@xxxxxxxxxxxx>
Signed-off-by: Menglong Dong <dong.menglong@xxxxxxxxxx>
include/linux/socket.h | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/include/linux/socket.h b/include/linux/socket.h
index d5ebfe30d96b..317b2933f499 100644
--- a/include/linux/socket.h
+++ b/include/linux/socket.h
@@ -312,17 +312,18 @@ struct ucred {
* plain text and require encryption

+#if defined(CONFIG_COMPAT)
+#define MSG_CMSG_COMPAT BIT(21) /* This message needs 32 bit fixups */
+#define MSG_CMSG_COMPAT 0 /* We never have 32 bit fixups */
#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
-#if defined(CONFIG_COMPAT)
-#define MSG_CMSG_COMPAT BIT(31) /* This message needs 32 bit fixups */
-#define MSG_CMSG_COMPAT 0 /* We never have 32 bit fixups */

/* Setsockoptions(2) level. Thanks to BSD these must match IPPROTO_xxx */