[PATCH v2] x86: Fix x32 System V message queue syscalls

[Jessica Clarke <jrtc27@jrtc27.com>]

POSIX specifies that the first field of the supplied msgp, namely mtype,
is a long, not a __kernel_long_t, and it's a user-defined struct due to
the variable-length mtext field so we can't even bend the spec and make
it a __kernel_long_t even if we wanted to. Thus we must use the compat
syscalls on x32 to avoid buffer overreads and overflows in msgsnd and
msgrcv respectively.

Due to erroneously including the first 4 bytes of mtext in the mtype
this would previously also cause non-zero msgtyp arguments for msgrcv to
search for the wrong messages, and if sharing message queues between x32
and non-x32 (i386 or x86_64) processes this would previously cause mtext
to "move" and, depending on the direction and ABI combination, lose the
first 4 bytes.

Signed-off-by: Jessica Clarke <jrtc27@jrtc27.com>
---

I have verified that the test at the end of [1] now gives the correct
result on x32 ("PAYL" not "PAY" as I erroneously claimed it should be in
the above email) and that both i386 and amd64 give the same output with
that test as before.

[1] <1156938F-A9A3-4EE9-B059-2294A0B9FBFE@jrtc27.com>

Changes since v1:
 * Uses the same syscall numbers for x32 as amd64 and the current x32
   rather than (further) breaking ABI by allocating new ones from the
   legacy x32 range

 arch/x86/entry/syscalls/syscall_64.tbl | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index f30d6ae9a..f462123f3 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -77,8 +77,10 @@
 66	common	semctl			sys_semctl
 67	common	shmdt			sys_shmdt
 68	common	msgget			sys_msgget
-69	common	msgsnd			sys_msgsnd
-70	common	msgrcv			sys_msgrcv
+69	64	msgsnd			sys_msgsnd
+69	x32	msgsnd			compat_sys_msgsnd
+70	64	msgrcv			sys_msgrcv
+70	x32	msgrcv			compat_sys_msgrcv
 71	common	msgctl			sys_msgctl
 72	common	fcntl			sys_fcntl
 73	common	flock			sys_flock
-- 
2.28.0