Re: [PATCH 1/7] Apply transparent_union attribute to union semun

From: Stafford Horne
Date: Mon Sep 19 2016 - 10:49:41 EST

On Mon, 19 Sep 2016, Jonas Bonn wrote:

NAK... this breaks other architectures.

The OpenRISC toolchain is broken with regard to this issue. Five years ago (last I looked) nobody seemed interesting in fixing it. Has anything changed here?

Hi Jonas,

This was also pointed out by the automated kbuild system and I replied on it. I am currently looking at a possible fix of adding an abi compat layer similar to what arm does with 'arch/arm/kernel/sys_oabi-compat.c'.

By toolchain being broken do you mean the openrisc abi spec is broken as well? Last I checked gcc does compile as per the spec. But it is not a very good spec. There have been discussions to change this as well but no progress.


On 09/16/2016 04:42 PM, Stafford Horne wrote:
..From: Jonas Bonn <jonas@xxxxxxxxxxxx>

The syscall handler for semctl is written under the assumption that the
toolchain will pass "small" unions as function parameters directly instead
of by reference. The union semun is "small" and thus fits this

Since it is assumed that the union will be passed directly and not by
reference, it is safe to access the union members without going via

The OpenRISC architecture, however, passes all unions by reference, thus
breaking the above assumption.

The technically correct fix here is to mark the union as being transparent
so that the ABI of the union's first element determines the parameter
passing method and thus make explicit what's already implied in the

Signed-off-by: Jonas Bonn <jonas@xxxxxxxxxxxx>
Signed-off-by: Stafford Horne <shorne@xxxxxxxxx>
include/uapi/linux/sem.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/uapi/linux/sem.h b/include/uapi/linux/sem.h
index dd73b90..aabe50f 100644
--- a/include/uapi/linux/sem.h
+++ b/include/uapi/linux/sem.h
@@ -48,7 +48,7 @@ union semun {
unsigned short __user *array; /* array for GETALL & SETALL */
struct seminfo __user *__buf; /* buffer for IPC_INFO */
void __user *__pad;
+} __attribute__ ((transparent_union));

struct seminfo {
int semmap;