s390: fix save and restore of the floating-point-control register

The FPC_VALID_MASK has been used to check the validity of the value
to be loaded into the floating-point-control register. With the
introduction of the floating-point extension facility and the
decimal-floating-point additional bits have been defined which need
to be checked in a non straight forward way. So far these bits have
been ignored which can cause an incorrect results for decimal-
floating-point operations, e.g. an incorrect rounding mode to be
set after signal return.

The static check with the FPC_VALID_MASK is replaced with a trial
load of the floating-point-control value, see test_fp_ctl.

In addition an information leak with the padding word between the
floating-point-control word and the floating-point registers in
the s390_fp_regs is fixed.

Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/uapi/asm/ptrace.h

@@ -199,6 +199,7 @@ typedef union
 typedef struct
 {
 	__u32   fpc;
+	__u32	pad;
 	freg_t  fprs[NUM_FPRS];              
 } s390_fp_regs;
 
@@ -206,7 +207,6 @@ typedef struct
 #define FPC_FLAGS_MASK          0x00F80000
 #define FPC_DXC_MASK            0x0000FF00
 #define FPC_RM_MASK             0x00000003
-#define FPC_VALID_MASK          0xF8F8FF03
 
 /* this typedef defines how a Program Status Word looks like */
 typedef struct 

arch/s390/include/uapi/asm/sigcontext.h

@@ -49,6 +49,7 @@ typedef struct
 typedef struct
 {
 	unsigned int fpc;
+	unsigned int pad;
 	double   fprs[__NUM_FPRS];
 } _s390_fp_regs;