ARM: 8643/3: arm/ptrace: Preserve previous registers for short regset write

Ensure that if userspace supplies insufficient data to
PTRACE_SETREGSET to fill all the registers, the thread's old
registers are preserved.

Cc: <stable@vger.kernel.org> # 3.0.x-
Fixes: 5be6f62b ("ARM: 6883/1: ptrace: Migrate to regsets framework")
Signed-off-by: NDave Martin <Dave.Martin@arm.com>
Acked-by: NRussell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/ptrace.c

@@ -600,7 +600,7 @@ static int gpr_set(struct task_struct *target,
 		   const void *kbuf, const void __user *ubuf)
 {
 	int ret;
-	struct pt_regs newregs;
+	struct pt_regs newregs = *task_pt_regs(target);
 
 	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf,
 				 &newregs,