uml: fix signal frame alignment

Use the same signal frame alignment calculations as the underlying
architecture.  x86_64 appeared to do this, but the "- 8" was really
subtracting 8 * sizeof(struct rt_sigframe) rather than 8 bytes.

UML/i386 might have been OK, but I changed the calculation to match
i386 just to be sure.
Signed-off-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

arch/um/sys-i386/signal.c

@@ -209,7 +209,7 @@ int setup_signal_stack_sc(unsigned long stack_top, int sig,
 	void *restorer;
 	int err = 0;
 
-	stack_top &= -8UL;
+	stack_top = ((stack_top + 4) & -16UL) - 4;
 	frame = (struct sigframe *) stack_top - 1;
 	if (!access_ok(VERIFY_WRITE, frame, sizeof(*frame)))
 		return 1;

arch/um/sys-x86_64/signal.c

@@ -174,8 +174,9 @@ int setup_signal_stack_si(unsigned long stack_top, int sig,
 	struct task_struct *me = current;
 
 	frame = (struct rt_sigframe __user *)
-		round_down(stack_top - sizeof(struct rt_sigframe), 16) - 8;
-        frame = (struct rt_sigframe *) ((unsigned long) frame - 128);
+		round_down(stack_top - sizeof(struct rt_sigframe), 16);
+	/* Subtract 128 for a red zone and 8 for proper alignment */
+        frame = (struct rt_sigframe *) ((unsigned long) frame - 128 - 8);
 
 	if (!access_ok(VERIFY_WRITE, fp, sizeof(struct _fpstate)))
 		goto out;