openrisk: fix altstack switching on sigreturn

do_sigaltstack() expects _userland_ address, TYVM...
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

arch/openrisc/kernel/signal.c

@@ -84,7 +84,6 @@ asmlinkage long _sys_rt_sigreturn(struct pt_regs *regs)
 {
 	struct rt_sigframe *frame = (struct rt_sigframe __user *)regs->sp;
 	sigset_t set;
-	stack_t st;
 
 	/*
 	 * Since we stacked the signal on a dword boundary,
@@ -104,11 +103,10 @@ asmlinkage long _sys_rt_sigreturn(struct pt_regs *regs)
 	if (restore_sigcontext(regs, &frame->uc.uc_mcontext))
 		goto badframe;
 
-	if (__copy_from_user(&st, &frame->uc.uc_stack, sizeof(st)))
-		goto badframe;
 	/* It is more difficult to avoid calling this function than to
 	   call it and ignore errors.  */
-	do_sigaltstack(&st, NULL, regs->sp);
+	if (do_sigaltstack(&frame->uc.uc_stack, NULL, regs->sp) == -EFAULT)
+		goto badframe;
 
 	return regs->gpr[11];