ARC: [Review] Prevent incorrect syscall restarts

Per Al Viro's "signals for dummies" https://lkml.org/lkml/2012/12/6/366
there are 3 golden rules for (not) restarting syscalls:

"	What we need to guarantee is
* restarts do not happen on signals caught in interrupts or exceptions
* restarts do not happen on signals caught in sigreturn()
* restart should happen only once, even if we get through do_signal()
  many times."

ARC Port already handled #1, this patch fixes raspberrypi#2 and raspberrypi#3.

We use the additional state in pt_regs->orig_r8 to ckh if restarting
has already been done once.

Thanks to Al Viro for spotting this.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>

arch/arc/include/asm/ptrace.h

@@ -100,6 +100,9 @@ struct callee_regs {
 #define in_syscall(regs)    (regs->event & orig_r8_IS_SCALL)
 #define in_brkpt_trap(regs) (regs->event & orig_r8_IS_BRKPT)
 
+#define syscall_wont_restart(regs) (regs->event |= orig_r8_IS_SCALL_RESTARTED)
+#define syscall_restartable(regs) !(regs->event &  orig_r8_IS_SCALL_RESTARTED)
+
 #define current_pt_regs()					\
 ({								\
 	/* open-coded current_thread_info() */			\

arch/arc/kernel/signal.c

@@ -128,6 +128,9 @@ SYSCALL_DEFINE0(rt_sigreturn)
 		if (restore_altstack(&sf->uc.uc_stack))
 			goto badframe;
 
+	/* Don't restart from sigreturn */
+	syscall_wont_restart(regs);
+
 	return regs->r0;
 
 badframe:
@@ -318,13 +321,13 @@ void do_signal(struct pt_regs *regs)
 
 	signr = get_signal_to_deliver(&info, &ka, regs, NULL);
 
-	/* Are we from a system call? */
-	restart_scall = in_syscall(regs);
+	restart_scall = in_syscall(regs) && syscall_restartable(regs);
 
 	if (signr > 0) {
-		if (restart_scall)
+		if (restart_scall) {
 			arc_restart_syscall(&ka, regs);
-
+			syscall_wont_restart(regs);	/* No more restarts */
+		}
 		handle_signal(signr, &ka, &info, regs);
 		return;
 	}
@@ -339,6 +342,7 @@ void do_signal(struct pt_regs *regs)
 			regs->r8 = __NR_restart_syscall;
 			regs->ret -= 4;
 		}
+		syscall_wont_restart(regs);	/* No more restarts */
 	}
 
 	/* If there's no signal to deliver, restore the saved sigmask back */