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 #2 and #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>

2 files changed, 11 insertions, 4 deletions

diff --git a/arch/arc/include/asm/ptrace.h b/arch/arc/include/asm/ptrace.h
index 3ec89f467ac3..a98957a95fb3 100644
--- a/arch/arc/include/asm/ptrace.h
+++ b/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() */                  \
diff --git a/arch/arc/kernel/signal.c b/arch/arc/kernel/signal.c
index ab8ed5a55461..ee6ef2f60a28 100644
--- a/arch/arc/kernel/signal.c
+++ b/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 */