[PATCH v2 21/76] ARC: [Review] Prevent incorrect syscall restarts

[Vineet Gupta]

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@xxxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
---
 arch/arc/include/asm/ptrace.h |    3 +++
 arch/arc/kernel/signal.c      |   12 ++++++++----
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/arch/arc/include/asm/ptrace.h b/arch/arc/include/asm/ptrace.h
index ddb9ce7..92ef198 100644
--- a/arch/arc/include/asm/ptrace.h
+++ b/arch/arc/include/asm/ptrace.h
@@ -130,6 +130,9 @@ struct user_regs_struct {
 #define in_syscall(regs) (regs->orig_r8 & orig_r8_IS_SCALL)
 #define in_brkpt_trap(regs) (regs->orig_r8 & orig_r8_IS_BRKPT)
 
+#define syscall_wont_restart(regs) (regs->orig_r8 |= orig_r8_IS_SCALL_RESTARTED)
+#define syscall_restartable(regs) !(regs->orig_r8 & orig_r8_IS_SCALL_RESTARTED)
+
 #endif /* !__ASSEMBLY__ */
 
 #define orig_r8_IS_SCALL		0x0001
diff --git a/arch/arc/kernel/signal.c b/arch/arc/kernel/signal.c
index 464af3f..6671968 100644
--- a/arch/arc/kernel/signal.c
+++ b/arch/arc/kernel/signal.c
@@ -137,6 +137,9 @@ SYSCALL_DEFINE0(rt_sigreturn)
 		if (do_sigaltstack(&sf->uc.uc_stack, NULL, regs->sp) == -EFAULT)
 			goto badframe;
 
+	/* Don't restart from sigreturn */
+	syscall_wont_restart(regs);
+
 	return regs->r0;
 
 badframe:
@@ -331,13 +334,13 @@ 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;
 	}
@@ -352,6 +355,7 @@ 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 */
-- 
1.7.4.1

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