diff --git a/winsup/cygwin/ChangeLog b/winsup/cygwin/ChangeLog
index 7f33687ec..31b429ba1 100644
--- a/winsup/cygwin/ChangeLog
+++ b/winsup/cygwin/ChangeLog
@@ -1,3 +1,10 @@
+2015-06-27  Corinna Vinschen  <corinna@vinschen.de>
+
+	* exceptions.cc (_cygtls::call_signal_handler): Drop manipulating
+	thread's ss_flags here.  It's not safe against longjmp.
+	* signal.cc (sigaltstack): Check if we're running on the alternate
+	stack and set ss_flags returned in oss to SS_ONSTACK.
+
 2015-06-26  Corinna Vinschen  <corinna@vinschen.de>
 
 	* include/cygwin/signal.h: Revert to define MINSIGSTKSZ and SIGSTKSZ
diff --git a/winsup/cygwin/exceptions.cc b/winsup/cygwin/exceptions.cc
index d493abb00..90a8ff25d 100644
--- a/winsup/cygwin/exceptions.cc
+++ b/winsup/cygwin/exceptions.cc
@@ -1599,8 +1599,6 @@ _cygtls::call_signal_handler ()
 	  uintptr_t new_sp = (uintptr_t) _my_tls.altstack.ss_sp
 			     + _my_tls.altstack.ss_size;
 	  new_sp &= ~0xf;
-	  /* Mark alternate stack as used. */
-	  _my_tls.altstack.ss_flags = SS_ONSTACK;
 	  /* In assembler: Save regs on new stack, move to alternate stack,
 	     call thisfunc, revert stack regs. */
 #ifdef __x86_64__
@@ -1671,8 +1669,6 @@ _cygtls::call_signal_handler ()
 		       [FUNC]	"o" (thisfunc)
 		   : "memory");
 #endif
-	  /* Revert alternate stack to unused. */
-	  _my_tls.altstack.ss_flags = 0;
 	}
       else
 	/* No alternate signal stack requested or available, just call
diff --git a/winsup/cygwin/signal.cc b/winsup/cygwin/signal.cc
index 94faaf308..467c919c3 100644
--- a/winsup/cygwin/signal.cc
+++ b/winsup/cygwin/signal.cc
@@ -667,7 +667,17 @@ sigaltstack (const stack_t *ss, stack_t *oss)
 	    }
 	}
       if (oss)
-	memcpy (oss, &me.altstack, sizeof *oss);
+	{
+	  char stack_marker;
+	  memcpy (oss, &me.altstack, sizeof *oss);
+	  if (!me.altstack.ss_flags && me.altstack.ss_sp)
+	    {
+	      if (&stack_marker >= (char *) me.altstack.ss_sp
+		  && &stack_marker < (char *) me.altstack.ss_sp
+				     + me.altstack.ss_size)
+		oss->ss_flags = SS_ONSTACK;
+	    }
+	}
     }
   __except (EFAULT)
     {