[Pal/Linux] make _DkTerminateSighandler robust

- atomic access to PAL_TCB_LINUX::pending_event
- replace IN_PAL() condition in _DkTerminateSighandler to avoid race
NOTE: there may still remain races.

Signed-off-by: Isaku Yamahata <isaku.yamahata@gmail.com>

Pal/src/host/Linux/db_exception.c

@@ -266,17 +266,15 @@ static void _DkTerminateSighandler (int signum, siginfo_t * info,
     if (event_num == -1)
         return;
 
-    uintptr_t rip = uc->uc_mcontext.gregs[REG_RIP];
+    PAL_TCB_LINUX * tcb = get_tcb_linux();
 
     // If the signal arrives in the middle of a PAL call, add the event
     // to pending in the current TCB.
-    if (ADDR_IN_PAL(rip)) {
-        PAL_TCB_LINUX * tcb = get_tcb_linux();
+    if (atomic_read(&tcb->in_pal)) {
         assert(tcb);
-        if (!tcb->pending_event) {
-            // Use the preserved pending event slot
-            tcb->pending_event = event_num;
-        } else {
+        // try to use the preserved pending event slot
+        int old = 0;
+        if (__atomic_compare_exchange_n(&tcb->pending_event, &old, event_num,false, __ATOMIC_RELAXED, __ATOMIC_RELAXED)) {
             // If there is already a pending event, add the new event to the queue.
             // (a relatively rare case.)
             struct event_queue * ev = malloc(sizeof(*ev));
@@ -309,6 +307,11 @@ static void _DkPipeSighandler (int signum, siginfo_t * info,
     return;
 }
 
+void __enter_pal_call(void) {
+    PAL_TCB_LINUX * tcb = get_tcb_linux();
+    assert(tcb);
+    atomic_inc(&tcb->in_pal);
+}
 /*
  * __check_pending_event(): checks the existence of a pending event in the TCB
  * and handles the event consequently.
@@ -317,9 +320,12 @@ void __check_pending_event (void)
 {
     PAL_TCB_LINUX * tcb = get_tcb_linux();
     assert(tcb);
-    if (tcb->pending_event) {
-        int event = tcb->pending_event;
-        tcb->pending_event = 0;
+
+    if (atomic_add_return(-1, &tcb->in_pal))
+        return;
+
+    int event = __atomic_exchange_n(&tcb->pending_event, 0, __ATOMIC_RELAXED);
+    if (event) {
         _DkGenericSignalHandle(event, NULL, NULL);
 
         if (!LISTP_EMPTY(&tcb->pending_queue)) {

Pal/src/host/Linux/pal_host.h

@@ -179,8 +179,11 @@ typedef struct pal_handle
 
 #define HANDLE_TYPE(handle)  ((handle)->hdr.type)
 
+extern void __enter_pal_call(void);
 extern void __check_pending_event (void);
 
+#define ENTER_PAL_CALL(name) __enter_pal_call()
+
 #define LEAVE_PAL_CALL() do { __check_pending_event(); } while (0)
 
 #define LEAVE_PAL_CALL_RETURN(retval) \

Pal/src/host/Linux/pal_linux.h

@@ -191,7 +191,9 @@ typedef struct pal_tcb_linux {
     PAL_TCB common;
     struct {
         /* private to Linux PAL */
-        int         pending_event;
+        struct atomic_int in_pal;
+        int         pending_event;  /* needs to be accessed by atomic
+                                     * operation */
         LISTP_TYPE(event_queue) pending_queue;
         PAL_HANDLE  handle;
         void *      alt_stack;