Eliminate g_alt_stack variable and void* runtime context

(just use OS to keep track of the altstack)

wasm2c/README.md

@@ -295,8 +295,8 @@ void wasm_rt_allocate_externref_table(wasm_rt_externref_table_t*, uint32_t eleme
 void wasm_rt_free_funcref_table(wasm_rt_table_t*);
 void wasm_rt_free_externref_table(wasm_rt_table_t*);
 uint32_t wasm_rt_call_stack_depth; /* on platforms that don't use the signal handler to detect exhaustion */
-void* wasm_rt_init_thread(void);
-void wasm_rt_free_thread(void*);
+void wasm_rt_init_thread(void);
+void wasm_rt_free_thread(void);
 ```
 
 `wasm_rt_init` must be called by the embedder before anything else, to

wasm2c/examples/threads/threads.c

@@ -65,7 +65,7 @@ void* do_thread(void* arg) {
   /* Initialize the per-thread context for the Wasm runtime (in practice, an
      altstack for catching segfaults caused by stack exhaustion or out-of-bounds
      memory access. */
-  void* context = wasm_rt_init_thread();
+  wasm_rt_init_thread();
 
   /* Instantiate the Wasm module. */
   w2c_sample inst;
@@ -85,7 +85,7 @@ void* do_thread(void* arg) {
       wasm2c_sample_free(&inst);
 
       /* Free the per-thread runtime context. */
-      wasm_rt_free_thread(context);
+      wasm_rt_free_thread();
 
       return arg;
     } else {

wasm2c/wasm-rt-impl.c

@@ -48,8 +48,6 @@ static void* g_sig_handler_handle = 0;
 #if WASM_RT_USE_STACK_DEPTH_COUNT
 WASM_RT_THREAD_LOCAL uint32_t wasm_rt_call_stack_depth;
 WASM_RT_THREAD_LOCAL uint32_t wasm_rt_saved_call_stack_depth;
-#elif WASM_RT_INSTALL_SIGNAL_HANDLER
-static char* g_alt_stack = 0;
 #endif
 
 WASM_RT_THREAD_LOCAL wasm_rt_jmp_buf g_wasm_rt_jmp_buf;
@@ -184,7 +182,7 @@ static bool os_has_altstack_installed() {
 }
 
 /* Use alt stack to handle SIGSEGV from stack overflow */
-static void* os_allocate_and_install_altstack(void) {
+static void os_allocate_and_install_altstack(void) {
   /* allocate altstack */
   void* alt_stack = malloc(SIGSTKSZ);
   if (alt_stack == NULL) {
@@ -202,7 +200,7 @@ static void* os_allocate_and_install_altstack(void) {
     abort();
   }
 
-  return alt_stack;
+  /* altstack pointer is stored by OS */
 }
 
 static void os_install_signal_handler(void) {
@@ -223,27 +221,27 @@ static void os_install_signal_handler(void) {
   }
 }
 
-static void os_disable_and_deallocate_altstack(void* alt_stack) {
+static void os_disable_and_deallocate_altstack(void) {
   /* verify altstack was still in place */
   stack_t ss;
   if (sigaltstack(NULL, &ss) != 0) {
     perror("sigaltstack failed");
     abort();
   }
-  if (!(ss.ss_flags & SS_DISABLE) && (ss.ss_sp == alt_stack) &&
-      (ss.ss_size == SIGSTKSZ)) {
-    /* disable */
+  if (!(ss.ss_flags & SS_DISABLE) && (ss.ss_size == SIGSTKSZ)) {
+    /* disable and free */
+    void* alt_stack = ss.ss_sp;
     ss.ss_flags = SS_DISABLE;
     if (sigaltstack(&ss, NULL) != 0) {
       perror("sigaltstack failed");
       abort();
     }
     assert(!os_has_altstack_installed());
-  } else {
-    fprintf(stderr, "Warning: wasm2c altstack was modified unexpectedly\n");
+    free(alt_stack);
+    return;
   }
 
-  free(alt_stack);
+  fprintf(stderr, "Warning: wasm2c altstack was modified unexpectedly\n");
 }
 
 static void os_cleanup_signal_handler(void) {
@@ -261,11 +259,8 @@ static void os_cleanup_signal_handler(void) {
 #endif
 
 void wasm_rt_init(void) {
+  wasm_rt_init_thread();
 #if WASM_RT_INSTALL_SIGNAL_HANDLER
-#if !WASM_RT_USE_STACK_DEPTH_COUNT
-  assert(!g_alt_stack);
-  g_alt_stack = os_allocate_and_install_altstack();
-#endif
   assert(!g_signal_handler_installed);
   os_install_signal_handler();
   g_signal_handler_installed = true;
@@ -294,27 +289,23 @@ void wasm_rt_free(void) {
 #if WASM_RT_INSTALL_SIGNAL_HANDLER
   os_cleanup_signal_handler();
   g_signal_handler_installed = false;
-#if !WASM_RT_USE_STACK_DEPTH_COUNT
-  assert(g_alt_stack);
-  os_disable_and_deallocate_altstack(g_alt_stack);
-  g_alt_stack = NULL;
-#endif
 #endif
+  wasm_rt_free_thread();
 }
 
-void* wasm_rt_init_thread(void) {
+void wasm_rt_init_thread(void) {
 #if WASM_RT_USE_STACK_DEPTH_COUNT || (!WASM_RT_INSTALL_SIGNAL_HANDLER)
-  return NULL;
+  return;
 #else
-  return os_allocate_and_install_altstack();
+  os_allocate_and_install_altstack();
 #endif
 }
 
-void wasm_rt_free_thread(void* alt_stack) {
+void wasm_rt_free_thread(void) {
 #if WASM_RT_USE_STACK_DEPTH_COUNT || (!WASM_RT_INSTALL_SIGNAL_HANDLER)
   return;
 #else
-  os_disable_and_deallocate_altstack(alt_stack);
+  os_disable_and_deallocate_altstack();
 #endif
 }
 

wasm2c/wasm-rt.h

@@ -335,15 +335,14 @@ void wasm_rt_free(void);
  * Initialize the multithreaded runtime for a given thread. Must be
  * called by each thread (other than the one that called wasm_rt_init)
  * before initializing a Wasm module or calling an exported
- * function. Returns an opaque state pointer.
+ * function.
  */
-void* wasm_rt_init_thread(void);
+void wasm_rt_init_thread(void);
 
 /*
- * Free the individual thread's state, given the state returned by
- * wasm_rt_init_thread().
+ * Free the individual thread's state.
  */
-void wasm_rt_free_thread(void*);
+void wasm_rt_free_thread(void);
 
 /**
  * A hardened jmp_buf that allows checking for initialization before use