Thread-safe library initialization

LICENSE.txt

@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2017-2024 Yury Gribov
+Copyright (c) 2017-2025 Yury Gribov
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal

README.md

@@ -52,10 +52,10 @@ where `TARGET` can be any of
 Script generates two files: `libxyz.so.tramp.S` and `libxyz.so.init.c` which need to be linked to your application (instead of `-lxyz`):
 
 ```
-$ gcc myfile1.c myfile2.c ... libxyz.so.tramp.S libxyz.so.init.c ... -ldl
+$ gcc myfile1.c myfile2.c ... libxyz.so.tramp.S libxyz.so.init.c ... -ldl -pthread
 ```
 
-Note that you need to link against libdl.so. On ARM in case your app is compiled to Thumb code (which e.g. Ubuntu's `arm-linux-gnueabihf-gcc` does by default) you'll also need to add `-mthumb-interwork`.
+Note that you need to link against libdl.so and libpthread.so (unless you disable thread safety via `--no-thread-safe`). On ARM in case your app is compiled to Thumb code (which e.g. Ubuntu's `arm-linux-gnueabihf-gcc` does by default) you'll also need to add `-mthumb-interwork`.
 
 Application can then freely call functions from `libxyz.so` _without linking to it_. Library will be loaded (via `dlopen`) on first call to any of its functions. If you want to forcedly resolve all symbols (e.g. if you want to avoid delays further on) you can call `void libxyz_init_all()`.
 

arch/common/init.c.tpl

@@ -1,5 +1,5 @@
 /*
- * Copyright 2018-2022 Yury Gribov
+ * Copyright 2018-2025 Yury Gribov
  *
  * The MIT License (MIT)
  *
@@ -11,12 +11,22 @@
 #define _GNU_SOURCE // For RTLD_DEFAULT
 #endif
 
+#define HAS_DLOPEN_CALLBACK $has_dlopen_callback
+#define HAS_DLSYM_CALLBACK $has_dlsym_callback
+#define NO_DLOPEN $no_dlopen
+#define LAZY_LOAD $lazy_load
+#define THREAD_SAFE $thread_safe
+
 #include <dlfcn.h>
 #include <stdlib.h>
 #include <string.h>
 #include <stdio.h>
 #include <assert.h>
 
+#if THREAD_SAFE
+#include <pthread.h>
+#endif
+
 // Sanity check for ARM to avoid puzzling runtime crashes
 #ifdef __arm__
 # if defined __thumb__ && ! defined __THUMB_INTERWORK__
@@ -36,21 +46,60 @@ extern "C" {
     } \
   } while(0)
 
-#define HAS_DLOPEN_CALLBACK $has_dlopen_callback
-#define HAS_DLSYM_CALLBACK $has_dlsym_callback
-#define NO_DLOPEN $no_dlopen
-#define LAZY_LOAD $lazy_load
-
-static void *lib_handle;
+static volatile void *lib_handle;
 static int do_dlclose;
-static int is_lib_loading;
 
 #if ! NO_DLOPEN
-static void *load_library() {
-  if(lib_handle)
-    return lib_handle;
 
-  is_lib_loading = 1;
+#if THREAD_SAFE
+
+// We need to consider two cases:
+// - different threads calling intercepted APIs in parallel
+// - same thread calling 2 intercepted APIs recursively
+//   due to dlopen calling library constructors
+//   (only if IMPLIB_EXPORT_SHIMS is used)
+
+static pthread_mutex_t mtx;
+
+static void init_lock() {
+  // We need recursive lock because dlopen will call library constructors
+  // which may call other intercepted APIs that will call load_library again.
+  // PTHREAD_RECURSIVE_MUTEX_INITIALIZER is not portable
+  // so we do it hard way.
+
+  pthread_mutexattr_t attr;
+  CHECK(0 == pthread_mutexattr_init(&attr), "failed to init mutex");
+  CHECK(0 == pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE), "failed to init mutex");
+
+  CHECK(0 == pthread_mutex_init(&mtx, &attr), "failed to init mutex");
+}
+
+static void lock() {
+  static pthread_once_t once = PTHREAD_ONCE_INIT;
+  CHECK(0 == pthread_once(&once, init_lock), "failed to init lock");
+
+  CHECK(0 == pthread_mutex_lock(&mtx), "failed to lock mutex");
+}
+
+static void unlock() {
+  CHECK(0 == pthread_mutex_unlock(&mtx), "failed to unlock mutex");
+}
+#else
+static void lock() {}
+static void unlock() {}
+#endif
+
+static void load_library() {
+  lock();
+
+  if (lib_handle) {
+    unlock();
+    return;
+  }
+
+  // With (non-default) IMPLIB_EXPORT_SHIMS we may call dlopen more than once,
+  // not sure if this is a problem. We could fix this with first checking
+  // with RTLD_NOLOAD?
 
 #if HAS_DLOPEN_CALLBACK
   extern void *$dlopen_callback(const char *lib_name);
@@ -62,14 +111,13 @@ static void *load_library() {
 #endif
 
   do_dlclose = 1;
-  is_lib_loading = 0;
 
-  return lib_handle;
+  unlock();
 }
 
 static void __attribute__((destructor)) unload_lib() {
   if(do_dlclose && lib_handle)
-    dlclose(lib_handle);
+    dlclose((void *)lib_handle);
 }
 #endif
 
@@ -93,14 +141,12 @@ extern void *_${lib_suffix}_tramp_table[];
 void _${lib_suffix}_tramp_resolve(int i) {
   assert((unsigned)i < SYM_COUNT);
 
-  CHECK(!is_lib_loading, "library function '%s' called during library load", sym_names[i]);
-
   void *h = 0;
 #if NO_DLOPEN
   // Library with implementations must have already been loaded.
   if (lib_handle) {
     // User has specified loaded library
-    h = lib_handle;
+    h = (void *)lib_handle;
   } else {
     // User hasn't provided us the loaded library so search the global namespace.
 #   ifndef IMPLIB_EXPORT_SHIMS
@@ -113,7 +159,8 @@ void _${lib_suffix}_tramp_resolve(int i) {
 #   endif
   }
 #else
-  h = load_library();
+  load_library();
+  h = (void *)lib_handle;
   CHECK(h, "failed to resolve symbol '%s', library failed to load", sym_names[i]);
 #endif
 

implib-gen.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python3
 
-# Copyright 2017-2024 Yury Gribov
+# Copyright 2017-2025 Yury Gribov
 #
 # The MIT License (MIT)
 #
@@ -382,6 +382,12 @@ def main():
   parser.add_argument('--no-lazy-load',
                       help="Load library at program start",
                       dest='lazy_load', action='store_false')
+  parser.add_argument('--thread-safe',
+                      help="Ensure thread-safety (default)",
+                      dest='thread_safe', action='store_true', default=True)
+  parser.add_argument('--no-thread-safe',
+                      help="Do not ensure thread-safety",
+                      dest='thread_safe', action='store_false')
   parser.add_argument('--vtables',
                       help="Intercept virtual tables (EXPERIMENTAL)",
                       dest='vtables', action='store_true', default=False)
@@ -419,6 +425,7 @@ def main():
   dlsym_callback = args.dlsym_callback
   dlopen = args.dlopen
   lazy_load = args.lazy_load
+  thread_safe = args.thread_safe
   if args.target.startswith('arm'):
     target = 'arm'  # Handle armhf-..., armel-...
   elif re.match(r'^i[0-9]86', args.target):
@@ -617,6 +624,7 @@ def is_data_symbol(s):
         has_dlsym_callback=int(bool(dlsym_callback)),
         no_dlopen=int(not dlopen),
         lazy_load=int(lazy_load),
+        thread_safe=int(thread_safe),
         sym_names=sym_names)
       f.write(init_text)
     if args.vtables:

scripts/ld

@@ -249,6 +249,7 @@ Flags can be specified directly or via IMPLIBSO_LD_OPTIONS environment variable.
       info.replaced = True
   if changed:
     args.append('-ldl')
+    args.append('-lpthread')
 
   for name, info in sorted(libs.items()):
     if not info.replaced:

scripts/travis.sh

@@ -45,3 +45,4 @@ if ! echo "$ARCH" | grep -q 'powerpc\|mips\|riscv'; then
   # TODO: support vector types for remaining platforms
   tests/vector-args/run.sh $ARCH
 fi
+tests/thread/run.sh $ARCH

tests/common.sh

@@ -87,7 +87,7 @@ else
 fi
 
 if uname | grep -q BSD; then
-  LIBS=
+  LIBS='-pthread'
 else
-  LIBS=-ldl
+  LIBS='-ldl -pthread'
 fi

tests/thread/interposed.c

@@ -0,0 +1,61 @@
+/*
+ * Copyright 2025 Yury Gribov
+ *
+ * The MIT License (MIT)
+ * 
+ * Use of this source code is governed by MIT license that can be
+ * found in the LICENSE.txt file.
+ */
+
+#include <stdio.h>
+#include "interposed.h"
+
+__attribute__((visibility("default")))
+int foo0(int x) {
+  return x + 0;
+}
+
+__attribute__((visibility("default")))
+int foo1(int x) {
+  return x + 1;
+}
+
+__attribute__((visibility("default")))
+int foo2(int x) {
+  return x + 2;
+}
+
+__attribute__((visibility("default")))
+int foo3(int x) {
+  return x + 3;
+}
+
+__attribute__((visibility("default")))
+int foo4(int x) {
+  return x + 4;
+}
+
+__attribute__((visibility("default")))
+int foo5(int x) {
+  return x + 5;
+}
+
+__attribute__((visibility("default")))
+int foo6(int x) {
+  return x + 6;
+}
+
+__attribute__((visibility("default")))
+int foo7(int x) {
+  return x + 7;
+}
+
+__attribute__((visibility("default")))
+int foo8(int x) {
+  return x + 8;
+}
+
+__attribute__((visibility("default")))
+int foo9(int x) {
+  return x + 9;
+}

tests/thread/interposed.h

@@ -0,0 +1,24 @@
+/*
+ * Copyright 2025 Yury Gribov
+ *
+ * The MIT License (MIT)
+ * 
+ * Use of this source code is governed by MIT license that can be
+ * found in the LICENSE.txt file.
+ */
+
+#ifndef INTERPOSED_H
+#define INTERPOSED_H
+
+extern int foo0(int x);
+extern int foo1(int x);
+extern int foo2(int x);
+extern int foo3(int x);
+extern int foo4(int x);
+extern int foo5(int x);
+extern int foo6(int x);
+extern int foo7(int x);
+extern int foo8(int x);
+extern int foo9(int x);
+
+#endif