Bug 1752703 - Add interposers for functions manipulating the environm…

…ent to prevent crashes r=glandium

This add interposers for getenv(), putenv(), setenv(), unsetenv() and
clearenv(). All interposers use a single lock for synchronization while
internally using the libc-provided functions. This is done to prevent races
that typically happen in Firefox code when multiple threads call getenv()
while others are changing variables using setenv() and putenv().

Differential Revision: https://phabricator.services.mozilla.com/D164471

mozglue/interposers/env_interposer.cpp

@@ -0,0 +1,78 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include <pthread.h>
+#include <unistd.h>
+
+#include "InterposerHelper.h"
+
+// The interposers in this file cover all the functions used to access the
+// environment (getenv(), putenv(), setenv(), unsetenv() and clearenv()). They
+// all use the mutex below for synchronization to prevent races that caused
+// startup crashes, see bug 1752703.
+static pthread_mutex_t gEnvLock = PTHREAD_MUTEX_INITIALIZER;
+
+static char* internal_getenv(const char* aName) {
+  if (environ == nullptr || aName[0] == '\0') {
+    return nullptr;
+  }
+
+  size_t len = strlen(aName);
+  for (char** env_ptr = environ; *env_ptr != nullptr; ++env_ptr) {
+    if ((aName[0] == (*env_ptr)[0]) && (strncmp(aName, *env_ptr, len) == 0) &&
+        ((*env_ptr)[len] == '=')) {
+      return *env_ptr + len + 1;
+    }
+  }
+
+  return nullptr;
+}
+
+extern "C" {
+
+MFBT_API char* getenv(const char* name) {
+  pthread_mutex_lock(&gEnvLock);
+  char* result = internal_getenv(name);
+  pthread_mutex_unlock(&gEnvLock);
+
+  return result;
+}
+
+MFBT_API int putenv(char* string) {
+  static const auto real_putenv = GET_REAL_SYMBOL(putenv);
+
+  pthread_mutex_lock(&gEnvLock);
+  int result = real_putenv(string);
+  pthread_mutex_unlock(&gEnvLock);
+  return result;
+}
+
+MFBT_API int setenv(const char* name, const char* value, int replace) {
+  static const auto real_setenv = GET_REAL_SYMBOL(setenv);
+
+  pthread_mutex_lock(&gEnvLock);
+  int result = real_setenv(name, value, replace);
+  pthread_mutex_unlock(&gEnvLock);
+  return result;
+}
+
+MFBT_API int unsetenv(const char* name) {
+  static const auto real_unsetenv = GET_REAL_SYMBOL(unsetenv);
+
+  pthread_mutex_lock(&gEnvLock);
+  int result = real_unsetenv(name);
+  pthread_mutex_unlock(&gEnvLock);
+  return result;
+}
+
+MFBT_API int clearenv(void) {
+  static const auto real_clearenv = GET_REAL_SYMBOL(clearenv);
+
+  pthread_mutex_lock(&gEnvLock);
+  int result = real_clearenv();
+  pthread_mutex_unlock(&gEnvLock);
+  return result;
+}
+
+}  // extern "C"

mozglue/interposers/moz.build

@@ -7,6 +7,10 @@ Library("interposers")
 
 DEFINES["IMPL_MFBT"] = True
 
+UNIFIED_SOURCES += [
+    "env_interposer.cpp",
+]
+
 if CONFIG["MOZ_CRASHREPORTER"]:
     UNIFIED_SOURCES += [
         "pthread_create_interposer.cpp",