attempt to make the libc urandom override work for non-glibc too

sandbox/linux/services/libc_urandom_override.cc

-// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "sandbox/linux/services/libc_urandom_override.h"
 
 #include <dlfcn.h>
 #include <pthread.h>
 #include <stdio.h>
 #include <sys/stat.h>
 #include <unistd.h>
 
 #include "base/logging.h"
 #include "base/posix/eintr_wrapper.h"
 #include "base/rand_util.h"
 
 // Note: this file is used by the zygote and nacl_helper.
 
+// This is used when mapping stat* calls to __xstat* calls.
+#define XSTAT_VERSION 3
+
 namespace sandbox {
 
 static bool g_override_urandom = false;
 
-void InitLibcUrandomOverrides() {
-  // Make sure /dev/urandom is open.
-  base::GetUrandomFD();
-  g_override_urandom = true;
-}
-
 // TODO(sergeyu): Currently this code doesn't work properly under ASAN
 // - it crashes content_unittests. Make sure it works properly and
 // enable it here. http://crbug.com/123263
 #if !defined(ADDRESS_SANITIZER)
 
 static const char kUrandomDevPath[] = "/dev/urandom";
 
 typedef FILE* (*FopenFunction)(const char* path, const char* mode);
 typedef int (*XstatFunction)(int version, const char *path, struct stat *buf);
 typedef int (*Xstat64Function)(int version, const char *path,
                                struct stat64 *buf);
+typedef int (*StatFunction)(const char *path, struct stat *buf);
+typedef int (*Stat64Function)(const char *path, struct stat64 *buf);
 
 static pthread_once_t g_libc_file_io_funcs_guard = PTHREAD_ONCE_INIT;
-static FopenFunction g_libc_fopen;
-static FopenFunction g_libc_fopen64;
-static XstatFunction g_libc_xstat;
-static Xstat64Function g_libc_xstat64;
+static FopenFunction g_libc_fopen = NULL;
+static FopenFunction g_libc_fopen64 = NULL;
+static XstatFunction g_libc_xstat = NULL;
+static Xstat64Function g_libc_xstat64 = NULL;
+static StatFunction g_libc_stat = NULL;
+static Stat64Function g_libc_stat64 = NULL;
 
+void InitLibcUrandomOverrides() {
+  // Make sure /dev/urandom is open.
+  base::GetUrandomFD();
+  g_override_urandom = true;
+}
+
+// Find the libc's real fopen* and *stat* functions.
 static void InitLibcFileIOFunctions() {
+
+  // TODO(mostynb): call this once near the start of the init sequence, don't
+  // call it repeatedly with pthread_once() - ZygoteMain is not early enough.
+
   g_libc_fopen = reinterpret_cast<FopenFunction>(
       dlsym(RTLD_NEXT, "fopen"));
   g_libc_fopen64 = reinterpret_cast<FopenFunction>(
       dlsym(RTLD_NEXT, "fopen64"));
 
   if (!g_libc_fopen) {
     LOG(FATAL) << "Failed to get fopen() from libc.";
   } else if (!g_libc_fopen64) {
 #if !defined(OS_OPENBSD) && !defined(OS_FREEBSD)
     LOG(WARNING) << "Failed to get fopen64() from libc. Using fopen() instead.";
 #endif  // !defined(OS_OPENBSD) && !defined(OS_FREEBSD)
     g_libc_fopen64 = g_libc_fopen;
   }
 
-#if defined(LIBC_GLIBC)
-  // TODO(sergeyu): This works only on systems with glibc. Fix it to
-  // work properly on other systems if necessary.
+  // Note: we attempt to dlsym both stat and __xstat but only expect one
+  // of the calls to succeed (and similarly for the *64 versions).  This
+  // should work with both glibc and uClibc, and hopefully other libc's.
+  // Background: glibc headers inline stat into __xstat which it exports,
+  // while uClibc exports stat and does not implement __xstat.
+
   g_libc_xstat = reinterpret_cast<XstatFunction>(
       dlsym(RTLD_NEXT, "__xstat"));
   g_libc_xstat64 = reinterpret_cast<Xstat64Function>(
       dlsym(RTLD_NEXT, "__xstat64"));
+  g_libc_stat = reinterpret_cast<StatFunction>(
+      dlsym(RTLD_NEXT, "stat"));
+  g_libc_stat64 = reinterpret_cast<Stat64Function>(
+      dlsym(RTLD_NEXT, "stat64"));
 
-  if (!g_libc_xstat) {
-    LOG(FATAL) << "Failed to get __xstat() from libc.";
+  if (!g_libc_xstat && !g_libc_stat) {
+    LOG(FATAL) << "Neither __xstat nor stat found.";
   }
-  if (!g_libc_xstat64) {
-    LOG(WARNING) << "Failed to get __xstat64() from libc.";
+  if (!g_libc_xstat64 && !g_libc_stat64) {
+    LOG(FATAL) << "Neither __xstat64 nor stat64 found.";
   }
 }
 
 // fopen() and fopen64() are intercepted here so that NSS can open
 // /dev/urandom to seed its random number generator. NSS is used by
 // remoting in the sendbox.
 
 // fopen() call may be redirected to fopen64() in stdio.h using
 // __REDIRECT(), which sets asm name for fopen() to "fopen64". This
 // means that we cannot override fopen() directly here. Instead the
@@ skipping 28 matching lines @@
       return NULL;
     }
     return fdopen(fd, mode);
   } else {
     CHECK_EQ(0, pthread_once(&g_libc_file_io_funcs_guard,
                              InitLibcFileIOFunctions));
     return g_libc_fopen64(path, mode);
   }
 }
 
-// stat() is subject to the same problem as fopen(), so we have to use
-// the same trick to override it.
+// The stat() family of functions are subject to the same problem as
+// fopen(), so we have to use the same trick to override them.
+
 __attribute__ ((__visibility__("default")))
 int xstat_override(int version,
                    const char *path,
                    struct stat *buf)  __asm__ ("__xstat");
 
 __attribute__ ((__visibility__("default")))
 int xstat_override(int version, const char *path, struct stat *buf) {
+  CHECK_EQ(0, pthread_once(&g_libc_file_io_funcs_guard,
+                           InitLibcFileIOFunctions));
+
   if (g_override_urandom && strcmp(path, kUrandomDevPath) == 0) {
-    int result = __fxstat(version, base::GetUrandomFD(), buf);
+    int result = fstat(base::GetUrandomFD(), buf);

[jln (very slow on Chromium), 2013/06/26 01:57:21]

Loosing "version" here, may lead into very subtle bugs, we should be extremely
careful.

[Mostyn Bramley-Moore, 2013/06/26 10:30:18]

Done (reverted).

     return result;
+  }
+  else if (!g_libc_xstat) {
+    return g_libc_stat(path, buf);
   } else {
-    CHECK_EQ(0, pthread_once(&g_libc_file_io_funcs_guard,
-                             InitLibcFileIOFunctions));
     return g_libc_xstat(version, path, buf);
   }
 }
 
 __attribute__ ((__visibility__("default")))
 int xstat64_override(int version,
                      const char *path,
                      struct stat64 *buf)  __asm__ ("__xstat64");
 
 __attribute__ ((__visibility__("default")))
 int xstat64_override(int version, const char *path, struct stat64 *buf) {
+  CHECK_EQ(0, pthread_once(&g_libc_file_io_funcs_guard,
+                           InitLibcFileIOFunctions));
+
   if (g_override_urandom && strcmp(path, kUrandomDevPath) == 0) {
-    int result = __fxstat64(version, base::GetUrandomFD(), buf);
+    int result = fstat64(base::GetUrandomFD(), buf);
     return result;
+  }
+  else if (!g_libc_xstat64) {
+    return g_libc_stat64(path, buf);
   } else {
-    CHECK_EQ(0, pthread_once(&g_libc_file_io_funcs_guard,
-                             InitLibcFileIOFunctions));
-    CHECK(g_libc_xstat64);
     return g_libc_xstat64(version, path, buf);
   }
 }
-#endif  // defined(LIBC_GLIBC)
+
+__attribute__ ((__visibility__("default")))
+int stat_override(const char *path,
+                  struct stat *buf)  __asm__ ("stat");
+
+__attribute__ ((__visibility__("default")))
+int stat_override(const char *path, struct stat *buf) {
+  if (g_override_urandom && strcmp(path, kUrandomDevPath) == 0) {
+    int result = fstat(base::GetUrandomFD(), buf);
+    return result;
+  }
+
+  CHECK_EQ(0, pthread_once(&g_libc_file_io_funcs_guard,
+                           InitLibcFileIOFunctions));
+
+  if (!g_libc_stat) {
+    return g_libc_xstat(XSTAT_VERSION, path, buf);
+  } else {
+    return g_libc_stat(path, buf);
+  }
+}
+
+__attribute__ ((__visibility__("default")))
+int stat64_override(const char *path,
+                    struct stat64 *buf)  __asm__ ("stat64");
+
+__attribute__ ((__visibility__("default")))
+int stat64_override(const char *path, struct stat64 *buf) {
+  if (g_override_urandom && strcmp(path, kUrandomDevPath) == 0) {
+    int result = fstat64(base::GetUrandomFD(), buf);
+    return result;
+  }
+
+  CHECK_EQ(0, pthread_once(&g_libc_file_io_funcs_guard,
+                           InitLibcFileIOFunctions));
+
+  if (!g_libc_stat64) {
+    return g_libc_xstat64(XSTAT_VERSION, path, buf);
+  } else {
+    return g_libc_stat64(path, buf);
+  }
+}
 
 #endif  // !defined(ADDRESS_SANITIZER)
 
 }  // namespace content