Replace readlink calls with a safer version that guarantees NULL-termination.

This patch is part of a bigger patch that helps merging the breakpad code
with the modified version in Chromium OS.

Specifically, this patch makes the following changes:
1. Add a SafeReadLink function that wraps sys_readlink() to resolve a
   symbolic link but guarantees the result is NULL-terminated on success.
2. Refactor other source code to use SafeReadLink instead of readlink()
   or sys_readlink().

BUG=455
TEST=Tested the following:
1. Build on 32-bit and 64-bit Linux with gcc 4.4.3 and gcc 4.6.
2. Build on Mac OS X 10.6.8 with gcc 4.2 and clang 3.0 (with latest gmock).
3. All unit tests pass.
4. Run minidump-2-core to covnert a minidump file to a core file.
Review URL: http://breakpad.appspot.com/334001

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@896 4c0a9323-5329-0410-9bdc-e9ce6186880e

src/common/linux/file_id_unittest.cc

@@ -33,12 +33,14 @@
 #include <stdlib.h>
 
 #include "common/linux/file_id.h"
+#include "common/linux/safe_readlink.h"
 #include "common/linux/synth_elf.h"
 #include "common/test_assembler.h"
 #include "common/tests/auto_tempdir.h"
 #include "breakpad_googletest_includes.h"
 
 using namespace google_breakpad;
+using google_breakpad::SafeReadLink;
 using google_breakpad::synth_elf::BuildIDNote;
 using google_breakpad::synth_elf::ELF;
 using google_breakpad::test_assembler::kLittleEndian;
@@ -61,9 +63,7 @@ TEST(FileIDStripTest, StripSelf) {
   // FileID::ElfFileIdentifier, then make a copy of this binary,
   // strip it, and ensure that the result is the same.
   char exe_name[PATH_MAX];
-  ssize_t len = readlink("/proc/self/exe", exe_name, PATH_MAX - 1);
-  ASSERT_NE(len, -1);
-  exe_name[len] = '\0';
+  ASSERT_TRUE(SafeReadLink("/proc/self/exe", exe_name));
 
   // copy our binary to a temp file, and strip it
   AutoTempDir temp_dir;

src/common/linux/safe_readlink.cc

@@ -0,0 +1,53 @@
+// Copyright (c) 2011, Google Inc.
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// safe_readlink.cc: Implement google_breakpad::SafeReadLink.
+// See safe_readlink.h for details.
+
+#include <stddef.h>
+
+#include "third_party/lss/linux_syscall_support.h"
+
+namespace google_breakpad {
+
+bool SafeReadLink(const char* path, char* buffer, size_t buffer_size) {
+  // sys_readlink() does not add a NULL byte to |buffer|. In order to return
+  // a NULL-terminated string in |buffer|, |buffer_size| should be at least
+  // one byte longer than the expected path length. Also, sys_readlink()
+  // returns the actual path length on success, which does not count the
+  // NULL byte, so |result_size| should be less than |buffer_size|.
+  ssize_t result_size = sys_readlink(path, buffer, buffer_size);
+  if (result_size >= 0 && static_cast<size_t>(result_size) < buffer_size) {
+    buffer[result_size] = '\0';
+    return true;
+  }
+  return false;
+}
+
+}  // namespace google_breakpad

src/common/linux/safe_readlink.h

@@ -0,0 +1,65 @@
+// Copyright (c) 2011, Google Inc.
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// safe_readlink.h: Define the google_breakpad::SafeReadLink function,
+// which wraps sys_readlink and gurantees the result is NULL-terminated.
+
+#ifndef COMMON_LINUX_SAFE_READLINK_H_
+#define COMMON_LINUX_SAFE_READLINK_H_
+
+#include <stddef.h>
+
+namespace google_breakpad {
+
+// This function wraps sys_readlink() and performs the same functionalty,
+// but guarantees |buffer| is NULL-terminated if sys_readlink() returns
+// no error. It takes the same arguments as sys_readlink(), but unlike
+// sys_readlink(), it returns true on success.
+//
+// |buffer_size| specifies the size of |buffer| in bytes. As this function
+// always NULL-terminates |buffer| on success, |buffer_size| should be
+// at least one byte longer than the expected path length (e.g. PATH_MAX,
+// which is typically defined as the maximum length of a path name
+// including the NULL byte).
+//
+// The implementation of this function calls sys_readlink() instead of
+// readlink(), it can thus be used in the context where calling to libc
+// functions is discouraged.
+bool SafeReadLink(const char* path, char* buffer, size_t buffer_size);
+
+// Same as the three-argument version of SafeReadLink() but deduces the
+// size of |buffer| if it is a char array of known size.
+template <size_t N>
+bool SafeReadLink(const char* path, char (&buffer)[N]) {
+  return SafeReadLink(path, buffer, sizeof(buffer));
+}
+
+}  // namespace google_breakpad
+
+#endif  // COMMON_LINUX_SAFE_READLINK_H_

src/common/linux/safe_readlink_unittest.cc

@@ -0,0 +1,89 @@
+// Copyright (c) 2011, Google Inc.
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// safe_readlink_unittest.cc: Unit tests for google_breakpad::SafeReadLink.
+
+#include "breakpad_googletest_includes.h"
+#include "common/linux/safe_readlink.h"
+
+using google_breakpad::SafeReadLink;
+
+TEST(SafeReadLinkTest, ZeroBufferSize) {
+  char buffer[1];
+  EXPECT_FALSE(SafeReadLink("/proc/self/exe", buffer, 0));
+}
+
+TEST(SafeReadLinkTest, BufferSizeTooSmall) {
+  char buffer[1];
+  EXPECT_FALSE(SafeReadLink("/proc/self/exe", buffer, 1));
+}
+
+TEST(SafeReadLinkTest, BoundaryBufferSize) {
+  char buffer[PATH_MAX];
+  EXPECT_TRUE(SafeReadLink("/proc/self/exe", buffer, sizeof(buffer)));
+  size_t path_length = strlen(buffer);
+  EXPECT_LT(0, path_length);
+  EXPECT_GT(sizeof(buffer), path_length);
+
+  // Buffer size equals to the expected path length plus 1 for the NULL byte.
+  char buffer2[PATH_MAX];
+  EXPECT_TRUE(SafeReadLink("/proc/self/exe", buffer2, path_length + 1));
+  EXPECT_EQ(path_length, strlen(buffer2));
+  EXPECT_EQ(0, strncmp(buffer, buffer2, PATH_MAX));
+
+  // Buffer size equals to the expected path length.
+  EXPECT_FALSE(SafeReadLink("/proc/self/exe", buffer, path_length));
+}
+
+TEST(SafeReadLinkTest, NonexistentPath) {
+  char buffer[PATH_MAX];
+  EXPECT_FALSE(SafeReadLink("nonexistent_path", buffer, sizeof(buffer)));
+}
+
+TEST(SafeReadLinkTest, NonSymbolicLinkPath) {
+  char actual_path[PATH_MAX];
+  EXPECT_TRUE(SafeReadLink("/proc/self/exe", actual_path, sizeof(actual_path)));
+
+  char buffer[PATH_MAX];
+  EXPECT_FALSE(SafeReadLink(actual_path, buffer, sizeof(buffer)));
+}
+
+TEST(SafeReadLinkTest, DeduceBufferSizeFromCharArray) {
+  char buffer[PATH_MAX];
+  char* buffer_pointer = buffer;
+  EXPECT_TRUE(SafeReadLink("/proc/self/exe", buffer_pointer, sizeof(buffer)));
+  size_t path_length = strlen(buffer);
+
+  // Use the template version of SafeReadLink to deduce the buffer size
+  // from the char array.
+  char buffer2[PATH_MAX];
+  EXPECT_TRUE(SafeReadLink("/proc/self/exe", buffer2));
+  EXPECT_EQ(path_length, strlen(buffer2));
+  EXPECT_EQ(0, strncmp(buffer, buffer2, PATH_MAX));
+}