Update ReadProcMaps() to reflect lack of atomicity when reading /proc/self/maps.

base/debug/proc_maps_linux.cc

 // 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 "base/debug/proc_maps_linux.h"
 
+#include <fcntl.h>
+
 #if defined(OS_LINUX)
 #include <inttypes.h>
 #endif
 
 #include "base/file_util.h"
 #include "base/strings/string_split.h"
 
 #if defined(OS_ANDROID)
 // Bionic's inttypes.h defines PRI/SCNxPTR as an unsigned long int, which
 // is incompatible with Bionic's stdint.h defining uintptr_t as a unsigned int:
 // https://code.google.com/p/android/issues/detail?id=57218
 #undef SCNxPTR
 #define SCNxPTR "x"
 #endif
 
 namespace base {
 namespace debug {
 
+// Local testing revealed that the size of /proc/<pid>/maps for an official
+// release build of chrome hovered around the ~45 KB mark with some processes
+// hitting 80-90 KB.
+//
+// We'll play it safe and use 1 MB.
+enum { kMaxProcMapsSize = 1024 * 1024 };
+
 bool ReadProcMaps(std::string* proc_maps) {
-  FilePath proc_maps_path("/proc/self/maps");
-  return file_util::ReadFileToString(proc_maps_path, proc_maps);
+  scoped_ptr<char[]> buffer(new char[kMaxProcMapsSize]);
+
+  int fd = HANDLE_EINTR(open("/proc/self/maps", O_RDONLY));
+  if (fd == -1) {
+    DPLOG(ERROR) << "Couldn't open /proc/self/maps";
+    return false;
+  }
+  file_util::ScopedFD fd_closer(&fd);
+
+  // XXX there's file_util::ReadFromFD() but it expects an exact # of bytes
+  // to be read whereas here we're reading until EOF. Perhaps we can change
+  // the API / add a new one / add an optional param?
+  ssize_t offset = 0;
+  while (true) {
+    ssize_t bytes_read = HANDLE_EINTR(
+        read(fd, buffer.get() + offset, kMaxProcMapsSize - offset));

[Alexander Potapenko, 2013/07/11 10:47:23]

There must be only one read() call, otherwise we're busted if someone fragments
the memory while we're reading the file chunk by chunk.

+    if (bytes_read < 0) {
+      DPLOG(ERROR) << "Couldn't read /proc/self/maps";
+      return false;
+    }
+
+    offset += bytes_read;
+    if (offset == kMaxProcMapsSize) {
+      DLOG(ERROR) << "/proc/self/maps is too large";
+      return false;
+    }
+
+    if (bytes_read == 0)
+      break;
+  }
+
+  // Use 2-arg version of assign() to avoid an unnecessary length computation
+  // of |buffer|.
+  buffer[offset] = '\0';
+  proc_maps->assign(buffer.get(), offset);
+  return true;
 }
 
 bool ParseProcMaps(const std::string& input,
                    std::vector<MappedMemoryRegion>* regions_out) {
   std::vector<MappedMemoryRegion> regions;
 
   // This isn't async safe nor terribly efficient, but it doesn't need to be at
   // this point in time.
   std::vector<std::string> lines;
   SplitString(input, '\n', &lines);
@@ skipping 54 matching lines @@
     regions.push_back(region);
     regions.back().path.assign(line + path_index);
   }
 
   regions_out->swap(regions);
   return true;
 }
 
 }  // namespace debug
 }  // namespace base