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

base/debug/proc_maps_linux.cc

Index: base/debug/proc_maps_linux.cc
diff --git a/base/debug/proc_maps_linux.cc b/base/debug/proc_maps_linux.cc
index 9557feb025994b1e88763b3c52f855127eaeda81..7257b4ac8197a2d0dfe10339f8b417ec1c1a62d1 100644
--- a/base/debug/proc_maps_linux.cc
+++ b/base/debug/proc_maps_linux.cc
@@ -4,6 +4,8 @@
 
 #include "base/debug/proc_maps_linux.h"
 
+#include <fcntl.h>
+
 #if defined(OS_LINUX)
 #include <inttypes.h>
 #endif
@@ -22,9 +24,50 @@
 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,