Add memory corruption checking to base::File().

base/files/file_posix.cc

Index: base/files/file_posix.cc
diff --git a/base/files/file_posix.cc b/base/files/file_posix.cc
index 43684b5dabfe3df9b2a6907cd5dfbecb8e3582c3..9fad64344d486accda992d55b34fe1baccf0b7e6 100644
--- a/base/files/file_posix.cc
+++ b/base/files/file_posix.cc
@@ -483,6 +483,49 @@ File::Error File::OSErrorToFileError(int saved_errno) {
   }
 }
 
+File::MemoryCheckingScopedFD::MemoryCheckingScopedFD() {
+  UpdateChecksum();
+}
+
+File::MemoryCheckingScopedFD::MemoryCheckingScopedFD(int fd) : file_(fd) {
+  UpdateChecksum();
+}
+
+File::MemoryCheckingScopedFD::~MemoryCheckingScopedFD() {}
+
+// static
+void File::MemoryCheckingScopedFD::ComputeMemoryChecksum(
+    unsigned int* out_checksum) const {
+  // Use a single iteration of a linear congruentional generator (lcg) to
+  // provide a cheap checksum unlikely to be accidentally matched by a random
+  // memory corruption.
+
+  // By choosing constants that satisfy the Hull-Duebell Theorem on lcg cycle
+  // length, we insure that each distinct fd value maps to a distinct checksum,
+  // which maximises the utility of our checksum.
+
+  // This code uses "unsigned int" throughout for its defined modular semantics,
+  // which implicitly gives us a divisor of 2**32 or 2**64, depending.

[Nico, 2014/11/07 00:12:08]

(I think unsigneds are 32bit on all platforms we support, even in 64bit builds:
http://en.wikipedia.org/wiki/64-bit_computing#64-bit_data_models)

[gavinp, 2014/11/07 00:37:36]

Good point. I've updated the comment to just say "power of two" since that's
actually what's important here.

+
+  const unsigned int kMultiplier = 13035 * 4 + 1;
+  COMPILE_ASSERT(((kMultiplier - 1) & 3) == 0, pred_must_be_multiple_of_four);
+  const unsigned int kIncrement = 1595649551;
+  COMPILE_ASSERT(kIncrement & 1, must_be_coprime_to_powers_of_two);
+
+  *out_checksum =
+      static_cast<unsigned int>(file_.get()) * kMultiplier + kIncrement;
+}
+
+void File::MemoryCheckingScopedFD::Check() const {
+  unsigned int computed_checksum;
+  ComputeMemoryChecksum(&computed_checksum);
+  CHECK_EQ(file_memory_checksum_, computed_checksum) << "corrupted fd memory";
+}
+
+void File::MemoryCheckingScopedFD::UpdateChecksum() {
+  ComputeMemoryChecksum(&file_memory_checksum_);
+}
+
 void File::SetPlatformFile(PlatformFile file) {
   DCHECK(!file_.is_valid());
   file_.reset(file);