Properly verify that input data is just 'pssh' boxes

media/formats/mp4/box_definitions.cc

Index: media/formats/mp4/box_definitions.cc
diff --git a/media/formats/mp4/box_definitions.cc b/media/formats/mp4/box_definitions.cc
index 72809cfdffdd542a7c97b2563f97bb935bc79779..9dfbf52cb5f8fc5e8fb86a2e9631ce704b067d9d 100644
--- a/media/formats/mp4/box_definitions.cc
+++ b/media/formats/mp4/box_definitions.cc
@@ -26,7 +26,9 @@ ProtectionSystemSpecificHeader::~ProtectionSystemSpecificHeader() {}
 FourCC ProtectionSystemSpecificHeader::BoxType() const { return FOURCC_PSSH; }
 
 bool ProtectionSystemSpecificHeader::Parse(BoxReader* reader) {
-  // Don't bother validating the box's contents.
+  // Don't bother validating the box's contents other than type().

[ddorwin, 2015/06/25 23:08:17]

It's not "don't bother" - it's Fail if not the correct type.

This seems like something that should be consistent for all box parsers.

Should they all contain this? Would that be a DCHECK in most cases? (We would do
that in a separate CL after branch cut.) It seems odd to have PSSH be the
exception as appears to currently be the case.

[jrummell, 2015/06/30 00:06:03]

The "don't bother" is in relation to all the other fields in the pssh box that
could be validated but aren't -- version, flags, KID_count, KIDs, datasize,
data.

There are a few places where it reads boxes that can have different types (see
SampleDescription::Parse, line 291 below that calls
ReadAllChildren(&audio_entries) and then AudioSampleEntry::Parse @ 568). So
adding the test in ReadAllChildren() (or all Boxes) would possibly break them.

Most of the parsing is done by calling ScanChildren() and then using ReadChild()
to read a particular type. So ReadAllChildren() is different. I could make a
variation (or add an additional parameter) for the validation case -- would that
be clearer?

+  RCHECK(reader->type() == BoxType());
+
   // Copy the entire box, including the header, for passing to EME as initData.
   DCHECK(raw_box.empty());
   raw_box.assign(reader->data(), reader->data() + reader->size());