Validate the tag numbers when parsing. (#1725)

There was a twist code path (that some times showed up due to what happened to
be in memory in failure cases), that would cast a bogus wire type into the
enum, and then fall through switch statements.

Resolve this by validating all wire types when parsing tags and throwing the
error at that point so it can't enter the system.

As added safety, stick in a few asserts for apis that get passed tags to ensure
they also are only seeing valid data.

Bonus: Tweak the parsing loop to skip some work when we get the end marker
(zero tag) instead of still looping through all the fields.

1 files changed, 7 insertions, 0 deletions

diff --git a/objectivec/GPBWireFormat.m b/objectivec/GPBWireFormat.m
index 193235d6..860a339f 100644
--- a/objectivec/GPBWireFormat.m
+++ b/objectivec/GPBWireFormat.m
@@ -49,6 +49,13 @@ uint32_t GPBWireFormatGetTagFieldNumber(uint32_t tag) {
   return GPBLogicalRightShift32(tag, GPBWireFormatTagTypeBits);
 }
 
+BOOL GPBWireFormatIsValidTag(uint32_t tag) {
+  uint32_t formatBits = (tag & GPBWireFormatTagTypeMask);
+  // The valid GPBWireFormat* values are 0-5, anything else is not a valid tag.
+  BOOL result = (formatBits <= 5);
+  return result;
+}
+
 GPBWireFormat GPBWireFormatForType(GPBDataType type, BOOL isPacked) {
   if (isPacked) {
     return GPBWireFormatLengthDelimited;