Maybe a silly question, but why don't we just hit a tunable size limit
immediately before we "try to read" that data? 256MB is very big.
Is this a real format, or a test case designed to push the boundaries?
________________________________
From: Steve Lawrence <slawre...@apache.org>
Sent: Friday, September 11, 2020 1:14 PM
To: dev@daffodil.apache.org <dev@daffodil.apache.org>
Subject: Large dfdl:length values on complex types
I recently came across an issue where we have something like this:
<xs:element name="length" type="xs:int" ... />
<xs:element name="data"
dfdl:lengthKind="explicit" dfdl:length="{ ../length }">
<xs:complexType>
<xs:sequence>
<xs:element name="field1" ... />
<xs:element name="field2" ... />
...
<xs:element name="fieldN" ... />
</xs:sequence>
</xs:complexType>
</xs:element>
So we have a length element and a complex data field that uses this
length, and the data field is made up of a bunch of fields.
The issue I come across is related to how we cache bytes in buckets for
backtracking. As we fill up buckets, we currently limit the total amount
cache size of the buckets to 256MB. So if someone ever parses more than
256MB of data and then tries to backtrack past that, we error. The idea
being that we don't want to keep an infinite cache for potential
backtracking and people should have realized that they went down the
wrong branch much earlier.
Though, a problem occurs with the complex types with a large specified
length like above. When we have the complex type with expression
../length, before trying to parse any of the fields, we read that length
number of bytes into our cache buckets to confirm that that number of
bytes exists. The problem occurs if length is more than 256MB. In this
case, we read length number of bytes, and start removing elements from
the cache once we read more than 256MB.
But once that succeeds and we read length bytes, we then try to start
parsing the fields within the complex type, but we've removed those
early cached bytes, and so we fail with an unhelpful backtracking exception.
I'm not sure of the right solution here.
Perhaps we shouldn't be throwing away these bytes when dealing with
complex lengths?
Or perhaps we shouldn't even be trying to determine if that many bytes
are available when we have a specified length. Instead, maybe we should
just set the bit limit to make sure we don't parse more than than that?
And if eventually something tries to read a byte and there aren't enough
and we hit that limit, only then do we fail. This feels like the right
solution, but wanted to start a discussion to see if maybe there's a
reason we try to read the full length, or maybe there's another alternative?
