Under SanityManager/debug control, pre-fill DDMWriter buffers with garbage to
expose memory corruption errors
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Key: DERBY-865
URL: http://issues.apache.org/jira/browse/DERBY-865
Project: Derby
Type: Improvement
Components: Network Server
Versions: 10.1.1.0, 10.2.0.0
Reporter: Bryan Pendleton
Priority: Minor
DDMWriter uses a byte array to manage a memory buffer of formatted, but not yet
sent, network messsages.
The behavior of this byte array is that it automatically expands to accomodate
the requests that are given to it, but does not shrink. When the byte array is
first expanded to a certain size, Java provides 0x00 byte values for the new
bytes automatically, but when the buffer is later re-used, the bytes have
whatever value they previously had.
When a bug should exist in the code, such as the off-by-one bug described in
DERBY-125, the bytes in the byte array may be used without being set. Analysis
of DERBY-125 was complicated because the already-existing value in the buffer
turned out to be 0x00, which is a very common value in DSS message buffers, and
hence the bug was hidden for a long time because the value turned out to be
correct, accidentally.
Exposing and diagnosing such bugs would be much easier if the buffer's contents
were automatically set to some non-zero, unusual value, which would be less
likely to be accidentally correct.
This should be as easy as just dropping a few lines of code into
DDMWriter.ensureLength(), however, since this could have a significant
performance impact, the code should be written carefully so that the
performance is reasonable. One way to do this would be to make the code
conditional, and only perform the memory pre-setting if the source tree has
been specially built with the SanityManager enabled, or if debug flags are set
to true, or some such configuration.
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