[quake3-bugzilla] [Bug 4281] New: Uninitialized words in interpreted VM instruction stream.

[bugzilla-daemon at icculus.org]

http://bugzilla.icculus.org/show_bug.cgi?id=4281

           Summary: Uninitialized words in interpreted VM instruction
                    stream.
           Product: ioquake3
           Version: SVN HEAD
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: minor
          Priority: P3
         Component: Platform
        AssignedTo: zakk at icculus.org
        ReportedBy: baggett.patrick at gmail.com
         QAContact: quake3-bugzilla at icculus.org


Ryan C. Gordon <icculus at icculus.org> changed:

           What    |Removed                     |Added
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   Attachment #2126|application/octet-stream    |text/plain
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   Attachment #2126|0                           |1
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Created an attachment (id=2126)
 --> (http://bugzilla.icculus.org/attachment.cgi?id=2126)
Fix uninitialized sections of VM instruction stream

I know I noted before (possibly informally in the mailing list) that in
vm_interpreted.c:VM_PrepareInterpreter(), the use of a single 'pc' variable
while expanding opcodes from a byte -> word (mainly for RISC platforms where
unaligned access causes a crash) leaves 3 uninitialized instructions in the
instruction stream.

The code uses the same PC for both the int array and the byte array when
loading 32-bit values. Since the PC is updated via += 4, this causes the
integer array to have 3 uninitialized values in them. It just so happens that
the interpreter loop also does a += 4 (i.e. skipping 4 integer values), so this
bug doesn't manifest itself as a crash. The solution is almost trivial -- use a
byte PC and an integer PC and update the interpreter to not skip 4 words on
certain opcode -- which is much more consistent.

Tested on IRIX 6.5.28 on Octane2 (interpreted QVM, no *.so files)

--- Comment #1 from Ryan C. Gordon <icculus at icculus.org> 2009-09-17 00:15:37 EDT ---
(From update of attachment 2126)
Changing mimetype of attachment.

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