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Date: Sun, 23 Sep 2007 02:53:31 -0700
From: Darren Reed <darrenr@freebsd.org>
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Subject: yacc bug in reader.c:end_rule()
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There's a fairly obvious bug in yacc's reader.c but I'm not sure what 
the right fix is.

Witness:
end_rule()
{
    int i;

    if (!last_was_action && plhs[nrules]->tag)
    {
       for (i = nitems - 1; pitem[i]; --i) continue;
       if (pitem[i + 1] == 0 || pitem[i+1]->tag != plhs[nrules]->tag)
...
}

...clearly if pitem[nitems-1] == NULL (and nitems is the size of the
array from [0,nitems-1]) then the if() will access beyond the bounds
of the array.

There's also the question of i being able to run below 0 too here.

I don't know if the bug is here or if the bug is elsewhere in yacc,
but I doubt that the "fix" is s/i + 1/i/. *Maybe* "i = nitems - 2;"?

The bug can be masked by using calloc instead of malloc and similar
other tricks, but there is something more fundamentaly wrong here.

Has anyone else run into this?

Darren