Re: [Firebird-devel] FB2b2 3221225477

[Vlad H. <hv...@us...>]

> > template <typename CharType>
> > static void preKmp(const CharType *x, int m, SLONG kmpNext[]) 
> > {
> >     SLONG i = 0;
> >     SLONG j = kmpNext[0] = -1;
> >     while (i < m) {
> >         while (j > -1 && x[i] != x[j])
> >             j = kmpNext[j];
> >         i++;
> >         j++;
> >         if (x[i] == x[j])         // AV here
> >             kmpNext[i] = kmpNext[j];
> >         else
> >         kmpNext[i] = j;
> >         }
> > }
> >
> > Some variables at AV time :     m == 0x80, i == 0x80, j == 0x00
> > Since i must be in range 0..0x7F (0..m-1) we have an AV when x[i] evaluated
> >
> >
> >     As i understand this is a part of CONTAINING evaluation. The algorithm 
> > was written by Nickolay and i unfortunately have no knowledge of how it must 
> > work. May be Adriano can say more ?
> >   
> I don't have knowledge of Knuth-Morris-Pratt algorithm, but I can say 
> that preKmp is always accessing a invalid location.
> The AV will occur with a "magic" string length, causing the invalid 
> location is in another memory page.
> 
> As it's probably that compare of garbage with part of the string is 
> false, I propose to change the line to "if (i < m && x[i] == x[j])".

    In that case we'll have invalid i (i == m) in kmpNext[i] = j expression.
It seems that we must change while (i < m) to while (i < m-1)

    Am i right ?

Regards,
Vlad