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Re: Bug? wcsxfrm causing memory corruption

On Jun  6 17:57, Corinna Vinschen wrote:
> > > On Wed, May 10, 2017 at 11:30:46AM +0200, Erik Bray wrote:
> > >> [...]
> > >> The attached test demonstrates the bug.  Given an output buffer of N
> > >> wide characters, wcsxfrm will cause bytes beyond the destination size
> > >> to be reversed. I believe it might actually be a bug in the underlying
> > >> LCMapStringW workhorse (this is on Windows 10; have not tested other
> > >> versions).
> > >>
> > >> According to its docs [1], the cchDest argument (size of the
> > >> destination buffer) is treated as a *byte* count when using
> > >> LCMAP_SORTKEY.  However, for the purposes of applying the
> > >> LCMAP_BYTEREV transformation it seems to be treating the output size
> > >> (in bytes) as character count.  So in the example I give, where the
> > >> output sort key is 7 bytes (including the null terminator), it swaps
> > >> *14* bytes--the bytes including the sort key as well as the next 7
> > >> adjacent bytes.  This is obviously a problem if the destination buffer
> > >> is allocated out of some larger memory pool.
> > >>
> > >> This definitely has to be a bug, right?  Or at least very poorly
> > >> documented on MS's part.  A workaround would either be to not use
> > >> LCMAP_BYTEREV and just swap the bytes manually, or in a second call to
> > >> LCMapStringW with LCMAP_BYTEREV and the correct character count...
> > >> [...]
> Incredible piece of detective work.  Yeah, this looks very much like a
> bug in LCMapStringW, embarrassingly one I didn't notice at the time of
> writing the code.  I just tested this on W7 and the problem was already
> present.
> I don't know if this could be fixed by documentation.  There's just no
> safe way to combine LCMAP_SORTKEY with LCMAP_BYTEREV it seems, unless
> you always allocate a buffer at least twice as big as actually required
> for the sort key.  
> So, yeah, I think manual swaping after calling LCMapStringW without
> LCMAP_BYTEREV is the way to go.  I'll prepare a fix.
> Thanks for tracking down and the testcase,
> Corinna

Latest snapshot from has the patch applied.
Please try.


Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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