[vbox-dev] vhd format not stable?

Frank Mehnert Frank.Mehnert at Sun.COM
Mon Jun 22 03:30:42 PDT 2009


On Friday 19 June 2009, Huihong Luo wrote:
> I tested it again, but it still crashed.
> I think I now found the cause, it's quite delicate, but you should be able
> to fix it right away. 
> a8bd6ffd ff ff ff ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ??
> The above is the bitmap when crashed. You can see the beginnig there are
> only 3 bytes are valid, the 4th byte will cause crash. 
> But when you call
> ASMBitTest(const volatile void *pvBitmap, int32_t iBit)
> it will the 1st 4-byte int, thus caused the crash.

Good catch!

> The easiest fix is to allocate 8 byte length more for the bitmap, and zero
> fill them, that will definitely solve the issue. You can also change
> ASMBitTest method impl. 

I think a better fix would be to ensure that ASMBitTest isn't called
with non-word-aligned memory addresses.

Kind regards,

Dr.-Ing. Frank Mehnert    Sun Microsystems, Inc.    www.sun.com
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