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SubjectRe: [PATCH] sector_t overflow in block layer
On Thu, 18 May 2006, Linus Torvalds wrote:
> On Thu, 18 May 2006, Andreas Dilger wrote:
> > struct bio *bio;
> > + unsigned long long sector;
> > int ret = 0;
> >
> > BUG_ON(!buffer_locked(bh));
> > BUG_ON(!buffer_mapped(bh));
> > BUG_ON(!bh->b_end_io);
> >
> > + /* Check if we overflow sector_t when computing the sector offset. */
> > + sector = (unsigned long long)bh->b_blocknr * (bh->b_size >> 9);
>
> Ok so far, looks fine.
>
> But what the heck is this:
>
> > +#if !defined(CONFIG_LBD) && BITS_PER_LONG == 32
> > + if (unlikely(sector != (sector_t)sector))
> > +#else
> > + if (unlikely(((bh->b_blocknr >> 32) * (bh->b_size >> 9)) >=
> > + 0xffffffff00000000ULL))
> > +#endif
>
> I don't understand the #ifdef at all.
>
> Why isn't that just a
>
> if (unlikely(sector != (sector_t)sector))
>
> and that's it? What does this have to do with CONFIG_LBD or BITS_PER_LONG,
> or anything at all?
>
> If the sector number fits in a sector_t, we're all good.

I think you missed that Andrewas said he is worried about 64-bit overflows
as well. And you would not catch those with the sector !=
(sector_t)sector test because you would be comparing two 64-bit values
together so they always match...

Hence why he shifts the value right by 32 bits then multiplies and tests
the result for overflowing 32-bits which if it does it means it would
overflow the 64-bit multiplication, too therefor your "sector" is
truncated.

Makes sense to me in a some very convoluted sickening way... (-;

Best regards,

Anton
--
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer, http://www.linux-ntfs.org/
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