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SubjectRe: [PATCH] sector_t overflow in block layer
On May 18, 2006  23:12 +0100, Anton Altaparmakov wrote:
> On Thu, 18 May 2006, Linus Torvalds wrote:
> > On Thu, 18 May 2006, Andreas Dilger wrote:
> > > + /* 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 Andreas 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.

Yes, this is exactly correct. At various times while writing this I had
more comments to explain everything, but then removed them as too wordy.
The actual implementation isn't what I care about - the resulting corruption
is more important and I thought it better to at least get some attention
on it instead of worrying over the details.

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.

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