Messages in this thread | | | Date | Thu, 18 May 2006 16:43:06 -0600 | From | Andreas Dilger <> | Subject | Re: [PATCH] sector_t overflow in block layer |
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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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