Re: [PATCH] sector_t overflow in block layer

[Linus Torvalds <torvalds@osdl.org>]

On Thu, 18 May 2006, Anton Altaparmakov wrote:
> 
> I think you missed that Andrewas said he is worried about 64-bit overflows 
> as well.

Ahh. Ok. However, then the test _really_ should be something like

	sector_t maxsector, sector;
	int sector_shift = get_sector_shift(bh->b_size);

	maxsector = (~(sector_t)0) >> sector_shift;
	if (unshifted_value > maxsector)
		return -EIO;
	sector = (sector_t) unshifted_value << sector_shift;

which is a lot clearer, and likely faster too, with a proper 
get_sector_shift. Something like this:

	/*
	 * What it the shift required to turn a bh of size
	 * "size" into a 512-byte sector count?
	 */
	static inline int get_sector_shift(unsigned int size)
	{
		int shift = -1;
		unsigned int n = 256;

		do {
			shift++;
		} while ((n += n) < size);
		return shift;
	}

which should generate good code on just about any architecture (it avoids 
actually using shifts on purpose), and I think the end result will end up 
being more readable (I'm not claiming that the "get_sector_shift()" 
implementation is readable, I'm claiming that the _users_ will be more 
readable).

Of course, even better would be to not have "b_size" at all, but use 
"b_shift", but we don't have that. And the sector shift calculation might 
be fast enough that it's even a win (turning a 64-bit multiply into a 
shift _tends_ to be better)

		Linus
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