Re: [PATCH] block: create ioctl to discard-or-zeroout a range of blocks

[Jens Axboe]

On 11/10/2015 11:14 PM, Darrick J. Wong wrote:
> On Wed, Nov 11, 2015 at 05:30:07AM +0000, Seymour, Shane M wrote:
> > A quick question about this part of the patch:
> >
> > > +	uint64_t end = start + len - 1;
> >
> > > +	if (end >= i_size_read(bdev->bd_inode))
> >   		return -EINVAL;
> >
> > > +	/* Invalidate the page cache, including dirty pages */
> > > +	mapping = bdev->bd_inode->i_mapping;
> > > +	truncate_inode_pages_range(mapping, start, end);
> >
> > blk_ioctl_zeroout accepts unsigned values for start and end (uint64_t) but
> > loff_t types are turned from i_size_read() and passed as the 2nd and 3rd
> > values to truncate_inode_pages_range() and loff_t is a signed value. It
> > should be possible to pass in some values would overflow the calculation of
> > end causing the test on the value of end and the result of i_size_read to
> > pass but then end up passing a large unsigned value for in start that would
> > be implicitly converted to signed in truncate_inode_pages_range. I was
> > wondering if you'd tested passing in data that would cause sign conversion
> > issues when passed into truncate_inode_pages_range (does it handle it
> > gracefully?) or should this code:
> >
> > 	if (start & 511)
> >   		return -EINVAL;
> >   	if (len & 511)
> >   		return -EINVAL;
> >
> > be something more like this (for better sanity checking of your arguments)
> > which will ensure that you don't have implicit conversion issues from
> > unsigned to signed and ensure that the result of adding them together won't
> > either:
> >
> > 	if ((start & 511) || (start > (uint64_t)LLONG_MAX))
> >   		return -EINVAL;
> >   	if ((len & 511) ) || (len > (uint64_t)LLONG_MAX))
> >   		return -EINVAL;
> > 	if (end > (uint64_t)LLONG_MAX)
> > 		return -EINVAL;
> >
> > My apologies in advance if I've made a mistake when looking at this and my
> > concerns about unsigned values being implicitly converted to signed are
> > unfounded (I would have hoped for compiler warnings about any implicit
> > conversions though).
>
> I don't have a device large enough to test for signedness errors, since passing
> huge values for start and len never make it past the i_size_read check.
> However, I do see that I forgot to check the padding values, so I'll update
> that.

modprobe null_blk nr_devices=1 gb=512000

will get you a /dev/nullb0 that is 500TB. Adjust 'gb' at will. Or use 
loop with a big ass sparse file.

--
Jens Axboe

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