linux-ext4 - Re: don't use ->bd_inode to access the block device size

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Message-ID: <202110140813.44C95229@keescook>
Date:   Thu, 14 Oct 2021 08:14:31 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Dave Kleikamp <dave.kleikamp@...cle.com>
Cc:     Anton Altaparmakov <anton@...era.com>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Coly Li <colyli@...e.de>, Mike Snitzer <snitzer@...hat.com>,
        Song Liu <song@...nel.org>, David Sterba <dsterba@...e.com>,
        Josef Bacik <josef@...icpanda.com>,
        Theodore Ts'o <tytso@....edu>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
        Ryusuke Konishi <konishi.ryusuke@...il.com>,
        Konstantin Komarov <almaz.alexandrovich@...agon-software.com>,
        Phillip Lougher <phillip@...ashfs.org.uk>,
        Jan Kara <jack@...e.com>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
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Subject: Re: don't use ->bd_inode to access the block device size

On Thu, Oct 14, 2021 at 08:13:59AM -0500, Dave Kleikamp wrote:
> On 10/14/21 4:32AM, Anton Altaparmakov wrote:
> > Hi Christoph,
> > 
> > > On 14 Oct 2021, at 07:28, Christoph Hellwig <hch@....de> wrote:
> > > 
> > > On Wed, Oct 13, 2021 at 07:10:13AM +0200, Christoph Hellwig wrote:
> > > > I wondered about adding a helper for looking at the size in byte units
> > > > to avoid the SECTOR_SHIFT shifts in various places.  But given that
> > > > I could not come up with a good name and block devices fundamentally
> > > > work in sector size granularity I decided against that.
> > > 
> > > So it seems like the biggest review feedback is that we should have
> > > such a helper.  I think the bdev_size name is the worst as size does
> > > not imply a particular unit.  bdev_nr_bytes is a little better but I'm
> > > not too happy.  Any other suggestions or strong opinions?
> > 
> > bdev_byte_size() would seem to address your concerns?
> > 
> > bdev_nr_bytes() would work though - it is analogous to bdev_nr_sectors() after all.
> > 
> > No strong opinion here but I do agree with you that bdev_size() is a bad choice for sure.  It is bound to cause bugs down the line when people forget what unit it is in.
> 
> I don't really mind bdev_size since it's analogous to i_size, but
> bdev_nr_bytes seems good to me.

I much prefer bdev_nr_bytes(), as "size" has no units.

-- 
Kees Cook