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From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: [RFC PATCH] vfs: Use 12:20 bit major:minor in stat everywhere
Date: Wed, 4 Mar 2015 01:22:12 +0000
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	Gwendal Grignou <gwendal@chromium.org>,
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To: Daniel Ehrenberg <dehrenberg@google.com>
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On Tue, Mar 03, 2015 at 05:10:22PM -0800, Daniel Ehrenberg wrote:
> > What's to guarantee that your ABI change won't break any of those?
> 
> I guess it's impossible to guarantee, but if there is an error, it'll
> be that an -EOVERFLOW error is suppressed and the high bits of the
> major:minor pair are shaved off by the userspace program. I would
> suspect that this would just make debugging harder, rather than
> actually break an automated program which counts on getting EOVERFLOW
> from a huge block device, but no way to know. The block device has to
> actually exist for this to happen, and all we're talking about is stat
> failing. So it's replacing an an error code with erroneous way to get
> data about a device node (erroneous just because userspace ignores
> some of the bits with the answer).

All it takes is more than 16 SCSI disks, AFAICS, and use of open-coded
MINOR() somewhere in old userland code...