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Date:	Thu, 03 Apr 2014 19:57:31 +0100
From:	Pádraig Brady <P@...igBrady.com>
To:	Dave Reisner <d@...conindy.com>
CC:	linux-kernel@...r.kernel.org, tj@...nel.org
Subject: Re: Initramfs FSID altered in 3.14

On 04/03/2014 06:57 PM, Dave Reisner wrote:
> Hi,
> 
> [This is a repost of a G+ post at Tejun's request]
> 
> With Linux 3.14, you might notice in /proc/self/mountinfo that your
> root's parent FSID is now 0, instead of the 1 that it's been for the
> last N years. Tejun wrote the change (9e30cc9595303b27b48) that caused
> this, but the change comes in a rather innocuous way. Instead of an
> internal kernel mount of sysfs being assigned 0, it's now the initramfs.
> 
> So far, this has already caused switch_root and findmnt (from
> util-linux) to break, cp (from coreutils) to break when using the -x
> flag in early userspace, and it's also been pointed out that systemd's
> readahead code makes assumptions about a device number of 0.

For reference we've changed coreutils not to assume 0 is an invalid device ID:
http://git.sv.gnu.org/gitweb/?p=coreutils.git;a=commit;h=d0294ff3

> Are we now supposed to go and change all the assumptions in userspace
> about 0 being special? I'm conflicted. The kernel isn't supposed to
> break userspace, but it seems to me that FSIDs were never something to
> rely on -- similar to the block device numbering scheme.

I would say the kernel doesn't care what the value is,
so to ease compat worries just use >= 1.

cheers,
Pádraig
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