linux-kernel - Re: Adding subroot information to /proc/mounts, or obtaining that through other means

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Date:	Thu, 21 Jun 2007 09:13:40 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Al Viro <viro@....linux.org.uk>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org, util-linux-ng@...r.kernel.org
Subject: Re: Adding subroot information to /proc/mounts, or obtaining that
 through other means

Al Viro wrote:
> On Wed, Jun 20, 2007 at 01:57:33PM -0700, H. Peter Anvin wrote:
>> ... or, alternatively, add a subfield to the first field (which would
>> entail escaping whatever separator we choose):
>>
>> /dev/md6 /export ext3 rw,data=ordered 0 0
>> /dev/md6:/users/foo /home/foo ext3 rw,data=ordered 0 0
>> /dev/md6:/users/bar /home/bar ext3 rw,data=ordered 0 0
> 
> Hell, no.  The first field is in principle impossible to parse unless
> you know the fs type.
> 
> How about making a new file with sane format?  From the very
> beginning.  E.g. mountpoint + ID + relative path + type + options,
> where ID uniquely identifies superblock (e.g. numeric st_dev)
> and backing device (if any) is sitting among the options...

The more I'm thinking about this, I think it's simplest to just add
fields to the right of the existing /proc/*/mounts.  Yes, the format is
ugly, and it will end up being uglier still, but it's also ugly to have
a bunch of different chunks of information formatted in different ways.

So, the existing fields are:

mnt_devname mnt_path filesystem_type options 0 0

... and we'd want to add ...

mnt_id propagation_info sb_dev path_to_fs_root

As previously stated, in order to avoid having to expose kernel
addresses to userspace, I suggest we simply add a counter field to
struct vfsmount and use that for mnt_id.

I'm not all that up on what is needed for propagation_info.  I presume
we want to be able to deduce the full mount lattice.  One particularly
important thing in my mind is to be able to distinguish overmounted
filesystems (which I think is possible in the current setup only by
ordering -- the filesystem on top I believe will end up last in
/proc/mounts, but I don't know if there actually is anything that
enforces that.)

	-hpa
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