Re: [RFC] fs: filesystem specific options and remount

[Phillip Susi <psusi@ubuntu.com>]

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On 2/6/2012 6:08 PM, Karel Zak wrote:
> On Mon, Feb 06, 2012 at 11:41:16PM +0100, Jan Kara wrote:
>> On Fri 03-02-12 16:22:23, Jerome Marchand wrote:
>>> $ mount -o remount,hidepid=2 /proc/ $ grep proc /proc/mounts 
>>> /proc /proc proc rw,relatime,hidepid=2 0 0 $ mount | grep proc 
>>> proc on /proc type proc (rw,hidepid=2)
>>> 
>>> $ mount -o remount /proc/ $ grep proc /proc/mounts /proc /proc
>>> proc rw,relatime,hidepid=2 0 0 $ mount | grep proc proc on
>>> /proc type proc (rw)
> man mount:
> 
> The  remount  functionality follows the standard way how the mount 
> command works with options from fstab. It means the mount command 
> doesn't read fstab (or mtab) only when  a  device  and dir are
> fully specified.
> 
> mount -o remount,rw /dev/foo /dir
> 
> After this call all old mount options are replaced and arbitrary
> stuff from fstab is ignored, except the loop= option which is
> internally generated and maintained by the mount command.
> 
> mount -o remount,rw  /dir
> 
> After this call mount reads fstab (or mtab) and merges these
> options with options  from command line ( -o ).

Based on what you say there, I would guess this means that Jan has
proc listed in fstab, so mount tried to reset the mount options to
those specified in fstab, and the kernel proc fs left the options
unchanged.

It seems to me that the buggy behavior is in the proc fs not clearing
the unspecified options.  With the classic bitmask mount flags, any
that are not set are cleared by the remount, not left at their current
status.  This is why mount merges the existing flags specified in
fstab or mtab with those given on the command line.  The behavior
should be the same for the string passed mount flags.  In other words,
mount does its merging and whatever options are passed to the kernel
should be enabled, and any that are not should be disabled.

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