On Wed, Nov 18, 2009 at 04:28:23PM +0100, Miklos Szeredi wrote:
> On Wed, 18 Nov 2009, Karel Zak wrote:
> > The "flags" argument of mount(2) always contains MS_BIND for bind
> > mounts (if you use standard mount(8) command and /etc/mtab) for -o
> > remount.
>
> I think that's a bug. I can see the reasoning behind it, but it's
Don't blame userspace :-) see kernel patch:
commit 2e4b7fcd926006531935a4c79a5e9349fe51125b
Author: Dave Hansen <haveblue@xxxxxxxxxx>
Date: Fri Feb 15 14:38:00 2008 -0800
[PATCH] r/o bind mounts: honor mount writer counts at remount
> wrong. mount(8) should not guess whether the user wants to change the
> per superblock flags or the per mount flags based on *how* the mount
> was created.
>
> Look:
>
> mount /dev/hda1 /mnt1; mount /dev/hda1 /mnt2
>
> is completely equivalent to
>
> mount /dev/hda1 /mnt1; mount --bind /mnt1 /mnt2
>
> Why should "mount -oremount" treat one differently than the other?
It's kernel who cares about M_REMOUNT & MS_BIND. And kernel
MS_REMOUNT behavior was silently changed one year ago.
The mount(8) command has exactly defined way how works with mount
options for many years. There is not any exception for MS_BIND. It
reads options from fstab/mtab and apply options from command line,
the result is send to kernel.
I think the current (undocumented:-) kernel behavior is not clear.
We use two options for three separate operations: MS_REMOUNT (remount
superblock), MS_BIND (loopback) and MS_REMOUNT & MS_BIND (change mnt
flags).
> It would be much cleaner to have a mount option, say
> --change-mnt-flags, that is equivalent to "--bind -oremount".
Maybe, the question is how many users already depend on mount --bind
/old /new; mount -o remount,ro /new.
Karel
--
Karel Zak <kzak@xxxxxxxxxx>
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