Mailinglist Archive: opensuse-kernel (75 mails)

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Re: [opensuse-kernel] Problems with bind mount on OpenSUSE 11.4
On Wed, Mar 16, 2011 at 3:19 PM, Petr Uzel <petr.uzel@xxxxxxx> wrote:
On Wed, Mar 16, 2011 at 11:57:01AM +0100, richard -rw- weinberger wrote:
On Wed, Mar 16, 2011 at 11:47 AM, Petr Uzel <petr.uzel@xxxxxxx> wrote:
Try this:
mount -o bind /home /mnt
mount -o remount,ro,bind /mnt

Can you please explain me why this works?

Simple 'mount -o bind,ro /olddir /newdir' just does not work, as
explained in mount(8) manpage:

       Note  that  the  filesystem  mount  options  will  remain the
       same as those on the original mount point, and cannot be
       changed by passing the -o option along with --bind/--rbind.
       The mount options can be changed by a separate remount
       command, ...

In the past, this seemed to work, but actually the situation was much
worse than it is now. What actually happened with earlier versions of
util-linux (mount), was that the kernel did bind mount, but silently
ignored the RO flag, so the bind mount was in fact RW. What's worse,
mount did not recognize this and reported the mount as RO.

Here's example on one of the older systems:

# mount -o bind,ro /proc/proc_bind
# mount | grep proc_bind
/proc on /proc_bind type none (ro,bind)

       OK, mount shows it as RO ...

# cat /etc/mtab | grep proc_bind
/proc /proc_bind none ro,bind 0 0

       ... Because it reads the info from /etc/mtab, but ...

# cat /proc/self/mounts | grep proc_bind
proc /proc_bind proc rw,relatime 0 0

       ... but it is actually RW!

So util-linux/mount(8) was fixed and now recognizes this situation
and reports the RW bind mount (when the user believes it is RO).

So you have to use additional 'mount -o remount,ro' to make it really
read only.

More info: http://lwn.net/Articles/281157/

BTW this is one example of possible discrepancies between /etc/mtab and
/proc/self/mounts and one of the reasons why we made /etc/mtab
a symlink to /proc/self/mounts.


The reason why you have to use bind flag with the remount command
is explained in my previous mail and man mount, I believe.

Does this help?

Yes. Thanks a lot!


BR,

       Petr

--
Petr Uzel
IRC: ptr_uzl @ freenode


--
Thanks,
//richard
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