Hi Peter,
CC: util-linux-ng mailing list
On Fri, Jul 24, 2009 at 04:41:06PM +0400, Peter Volkov wrote:
> Hi, Karel. I found strange behavior of mount -o bind. If I create two
> directories, 1 and 2, after bind-mount 1 to 2 and after that issue mount
> 2 - 1 will be mounted twice on 2:
>
> # mkdir 1 2
> # mount -o bind 1 2
> # mount 2
> # mount | grep 2
> /root/1 on /root/2 type none (rw,bind)
> /root/1 on /root/2 type none (rw,bind)
This is very old (10+years) feature (but undocumented). The mount(8)
is trying to found mountpoint/device in fstab and then in mtab.
I guess the original purpose for this feature is user-friendly "mount
/node -o remount". Unfortunately, the code does not check for
MS_REMOUNT flag and consults mtab in all cases. (The code is
consulting fstab/mtab before mount options parsing.)
> The same if I do mount 1:
> # mount 1
> # mount | grep 2
> /root/1 on /root/2 type none (rw,bind)
> /root/1 on /root/2 type none (rw,bind)
> /root/1 on /root/2 type none (rw,bind)
>
> Looks like mount is consulting /etc/mtab for some reason, which is at
> least non-expected behavior (or is not-documented).
>
> BTW if I substitute /etc/mtab with symlink on /proc/mounts I get
> expected error:
>
> # LC_ALL=C mount 2
> mount: /dev/sda3 already mounted or /root/2 busy
Sure, /proc/mounts does not provide proper information about bind
mounds, so your "mount 2" is interpreted as "mount /dev/sda3 /root/2"
-- it's not possible to mount the same device on the same mountpoint
two times. This restriction does not exist for bind mounts.
> mount: according to mtab, /dev/root is already mounted on /root/2
>
> Is this a bug or /me missing something?
It's bug in the mount.8 man page -- this behaviour should be
documented. Patches are welcomed :-)
Karel
--
Karel Zak <kzak@xxxxxxxxxx>
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