linux-kernel - Re: [PATCH] umount.2: Document the effect of shared subtrees on umount

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Date:	Mon, 02 Feb 2015 16:36:13 +0100
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	mtk.manpages@...il.com, Andy Lutomirski <luto@...capital.net>,
	Andrey Wagin <avagin@...il.com>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] umount.2: Document the effect of shared subtrees on umount

On 10/28/2014 06:33 PM, Eric W. Biederman wrote:
> 
> Signed-off-by: Eric W. Biederman <ebiederm@...ssion.com>

Thanks, Eric. Again, sorry for the delay. I've applied this 
patch.

Cheers,

Michael

> ---
>  man2/umount.2 | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/man2/umount.2 b/man2/umount.2
> index aea39d8306fe..a0120b2fc811 100644
> --- a/man2/umount.2
> +++ b/man2/umount.2
> @@ -97,6 +97,24 @@ Don't dereference
>  if it is a symbolic link.
>  This flag allows security problems to be avoided in set-user-ID-\fIroot\fP
>  programs that allow unprivileged users to unmount filesystems.
> +
> +.P
> +Shared mount points cause any mount activity on that mount point
> +including umounts to be forwarded to every shared mount point in it's
> +peer group and every slave mount of that peer group.  This means that
> +umount of any peer in a set of shared mounts will cause all of it's
> +peers to be unmounted and all of their slaves to be unmounted as well.
> +
> +This propogagtion of unmount activity can be particularly surprising
> +on systems where every mount point is shared by default.  On such
> +systems recursively bind mounting the root directory of the filesystem
> +onto a subdirectory and then later unmounting that subdirectory with
> +.BR MNT_DETACH
> +will cause every mount in the mount namespace to be lazily unmounted.
> +
> +To ensure umount does not propagate the mount point may be
> +remounted with MS_REC | MS_PRIVATE prior to umount being called.
> +
>  .SH RETURN VALUE
>  On success, zero is returned.
>  On error, \-1 is returned, and
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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