Re: [PATCH] umount.2: Correct the description of MNT_DETACH

From: Andy Lutomirski
Date: Mon Sep 29 2014 - 22:16:50 EST

On Mon, Sep 29, 2014 at 7:15 PM, Eric W. Biederman
<ebiederm@xxxxxxxxxxxx> wrote:
> Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:
>> On Mon, Sep 29, 2014 at 6:04 PM, Eric W. Biederman
>> <ebiederm@xxxxxxxxxxxx> wrote:
>>> I recently realized that I had been reasoning improperly about what
>>> umount(MNT_DETACH) did based on an insufficient description in
>>> the umount.2 man page, that matched my intuition but not the
>>> implementation.
>>> When there are no submounts MNT_DETACH is essentially harmless to
>>> applications. Where there are submounts MNT_DETACH changes what
>>> is visible to applications using the detach directories.
>>> Signed-off-by: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
>>> ---
>>> man2/umount.2 | 7 ++++---
>>> 1 file changed, 4 insertions(+), 3 deletions(-)
>>> diff --git a/man2/umount.2 b/man2/umount.2
>>> index 5ff88152c738..aea39d8306fe 100644
>>> --- a/man2/umount.2
>>> +++ b/man2/umount.2
>>> @@ -66,9 +66,10 @@ This can cause data loss.
>>> (Only for NFS mounts.)
>>> .TP
>>> .BR MNT_DETACH " (since Linux 2.4.11)"
>>> -Perform a lazy unmount: make the mount point unavailable for
>>> -new accesses, and actually perform the unmount when the mount point
>>> -ceases to be busy.
>>> +Perform a lazy unmount: make the mount point unavailable for new
>>> +accesses, immediately disconnect the filesystem and all filesystems
>>> +mounted below it from each other and from the mount table, and
>>> +actually perform the unmount when the mount point ceases to be busy.
>> Want to add something like:
>> MNT_DETACH on a shared mount will propagate unmount events to its peer
>> group. That means that recursively bind mounting a shared mount and
>> then unmounting that recursive bind mount will unmount all submounts
>> of the original mount. This behavior can be avoided by remounting a
>> directory with MS_REC | MS_PRIVATE before unmounting it.
> Make that any unmount on a shared mount that will propogate unmount
> events to it's peer group.

I don't understand your proposed edit. Can you type it out explicitly?


> Eric

Andy Lutomirski
AMA Capital Management, LLC
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