* Duplicate network filesystems
@ 2019-02-11 17:58 Aurélien Aptel
2019-02-15 3:40 ` NeilBrown
0 siblings, 1 reply; 3+ messages in thread
From: Aurélien Aptel @ 2019-02-11 17:58 UTC (permalink / raw)
To: linux-fsdevel
Hi,
In fs/namespace.c do_add_mount() we do this check:
/* Refuse the same filesystem on the same mount point */
err = -EBUSY;
if (path->mnt->mnt_sb == newmnt->mnt.mnt_sb &&
path->mnt->mnt_root == path->dentry)
goto unlock;
So that mount fails with EBUSY. But for networked filesystems (at least
cifs and nfs) you can do this:
mount //foo /mnt -o A
mount //foo /mnt -o B # different options
Since the SB are different it works, fine.
But mounting a 3rd time with options A succeeds, where from a user POV I
would have expected to fail.
So to recap:
mount //foo /mnt -o A
mount //foo /mnt -o A
# EBUSY => expected behaviour
mount //foo /mnt -o A
mount //foo /mnt -o B
# ok => expected behaviour
mount //foo /mnt -o A
mount //foo /mnt -o B
mount //foo /mnt -o A
# ok => what?
Shouldn't we check the stack of filesystems mounted at the path instead of
just the last one?
Cheers,
--
Aurélien Aptel / SUSE Labs Samba Team
GPG: 1839 CB5F 9F5B FB9B AA97 8C99 03C8 A49B 521B D5D3
SUSE Linux GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Duplicate network filesystems
2019-02-11 17:58 Duplicate network filesystems Aurélien Aptel
@ 2019-02-15 3:40 ` NeilBrown
2019-02-15 10:18 ` Aurélien Aptel
0 siblings, 1 reply; 3+ messages in thread
From: NeilBrown @ 2019-02-15 3:40 UTC (permalink / raw)
To: Aurélien Aptel, linux-fsdevel
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On Mon, Feb 11 2019, Aurélien Aptel wrote:
> Hi,
>
> In fs/namespace.c do_add_mount() we do this check:
>
> /* Refuse the same filesystem on the same mount point */
> err = -EBUSY;
> if (path->mnt->mnt_sb == newmnt->mnt.mnt_sb &&
> path->mnt->mnt_root == path->dentry)
> goto unlock;
>
> So that mount fails with EBUSY. But for networked filesystems (at least
> cifs and nfs) you can do this:
>
> mount //foo /mnt -o A
> mount //foo /mnt -o B # different options
>
> Since the SB are different it works, fine.
>
> But mounting a 3rd time with options A succeeds, where from a user POV I
> would have expected to fail.
Why?
>
> So to recap:
>
> mount //foo /mnt -o A
> mount //foo /mnt -o A
> # EBUSY => expected behaviour
>
> mount //foo /mnt -o A
> mount //foo /mnt -o B
> # ok => expected behaviour
>
> mount //foo /mnt -o A
> mount //foo /mnt -o B
> mount //foo /mnt -o A
> # ok => what?
>
> Shouldn't we check the stack of filesystems mounted at the path instead of
> just the last one?
Why?
I think that the main reason that -EBUSY is important is that people
often run "mount -a" and don't expect filesystems that are already
mounted to be mounted again. The current behaviour achieves that.
What is your use-case for wanted -EBUSY in some other circumstance?
NeilBrown
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^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Duplicate network filesystems
2019-02-15 3:40 ` NeilBrown
@ 2019-02-15 10:18 ` Aurélien Aptel
0 siblings, 0 replies; 3+ messages in thread
From: Aurélien Aptel @ 2019-02-15 10:18 UTC (permalink / raw)
To: NeilBrown, linux-fsdevel
NeilBrown <neilb@suse.com> writes:
>> mount //foo /mnt -o A
>> mount //foo /mnt -o B # different options
>>
>> Since the SB are different it works, fine.
>>
>> But mounting a 3rd time with options A succeeds, where from a user POV I
>> would have expected to fail.
>
> Why?
Because it's already mounted.
>> mount //foo /mnt -o A
>> mount //foo /mnt -o B
>> mount //foo /mnt -o A
>> # ok => what?
>>
>> Shouldn't we check the stack of filesystems mounted at the path instead of
>> just the last one?
>
> Why?
>
> I think that the main reason that -EBUSY is important is that people
> often run "mount -a" and don't expect filesystems that are already
> mounted to be mounted again. The current behaviour achieves that.
A is already mounted and mouting again succeeds. The current behaviour is
different from what you described. This is exactly what I was saying
earlier "from a user POV".
Side-note:
mount from coreutils has some logic to deal with -a and network
filesystems, because there are some issues with the default behaviour. I
can't recall the details about that, would have to look at the code
again.
Cheers,
--
Aurélien Aptel / SUSE Labs Samba Team
GPG: 1839 CB5F 9F5B FB9B AA97 8C99 03C8 A49B 521B D5D3
SUSE Linux GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
^ permalink raw reply [flat|nested] 3+ messages in thread
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2019-02-11 17:58 Duplicate network filesystems Aurélien Aptel
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