Bind mount bug?

Bind mount bug?

[Frans Pop]

I'm not sure whether this is a bug or expected behavior.
Suppose I create a "looped" bind mount situation as follows.

# mkdir test
# touch test/foo
# mkdir bindtest
# touch bindtest/bar
# mkdir bindtest/test
# mount --bind test/ bindtest/test/
# ls bindtest/test/
foo
# mount --bind bindtest/ test/
# ls test/
bar  test
# ls test/test/
#

I'd expected the last command to list "foo", but it shows an empty dir.
Shouldn't it also show the original contents of test (as they were before 
the first bind mount)?

# mount | grep test
/root/test on /root/bindtest/test type none (rw,bind)
/root/bindtest on /root/test type none (rw,bind)

Cheers,
Frans Pop

Re: Bind mount bug?

[Frans Pop]

On Sunday 11 November 2007, Frans Pop wrote:
> I'd expected the last command to list "foo", but it shows an empty dir.
> Shouldn't it also show the original contents of test (as they were before
> the first bind mount)?

The problem is of course also that any changes made into /test/test will get 
lost into thin space:

# touch test/test/baz
# ls bindtest/test
foo
# umount test
# ls test
foo
#

Re: Bind mount bug?

[Roland Kuhn]

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Hi Frans!

Let's see whether I can explain this (I'm not a guru...)

On 11 Nov 2007, at 11:06, Frans Pop wrote:

> I'm not sure whether this is a bug or expected behavior.
> Suppose I create a "looped" bind mount situation as follows.
>
> # mkdir test
> # touch test/foo
> # mkdir bindtest
> # touch bindtest/bar
> # mkdir bindtest/test
> # mount --bind test/ bindtest/test/
> # ls bindtest/test/
> foo
> # mount --bind bindtest/ test/

This mounts the bindtest/ tree on test/ _without_ copying the mount  
points which are found on subtrees. This is necessary to avoid loops  
in the filesystem (bind mounts are somewhat like hardlinks on  
directories, just without the headaches).

> # ls test/
> bar  test
> # ls test/test/
> #
>
This lists the contents of the original bintest/test/ directory which  
you created above. Creating e.g. a file in there stores that file  
physically in bindtest/test/bla, where "test" does _not_ mean the  
bind mount but the underlying directory here.

> I'd expected the last command to list "foo", but it shows an empty  
> dir.
> Shouldn't it also show the original contents of test (as they were  
> before
> the first bind mount)?
>
> # mount | grep test
> /root/test on /root/bindtest/test type none (rw,bind)
> /root/bindtest on /root/test type none (rw,bind)
>
You see, the bindtest/test/ mount was not propagated to test/test/.  
This is very much by design. You can e.g. do

# mkdir -p test/test
# mount --bind test test/test
# ls test/test
test
# ls test/test/test
#

so there is no loop (`find test` would actually say that it  
terminates because it has detected a loop, so it cannot be used to  
test this).

# touch test/test/test/a
# ls test/test/test
a
# ls test/test
# umount test/test
# ls test/test
a
#

So, you see, test/test/test/a was (as it should) physically created  
in test/test, where it is shadowed by the bind mount as long as that  
is not removed. Nothing vanishes into "thin air" ;-)

Ciao,
                     Roland

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Re: Bind mount bug?

[Jan Engelhardt]

>> I'm not sure whether this is a bug or expected behavior.
>> Suppose I create a "looped" bind mount situation as follows.
>>
>> # mkdir test
>> # touch test/foo
>> # mkdir bindtest
>> # touch bindtest/bar
>> # mkdir bindtest/test
>> # mount --bind test/ bindtest/test/
>> # ls bindtest/test/
>> foo
>> # mount --bind bindtest/ test/
>
>This mounts the bindtest/ tree on test/ _without_ copying the mount  
>points which are found on subtrees. This is necessary to avoid loops  
>in the filesystem (bind mounts are somewhat like hardlinks on  
>directories, just without the headaches).

What you seek is mount --rbind.

Re: Bind mount bug?

[Frans Pop]

On Sunday 11 November 2007, Roland Kuhn wrote:
> This mounts the bindtest/ tree on test/ _without_ copying the mount
> points which are found on subtrees.

Right. I guess this is the key point that tripped me up.

> So, you see, test/test/test/a was (as it should) physically created  
> in test/test, where it is shadowed by the bind mount as long as that  
> is not removed.

Which means that to continue the example from my follw-up mail:

> # ls bindtest/test
> foo
> # umount test
> # ls test
> foo
# umount bindtest/test
# ls bindtest/test
baz

And there indeed is the missing file.

> Nothing vanishes into "thin air" ;-)

Yeah, I saw that syntax error after sending the mail "into space" would have 
been OK too, but not my combining the two :-)

Thanks for the explanation!

Cheers,
FJP

Re: Bind mount bug?

[Frans Pop]

On Sunday 11 November 2007, Jan Engelhardt wrote:
> >This mounts the bindtest/ tree on test/ _without_ copying the mount
> >points which are found on subtrees. This is necessary to avoid loops
> >in the filesystem (bind mounts are somewhat like hardlinks on
> >directories, just without the headaches).
>
> What you seek is mount --rbind.

Thanks! That works fine with mount from util-linux-ng.

Busybox' mount (at least Debian's admittedly old 1.1.3 version) seems to 
fail to do the recursing. I'll file a BR against busybox in Debian and 
check that again when we have a more current version.