[Bug 12371] New: df & mount return nothing in Docker containers

[bugzilla at busybox.net]

https://bugs.busybox.net/show_bug.cgi?id=12371

            Bug ID: 12371
           Summary: df & mount return nothing in Docker containers
           Product: Busybox
           Version: 1.25.x
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P5
         Component: Other
          Assignee: unassigned at busybox.net
          Reporter: busybox at bacto.net
                CC: busybox-cvs at busybox.net
  Target Milestone: ---

Hi,

When using Busybox in a container, under certain conditions `df` and `mount`
return nothing.

I have the problem with the last Alpine release (`alpine:3.10.3`) as well as
the Busybox one (`busybox:1.31.1-musl`).

I tried some old Busybox releases and it works with `busybox:1.24.2-musl` but
no more starting with `busybox:1.25.0-musl`.


Here is a simple example:
```dockerfile
FROM busybox:1.25.0-musl

RUN mkdir /test0
RUN mkdir /test1
RUN mkdir /test2
RUN mkdir /test3
RUN mkdir /test4
RUN mkdir /test5
RUN mkdir /test6
RUN mkdir /test7
RUN mkdir /test8
RUN mkdir /test9
RUN mkdir /test10
RUN mkdir /test11
RUN mkdir /test12
```

Running this container gives that:
```bash
/ # mount
/ # df
Filesystem                Size      Used Available Use% Mounted on
/ #
```

But if I remove one `mkdir`, `mount` and `df` work.

With mkdir repeated 12 times:
```
# grep overlay /proc/mounts | wc -c
1006
```

With mkdir repeated 13 times:
```
# grep overlay /proc/mounts | wc -c
1060
```

I think it is related to the fact that the overlay line is more than 1024
characters.


Note that I think it is related to this issues too:
- https://gitlab.alpinelinux.org/alpine/aports/issues/5703
- https://gitlab.alpinelinux.org/alpine/aports/issues/7093

Best,
Adrien

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