[lxc-devel] [PATCH 1/1] pivot_root: switch to a new mechanism (v2)

[Andy Lutomirski]

On Mon, Sep 29, 2014 at 1:55 PM, Serge Hallyn <serge.hallyn at ubuntu.com> wrote:
> Quoting Dwight Engen (dwight.engen at oracle.com):
>> On Sat, 20 Sep 2014 03:15:44 +0000
>> Serge Hallyn <serge.hallyn at ubuntu.com> wrote:
>>
>> > This idea came from Andy Lutomirski.  Instead of using a
>> > temporary directory for the pivot_root put-old, use "." both
>> > for new-root and old-root.  Then fchdir into the old root
>> > temporarily in order to unmount the old-root, and finally
>> > chdir back into our '/'.
>> >
>> > Drop lxc.pivotdir from the lxc.container.conf manpage.
>> >
>> > Warn when we see a lxc.pivotdir entry (but keep it in the
>> > lxc.conf for now).
>> >
>> > Signed-off-by: Serge Hallyn <serge.hallyn at ubuntu.com>
>>
>> Hey Serge
>>
>> Just a heads up that this change makes my containers not start on a
>> 3.8.13 kernel. Reverting this change or using 3.13.11 kernel (just
>> tried a newer one I had handy) works. I'll try to debug further.
>> The log from trying to start a busybox container:
>
> Fascinating.
>
> It seems this is the case from at least v3.2 to v3.11.
>
> If my systemtap experiment isn't completely wrong, do_mount's
> call to kern_path() for any path to mount is returning a path
> with mnt->mnt_ns = NULL.  But it doesn't return an error.  So
> later on, when do_loopback checks for the path's mnt_ns being
> the same as current's, that fails since mnt_ns is NULL.
>
> The easiest way to reproduce is to use the program below, call it
> 'pivot.c", build it, and run "sudo ./pivot", then in the resulting bash
> shell try to, say, "mount --bind /mnt /mnt".
>
> I've reached the end of my ability to system-tap, so I'm going
> to build a kernel with some printk'ing :) and continue in awhile.
>
> Note that editing the program to do pivot_root(".", "/mnt") and
> umount2("/", MNT_DETACH) does NOT cause a problem.
>
> ===================================================================
>
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
> #include <sys/mount.h>
> #include <linux/sched.h>
>
>
> int main()
> {
>         int ret;
>         ret = unshare(CLONE_NEWNS);
>         if (ret)
>                 exit(1);
>         int oldroot, newroot;
>
>         ret = mount("/", "/mnt", "none", MS_BIND | MS_REC, NULL);
>         if (ret)
>                 exit(1);
>
>         oldroot = open("/", O_DIRECTORY | O_RDONLY);
>         if (oldroot < 0)
>                 exit(1);
>         newroot = open("/mnt", O_DIRECTORY | O_RDONLY);
>         if (newroot < 0)
>                 exit(1);
>         if (fchdir(newroot))
>                 exit(1);
>
>         if (pivot_root(".", "."))
>                 exit(2);
>         printf("pivoted");
>
>         if (fchdir(oldroot) < 0)
>                 exit(3);
>         if (umount2("/", MNT_DETACH) < 0)
>                 exit(3);

Shouldn't that be umount2(".", MNT_DETACH)?

I'm currently having trouble finding an old enough box.  Can you try
the attached fancier test and see what it prints?

Thanks,
Andy

>         if (fchdir(newroot) < 0)
>                 exit(4);
>         printf("in new root");
>         close(oldroot);
>         close(newroot);
>         printf("I am %d\n", getpid());
>         execl("/bin/bash", "/bin/bash", NULL);
> }



-- 
Andy Lutomirski
AMA Capital Management, LLC
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