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Bug#756593: busybox's switch_root makes read-only NFS root read/write



Am 06.08.2014 um 07:42 schrieb Michael Tokarev <mjt@tls.msk.ru>:

> 05.08.2014 17:36, Zimmermann, Alexander wrote:
> 
>> Despite the fact that I was unable to write a proper wrapper :-) - the kernel crashes - 
>> I know now that neither busybox nor AUFS is the culprit. See below:
> 
> Um.  The wrapper should be something like:
> 
> #! /bin/sh
> echo mounts before-init:
> mount
> exec /sbin/init "$@"
> 
> The key point is, I think, the `exec' keyword.  Init should be started as pid=1.

I see. I miss the exec command.

> 
>> (initramfs) mount
>> rootfs on / type rootfs (rw)
>> sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,relatime)
>> proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
>> udev on /root/dev type devtmpfs (rw,relatime,size=10240k,nr_inodes=2051439,mode=755)
>> devpts on /root/dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620)
>> tmpfs on /root/run type tmpfs (rw,nosuid,relatime,size=3282972k,mode=755)
>> 192.168.0.10:/muclab/image/debian-sid on /root type nfs (ro,relatime,vers=3,rsize=65536,wsize=65536,namlen=255,hard,nolock,proto=tcp,port=2049,timeo=7,retrans=10,sec=sys,local_lock=all,addr=192.168.0.10)
>> (initramfs) exit
>> sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,relatime)
>> proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
>> udev on /dev type devtmpfs (rw,relatime,size=10240k,nr_inodes=2051439,mode=755)
>> devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620)
>> tmpfs on /run type tmpfs (rw,nosuid,relatime,size=3282972k,mode=755)
>> 192.168.0.10:/muclab/image/debian-sid on / type nfs (ro,relatime,vers=3,rsize=65536,wsize=65536,namlen=255,hard,nolock,proto=tcp,port=2049,timeo=7,retrans=10,sec=sys,local_lock=all,addr=192.168.0.10)
>> Usage: init {-e VAR[=VAL] | [-t SECONDS] {0|1|2|3|4|5|6|S|s|Q|q|A|a|B|b|C|c|U|u}}
>> Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000100

Actually I was too fast. Since the node doesn’t came up in this test, I
cannot be sure if the NFS mount will be RO or RW after the booting was
completed. Let me rerun the test again. I will come back w/ the results.

> 
> But I see.  Busybox switch_root worked, ran your myinit, and the mount
> in question - nfs mount - is still readonly like it should be...
> 
> Now this is really interrrrrrresting.
> 
> Do you have /etc/fstab entry for this mount?

No. stab is empty.

I will create 

> 
> Thanks,
> 
> /mjt
> 

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