[lxc-devel] lxc-start leaves temporary pivot dir behind

[Ferenc Wagner]

Daniel Lezcano <daniel.lezcano at free.fr> writes:

> Ferenc Wagner wrote:
>
>> Ferenc Wagner <wferi at niif.hu> writes:
>>   
>>> Daniel Lezcano <dlezcano at fr.ibm.com> writes:
>>>     
>>>> Ferenc Wagner wrote:
>>>>
>>>>> Daniel Lezcano <daniel.lezcano at free.fr> writes:
>>>>>
>>>>>> Ferenc Wagner wrote:
>>>>>>
>>>>>>> While playing with lxc-start, I noticed that /tmp is infested by
>>>>>>> empty lxc-r* directories: [...] Ok, this name comes from lxc-rootfs
>>>>>>> in conf.c:setup_rootfs.  After setup_rootfs_pivot_root returns, the
>>>>>>> original /tmp is not available anymore, so rmdir(tmpname) at the
>>>>>>> bottom of setup_rootfs can't achieve much.  Why is this temporary
>>>>>>> name needed anyway?  Is pivoting impossible without it?
>>>>>>             
>>>>>> That was put in place with chroot, before pivot_root, so the distro's
>>>>>> scripts can remount their '/' without failing.
>>>>>>
>>>>>> Now we have pivot_root, I suppose we can change that to something cleaner...
>>>>>           
>>>>> Like simply nuking it?  Shall I send a patch?
>>>>         
>>>> Sure, if we can kill it, I will be glad to take your patch :)
>>>       
>>> I can't see any reason why lxc-start couldn't do without that temporary
>>> recursive bind mount of the original root.  If neither do you, I'll
>>> patch it out and see if it still flies.
>>
>> For my purposes the patch below works fine.  I only run applications,
>> though, not full systems, so wider testing is definitely needed.
>>
>> From 98b24c13f809f18ab8969fb4d84defe6f812b25c Mon Sep 17 00:00:00 2001
>> From: Ferenc Wagner <wferi at niif.hu>
>> Date: Thu, 6 May 2010 14:47:39 +0200
>> Subject: [PATCH] no need to use a temporary directory for pivoting
>> [...]
>
> We can't simply remove it because of the pivot_root which returns EBUSY.
> I suppose it's coming from: "new_root and put_old must not be on the
> same file system as the current root."

Hmm, this could indeed be a problem if lxc.rootfs is on the current root
file system.  I didn't consider pivoting to the same FS, but looks like
this is the very reason for the current complexity in the architecture.

Btw. is this really a safe thing to do, to pivot into a subdirectory of
a file system?  Is there really no way out of that?

> But as we will pivot_root right after, we won't reuse the real rootfs,
> so we can safely use the host /tmp.

That will cause problems if rootfs is under /tmp, don't you think?
Actually, I'm not sure you can fully solve this.  If rootfs is a
separate file system, this is only much ado about nothing.  If rootfs
isn't a separate filesystem, you can't automatically find a good place
and also clean it up.  So why not require that rootfs is a separate
filesystem, and let the user deal with it by doing the necessary bind
mount in the lxc config?

> --- lxc.orig/src/lxc/conf.c
> +++ lxc/src/lxc/conf.c
> @@ -581,37 +581,24 @@ static int setup_rootfs_pivot_root(const
>  
>  static int setup_rootfs(const char *rootfs, const char *pivotdir)
>  {
> -	char *tmpname;
> -	int ret = -1;
> +	const char *tmpfs = "/tmp";
>  
>  	if (!rootfs)
>  		return 0;
>  
> -	tmpname = tempnam("/tmp", "lxc-rootfs");
> -	if (!tmpname) {
> -		SYSERROR("failed to generate temporary name");
> +	if (mount(rootfs, tmpfs, "none", MS_BIND|MS_REC, NULL)) {
> +		SYSERROR("failed to mount '%s'->'%s'", rootfs, "/tmp");

You probably meant tmpfs instead of "/tmp" in SYSERROR() above.

-- 
Thanks,
Feri.