[Devel] Re: C/R and stdio redirection

Greg Kurz gkurz at fr.ibm.com
Wed Sep 8 02:44:52 PDT 2010

On Tue, 2010-09-07 at 13:03 -0700, Sukadev Bhattiprolu wrote:
> Suppose we create a container and redirect its stdout/stderr as follows:
> 	lxc-execute -name foo -- /path/to/app > /tmp/xyz.out 2>&1
> If we attempt to checkpoint the container 'foo', we fail bc one of the
> fds in the application refers to /tmp/xyz.out, which is also in use
> outside the container (specifically sys_checkpoint() fails due to the
> "alien mount ns" check in ckpt_fill_fname()).
> It can be argued, 'foo' is not a strict container (since it shares the
> fd with another container).  For this reason, we currently need the
> CHECKPOINT_SUBTREE flag in lxc-checkpoint.
> We initially thought that solving mount-namespaces will solve this, but
> realized that they are both separate problems. Mount-namespace C/R addresses
> preserving the mounts within the container and /tmp/xyz.out is outside
> the container.
> So if an application container needs to redirect stdio as above, we should
> either 
> 	a) disable/ignore the alien-mount-ns check or 
> 	b) try and start the application something like:
> 		$ cat /tmp/wrapper
> 		/path/to/app > /tmp/xyz.out 2>&1
> 		$ lxc-execute --name foo --  /tmp/wrapper
> with the difference being /tmp/xyz.out is now inside the container's /tmp
> filesystem rather than in the parent container.
> Maybe we can go with approach 'a' above only if CHECKPOINT_SUBTREE is also
> set - we had discussed this before and considered it hacky.
> Or are there other solutions to this stdio redirection issue ?

To be more accurate, this issue is about fd leaking from a parent
container to its descendants. The fd numbers may be anything else than
0,1 or 2 and the underlying files may be regular files, pipes,
sockets... For example, in the HPC world, stdio are often sockets
inheritated from a rshd like daemon.

Gregory Kurz                                     gkurz at fr.ibm.com
Software Engineer @ IBM/Meiosys                  http://www.ibm.com
Tel +33 (0)534 638 479                           Fax +33 (0)561 400 420

"Anarchy is about taking complete responsibility for yourself."
        Alan Moore.

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