[Devel] Re: C/R and stdio redirection

[Louis Rilling]

On 05/10/10 22:50 -0700, Sukadev Bhattiprolu wrote:
> Greg Kurz [gkurz at fr.ibm.com] wrote:
> | 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.
> 
> I agree that fd substitution is the right way to go.
> 
> However, Matt Helsley and I were discussing this and wondered if we should
> ignore the redirection and expect to user to specify it during restart.
> 
> i.e if container was created like this:
> 
> 	lxc-execute -name foo -- /path/to/app > /tmp/xyz.out 2>&1
> 
> and checkpointed, can we expect the user to restart it like this ?
> 
> 	lxc-restart --name foo --statefile ckpt.img >> /tmp/xyz.out 
> 
> i.e user has to redo the redirection or the output would go to stdout.
> 
> Doing this would somehow seem to match a (bogus container) like:
> 
> 	lxc-execute --name foo -- /path/to/app | sort
> 
> If this container is checkpointed/restarted, we can't really redirect
> the output of the app to 'sort' right ? So expecting the user to
> redo the redirection on restart would treat both redirections ('>'
> and '|') in a consistent way ?

From the fd substitution point of view, this means that lxc-restart would
automatically request the substitution of its stdout to the checkpointed
container init's stdout?

This sounds reasonable to me at least. Especially since the container is usually
not supposed to know where the host is redirecting its stdout.

Thanks,

Louis

-- 
Dr Louis Rilling			Kerlabs
Skype: louis.rilling			Batiment Germanium
Phone: (+33|0) 6 80 89 08 23		80 avenue des Buttes de Coesmes
http://www.kerlabs.com/			35700 Rennes
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