[Devel] [RFC][PATCH 4/4]: Enable cloning PTY namespaces

[Serge E. Hallyn]

Quoting Cedric Le Goater (clg at fr.ibm.com):
> >>>>>>  
> >>>>>> +struct pts_namespace *new_pts_ns(void)
> >>>>>> +{
> >>>>>> +	struct pts_namespace *ns;
> >>>>>> +
> >>>>>> +	ns = kmalloc(sizeof(*ns), GFP_KERNEL);
> >>>>>> +	if (!ns)
> >>>>>> +		return ERR_PTR(-ENOMEM);
> >>>>>> +
> >>>>>> +	ns->mnt = kern_mount_data(&devpts_fs_type, ns);
> >>>>> You create a circular references here - the namespace
> >>>>> holds the vfsmnt, the vfsmnt holds a superblock, a superblock
> >>>>> holds the namespace.
> >>>> Hmm, yeah, good point.  That was probably in my original version last
> >>>> year, so my fault not Suka's.  Suka, would it work to have the
> >>>> sb->s_info point to the namespace but not grab a reference, than have
> >>> If you don't then you may be in situation, when this devpts
> >>> is mounted from userspace and in case the namespace is dead
> >>> superblock will point to garbage... Superblock MUST hold the
> >>> namespace :)
> >> But when the ns is freed sb->s_info would be NULL.  Surely the helpers
> >> can be made to handle that safely?
> > 
> > Hm... How do we find the proper superblock? Have a reference on
> > it from the namespace? I'm afraid it will be easy to resolve the
> > locking issues here.
> > 
> > I propose another scheme - we simply don't have ANY references
> > from namespace to superblock/vfsmount, but get the current
> > namespace in devpts_get_sb() and put in devpts_free_sb().
> 
> I've choosen another path in mq_ns. 
> 
> I also don't take any refcount on superblock/vfsmount of the new mq_ns 
> bc of the circular ref. I've considered that namespaces only apply to 
> processes : the refcount of a namespace is incremented each time a new 
> task is cloned and the namespace (in my case mq_ns) is released when  
> the last tasks exists. But this becomes an issue with user mounts which 
> survives task death. you end up having a user mount pointing to a bogus
> mq_ns. 
> 
> unless you require to have CLONE_NEWNS at the sametime.
> 
> Now, this CLONE_NEWNS enforcement seems to be an issue with bind mount.
> 
> ... jumping to the other thread :)

But once again, given that the mnt/sb is a view into a namespace bound
to a set of tasks, if all those tasks have exited, I see nothing wrong
with having sb->s_info being made NULL, so that a task in another
namespace attempting to access the exited namespace through a user mount
sees an empty directory.

So again I recommend that we should simply have sb->s_info point to
the namespace but without taking a reference, and have free_x_ns() set
x_ns->mnt->sb->s_info to NULL.  (That'll take a barrier of some kind,
which we can maybe build into the common helper)

-serge
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