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

[Pavel Emelyanov <xemul-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>]

Serge E. Hallyn wrote:
> Quoting Pavel Emelyanov (xemul-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org):
>> Serge E. Hallyn wrote:
>>> Quoting Pavel Emelyanov (xemul-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org):
>>>> sukadev-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org wrote:
>>>>> From: Sukadev Bhattiprolu <sukadev-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
>>>>> Subject: [RFC][PATCH 4/4]: Enable cloning PTY namespaces
>>>>>
>>>>> Enable cloning PTY namespaces.
>>>>>
>>>>> TODO:
>>>>> 	This version temporarily uses the clone flag '0x80000000' which
>>>>> 	is unused in mainline atm, but used for CLONE_IO in -mm. 
>>>>> 	While we must extend clone() (urgently) to solve this, it hopefully
>>>>> 	does not affect review of the rest of this patchset.
>>>>>
>>>>> Changelog:
>>>>> 	- Version 0: Based on earlier versions from Serge Hallyn and
>>>>> 	  Matt Helsley.
>>>>>
>>>>> Signed-off-by: Sukadev Bhattiprolu <sukadev-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
>>>>> ---
>>>>>  fs/devpts/inode.c         |   84 +++++++++++++++++++++++++++++++++++++++-------
>>>>>  include/linux/devpts_fs.h |   52 ++++++++++++++++++++++++++++
>>>>>  include/linux/init_task.h |    1 
>>>>>  include/linux/nsproxy.h   |    2 +
>>>>>  include/linux/sched.h     |    2 +
>>>>>  kernel/fork.c             |    2 -
>>>>>  kernel/nsproxy.c          |   17 ++++++++-
>>>>>  7 files changed, 146 insertions(+), 14 deletions(-)
>>>>>
>>>>> Index: linux-2.6.24/fs/devpts/inode.c
>>>>> ===================================================================
>>>>> --- linux-2.6.24.orig/fs/devpts/inode.c	2008-02-05 19:16:39.000000000 -0800
>>>>> +++ linux-2.6.24/fs/devpts/inode.c	2008-02-05 20:27:41.000000000 -0800
>>>>> @@ -25,18 +25,25 @@
>>>>>  #define DEVPTS_SUPER_MAGIC 0x1cd1
>>>>>  
>>>>>  extern int pty_limit;		/* Config limit on Unix98 ptys */
>>>>> -static DEFINE_IDR(allocated_ptys);
>>>>>  static DECLARE_MUTEX(allocated_ptys_lock);
>>>>> +static struct file_system_type devpts_fs_type;
>>>>> +
>>>>> +struct pts_namespace init_pts_ns = {
>>>>> +	.kref = {
>>>>> +		.refcount = ATOMIC_INIT(2),
>>>>> +	},
>>>>> +	.allocated_ptys = IDR_INIT(init_pts_ns.allocated_ptys),
>>>>> +	.mnt = NULL,
>>>>> +};
>>>>>  
>>>>>  static inline struct idr *current_pts_ns_allocated_ptys(void)
>>>>>  {
>>>>> -	return &allocated_ptys;
>>>>> +	return &current->nsproxy->pts_ns->allocated_ptys;
>>>>>  }
>>>>>  
>>>>> -static struct vfsmount *devpts_mnt;
>>>>>  static inline struct vfsmount *current_pts_ns_mnt(void)
>>>>>  {
>>>>> -	return devpts_mnt;
>>>>> +	return current->nsproxy->pts_ns->mnt;
>>>>>  }
>>>>>  
>>>>>  static struct {
>>>>> @@ -59,6 +66,42 @@ static match_table_t tokens = {
>>>>>  	{Opt_err, NULL}
>>>>>  };
>>>>>  
>>>>> +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().

>>> free_pts_ns() null out its sb->s_info, i.e. something like
>>>
>>> void free_pts_ns(struct kref *ns_kref)
>>> {
>>>    struct pts_namespace *ns;
>>>    struct super_block *sb;
>>>
>>>    ns = container_of(ns_kref, struct pts_namespace, kref);
>>>    BUG_ON(ns == &init_pts_ns);
>>>    sb = ns->mnt->mnt_sb;
>>>
>>>    mntput(ns->mnt);
>>>    sb->s_info = NULL;
>>>
>>>    /*
>>>     * TODO:
>>>     *      idr_remove_all(&ns->allocated_ptys); introduced in
>>> .6.23
>>>     */
>>>    idr_destroy(&ns->allocated_ptys);
>>>    kfree(ns);
>>> }
>>>
>>>
>