Namespaces exhausted CLONE_XXX bits problem

Cedric Le Goater clg at fr.ibm.com
Mon Jan 14 06:44:25 PST 2008


Hello Pavel !

Pavel Emelyanov wrote:
> Hi, guys!
> 
> I started looking at PTYs/TTYs/Console to make the appropriate
> namespace and suddenly remembered that we have already
> exhausted all the CLONE_ bits in 32-bit mask.

yes nearly. 1 left with the mq_namespace i'm going to send.

> So, I recalled the discussions we had and saw the following 
> proposals of how to track this problem (with their disadvantages):
> 
> 1. make the clone2 system call with 64-bit mask
>    - this is a new system call

sys_clone2 is used on ia64 ... so we would need another name.
 
clone_ns() would be nice but it's too specific to namespaces unless 
we agree that we need a new syscall specific to namespaces. 

clone_new or clone_large ? 

> 2. re-use CLONE_STOPPED
>    - this will give us only one bit

not enough.

> 3. merge existing bits into one
>    - we lose the ability to create them separately

it would be useful to have such a flag though, something like CLONE_ALLN,
because it's the one everyone is going to use.

what i've been looking at in December is 1. and 3. : a new general purpose 
clone syscall with extend flags. The all-in-on flag is not an issue but it
would be nice to keep the last clone flag for this purpose. 

Now, if we use 64bits, we have a few issue/cleanups to solve. First, in 
kernel land, the clone_flags are passed down to the security modules
 
	security_task_create()

so we'll have to change to kernel api. I don't remember anything else 
blocking.

In user land, we need to choose a prototype supporting also 32bits arches. 
so it could be :

	long sys_clone_new(struct clone_new_args)

or

	long sys_clone_new(... unsigned long flags_high, unsigned long flag_low ...)

Second option might be an issue because clone already has 6 arguments.
right ?


> 4. implement a sys_unshare_ns system call with 64bit/arbitrary mask
>    - this is anew system call

I think that a new clone deserves a new unshare.
 
>    - this will bring some dissymmetry between namespaces

what do you mean ?

> 5. use sys_indirect
>    - this one is not in even -mm tree yet and it's questionable
>      whether it will be at all

I don't know much about that one.

C.

> I have one more suggestion:
> 
> 6. re-use bits, that don't make sense in sys_unshare (e.g.
>    CLONE_STOPPED, CLONE_PARENT_SETTID, CLONE_VFORK etc)
>    This will give us ~16 new bits, but this will look not very nice.
> 
> What do you think about all of this?
> 
> Thanks,
> Pavel
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