linux-kernel - Re: [PATCH 0/4] - v2 - Object creation with a specified id

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Date:	Fri, 18 Apr 2008 10:07:07 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Nadia.Derbey@...l.net
Cc:	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, nick@...k-andrew.net
Subject: Re: [PATCH 0/4] - v2 -  Object creation with a specified id

On Fri, 2008-04-18 at 07:44 +0200, Nadia.Derbey@...l.net wrote:
> The syntax is one of:
>    . echo "LONG XX" > /proc/self/task/<my_tid>/next_id
>      next object to be created will have an id set to XX
>    . echo "LONG<n> X0 ... X<n-1>" > /proc/self/task/<my_tid>/next_id
>      next object to be created will have its ids set to XX0, ... X<n-1>
>      This is particularly useful for processes that may have several ids if
>      they belong to nested namespaces.

I still think, in its current form, this is pretty dangerous.  

It might be nice to have a central place to set ids, but there's no real
safety here for when we get a mishmash of 50 of these things.  We have a
queue and we fix the order in which things are inserted, but how do we
control the order in which the kernel extracts them?  Can we 100%
conform to that order in the future and be consistent?  Won't that also
be dictated by the way userspace behaves?

If we're going to go this route, I think we need to do a few things.
First, these need to be more "type safe".  That means that sticking an
ids in here that you mean to go to an IPC should never, ever, ever be
able to get interpreted as a PID or as an inode number or anything else.

	echo "IPC_SEM_ID NN" > /proc/self/task/<my_tid>/next_id
	echo "PID MM" > /proc/self/task/<my_tid>/next_id

Maybe we should include the types in there as well, but those are kinda
implied by the kind of id you're setting.  Have you thought about 32-bit
userspace with 64-bit kernels?  Does that cause any issues?

I also feel like we should have the kernel able to enumerate which
things in a process *are* settable.  What are the valid values for that
first word being echoed in the file?

Does anyone have a list of these ids that we're going to want to set
like this?  Oren?  I'm a bit worried that we might want to set things
other than ids with an interface like this and that it won't transition
well.

-- Dave

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