linux-kernel - Re: [RFC][v7][PATCH 0/9] Implement clone2() system call

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Date:	Thu, 24 Sep 2009 19:19:19 -0400
From:	Oren Laadan <orenl@...rato.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org,
	Containers <containers@...ts.linux-foundation.org>,
	Nathan Lynch <nathanl@...tin.ibm.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>, hpa@...or.com,
	mingo@...e.hu, torvalds@...ux-foundation.org,
	Alexey Dobriyan <adobriyan@...il.com>,
	Pavel Emelyanov <xemul@...nvz.org>
Subject: Re: [RFC][v7][PATCH 0/9] Implement clone2() system call



Arnd Bergmann wrote:
> On Friday 25 September 2009, Oren Laadan wrote:
>> In this form, you place a constraints on where userspace may
>> place the {parent,child}_tid variable, and require that this
>> particular clone_struct remain valid memory in the parent until
>> the child terminates.  This may break existing programs that
>> use this (threads libraries ?)
> 
> No existing program uses sys_clone2, and the kernel function
> may well differ from the user space calling conventions, which
> are not bound by the six-argument limitation.
> 
> So a clone2 library call could set up the structure with the
> arguments to the real syscall, call into the kernel and
> copy the output data back into the pointers it was given by
> the user.

That may work well for parent_tid, however child_tid is also
kept on the task_struct and written to when the child exits,
and there is no explicit user-space wrapper on that.

Also, I may be mistaken, but I thought that the idea of these
was that the kernel writes them to user space, so other threads
may see them quickly, _before_ the parent returns to userspace;
otherwise, the parent (at the library) could himself copy the
return value.

Oren.

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