linux-kernel - Re: [RFC][v7][PATCH 8/9]: Define clone2() syscall

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Date:	Wed, 30 Sep 2009 12:14:23 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Arnd Bergmann <arnd@...db.de>
cc:	"H. Peter Anvin" <hpa@...or.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Roland McGrath <roland@...hat.com>,
	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
	Containers <containers@...ts.linux-foundation.org>,
	Nathan Lynch <nathanl@...tin.ibm.com>,
	linux-kernel@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>, mingo@...e.hu,
	Alexey Dobriyan <adobriyan@...il.com>,
	Pavel Emelyanov <xemul@...nvz.org>, linux-api@...r.kernel.org,
	kosaki.motohiro@...fujitsu.com
Subject: Re: [RFC][v7][PATCH 8/9]: Define clone2() syscall

On Wed, 30 Sep 2009, Arnd Bergmann wrote:
> 
> Right, you still need to save all the registers from the entry code.
> I was under the wrong assumption that task_pt_regs(current)
> would give the full register set on all architectures.
> 
> However, I'd still hope that a new system call can be defined in
> a way that you only need to have an assembly wrapper to save
> the full pt_regs, but no arch specific code to get the syscall arguments
> out of that again. In do_clone(), you need a pointer to pt_regs and
> the user stack pointer, but that can be generated from
> user_stack_pointer(regs).

I don't think it can. You don't know what the system call stack layout is. 

> Does task_pt_regs(current) give the right pointer on all architectures
> or do we also need to pass the regs into the syscall?

I do not believe that it gives the right pointer in general. In fact, I 
can guarantee it doesn't. Even on x86 it only works for certain contexts 
(non-vm86 mode at a minimum), and on architectures like alpha it's not at 
all sufficient, because even if you can locate the 'pt_regs' structure, 
you _also_ need the extra guarantees of the pt_regs being next to the 
extended signal state register structure - and that only happens for magic 
sequences like signal handling and explicit setups like fork/clone.

So I do repeat: if you think you can do all of this in generic code, then 
you're sadly and totally mistaken. Don't even try. It may work on some 
architectures, but it's simply fundamentally _wrong_.

		Linus
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