From: Arnd Bergmann <arnd@arndb.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [RFC][v7][PATCH 8/9]: Define clone2() syscall
Date: Wed, 30 Sep 2009 19:59:40 +0200
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Cc: "H. Peter Anvin" <hpa@zytor.com>, Arjan van de Ven <arjan@infradead.org>,
       Roland McGrath <roland@redhat.com>,
       Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>,
       Containers <containers@lists.linux-foundation.org>,
       Nathan Lynch <nathanl@austin.ibm.com>, linux-kernel@vger.kernel.org,
       "Eric W. Biederman" <ebiederm@xmission.com>, mingo@elte.hu,
       Alexey Dobriyan <adobriyan@gmail.com>,
       Pavel Emelyanov <xemul@openvz.org>, linux-api@vger.kernel.org,
       kosaki.motohiro@jp.fujitsu.com
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On Wednesday 30 September 2009, Linus Torvalds wrote:
> Umm. I don't think that is possible.
> 
> You need architecture-specific code to even get access to all registers to 
> copy and get a signal-handler-compatible stack frame. See for example 
> arch/alpha/kernel/entry.S with the switch-stack thing etc.  I don't think 
> there is any way to make that even remotely architecture-neutral.

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).

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

	Arnd <><
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