From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757363Ab2JQQHW (ORCPT ); Wed, 17 Oct 2012 12:07:22 -0400 Received: from zeniv.linux.org.uk ([195.92.253.2]:55851 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753267Ab2JQQHV (ORCPT ); Wed, 17 Oct 2012 12:07:21 -0400 Date: Wed, 17 Oct 2012 17:07:03 +0100 From: Al Viro To: Michal Simek Cc: Jonas Bonn , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, Linus Torvalds , Catalin Marinas , Haavard Skinnemoen , Mike Frysinger , Jesper Nilsson , David Howells , Tony Luck , Benjamin Herrenschmidt , Hirokazu Takata , Geert Uytterhoeven , "James E.J. Bottomley" , Richard Kuo , Martin Schwidefsky , Lennox Wu , "David S. Miller" , Paul Mundt , Chris Zankel , Chris Metcalf , Yoshinori Sato , Guan Xuetao Subject: Re: new execve/kernel_thread design Message-ID: <20121017160702.GY2616@ZenIV.linux.org.uk> References: <20121016223508.GR2616@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 17, 2012 at 04:27:06PM +0200, Michal Simek wrote: > In the patch above there is directly used current_pt_regs() function > which works good for newly created threads > when pt_regs are exactly in current_pt_regs() position but not for > pt_regs which are saved on the stack > which is the init task case. init_task does *not* do kernel_execve(). It's PID 0, not PID 1. init is spawned by it. > My question is how should /init be called? Because I need to save > pt_regs to current_pt_regs() position where > generic kernel_execve expects it. What happens during boot is this: * init_task (not to be confused with init) is used as current during infrastructure initializations. Once everything needed for scheduler and for working fork is set, we spawn two threads - future init and future kthreadd. The last thing we do with init_task is telling init that kthreadd has been spawned. After that init_task turns itself into an idle thread. * future init waits for kthreadd to be spawned (it would be more natural to fork them in opposite order, but we want init to have PID 1 - too much stuff in userland depends on that). Then it does the rest of initialization, including setting up initramfs contents. And does kernel_execve() on /init. Note that this is a task that had been created by kernel_thread() and is currently in function called from ret_from_kernel_thread(). Its kernel stack has been set up by copy_thread(). That's where pt_regs need to be set up; note that they'll be passed to start_thread() before you return to userland. If there are any magic bits in pt_regs needed by return-from-syscall code, set them in kthread case of copy_thread(). From mboxrd@z Thu Jan 1 00:00:00 1970 From: Al Viro Subject: Re: new execve/kernel_thread design Date: Wed, 17 Oct 2012 17:07:03 +0100 Message-ID: <20121017160702.GY2616@ZenIV.linux.org.uk> References: <20121016223508.GR2616@ZenIV.linux.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from zeniv.linux.org.uk ([195.92.253.2]:55851 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753267Ab2JQQHV (ORCPT ); Wed, 17 Oct 2012 12:07:21 -0400 Content-Disposition: inline In-Reply-To: Sender: linux-arch-owner@vger.kernel.org List-ID: To: Michal Simek Cc: Jonas Bonn , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, Linus Torvalds , Catalin Marinas , Haavard Skinnemoen , Mike Frysinger , Jesper Nilsson , David Howells , Tony Luck , Benjamin Herrenschmidt , Hirokazu Takata , Geert Uytterhoeven , "James E.J. Bottomley" , Richard Kuo , Martin Schwidefsky , Lennox Wu , "David S. Miller" , Paul Mundt , Chris Zankel , Chris Metcalf , Yoshinori Sato , Guan Xuetao On Wed, Oct 17, 2012 at 04:27:06PM +0200, Michal Simek wrote: > In the patch above there is directly used current_pt_regs() function > which works good for newly created threads > when pt_regs are exactly in current_pt_regs() position but not for > pt_regs which are saved on the stack > which is the init task case. init_task does *not* do kernel_execve(). It's PID 0, not PID 1. init is spawned by it. > My question is how should /init be called? Because I need to save > pt_regs to current_pt_regs() position where > generic kernel_execve expects it. What happens during boot is this: * init_task (not to be confused with init) is used as current during infrastructure initializations. Once everything needed for scheduler and for working fork is set, we spawn two threads - future init and future kthreadd. The last thing we do with init_task is telling init that kthreadd has been spawned. After that init_task turns itself into an idle thread. * future init waits for kthreadd to be spawned (it would be more natural to fork them in opposite order, but we want init to have PID 1 - too much stuff in userland depends on that). Then it does the rest of initialization, including setting up initramfs contents. And does kernel_execve() on /init. Note that this is a task that had been created by kernel_thread() and is currently in function called from ret_from_kernel_thread(). Its kernel stack has been set up by copy_thread(). That's where pt_regs need to be set up; note that they'll be passed to start_thread() before you return to userland. If there are any magic bits in pt_regs needed by return-from-syscall code, set them in kthread case of copy_thread().