From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753594AbcEWPXk (ORCPT ); Mon, 23 May 2016 11:23:40 -0400 Received: from mx1.redhat.com ([209.132.183.28]:51750 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752206AbcEWPXh (ORCPT ); Mon, 23 May 2016 11:23:37 -0400 Date: Mon, 23 May 2016 10:23:35 -0500 From: Josh Poimboeuf To: Brian Gerst Cc: x86@kernel.org, linux-kernel@vger.kernel.org, Ingo Molnar , "H. Peter Anvin" , Denys Vlasenko , Andy Lutomirski , Borislav Petkov , Thomas Gleixner Subject: Re: [PATCH 4/4] x86: Pass kernel thread parameters in fork_frame Message-ID: <20160523152335.gax7icvauxohyf3o@treble> References: <1463846691-18498-1-git-send-email-brgerst@gmail.com> <1463846691-18498-5-git-send-email-brgerst@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <1463846691-18498-5-git-send-email-brgerst@gmail.com> User-Agent: Mutt/1.6.0.1 (2016-04-01) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Mon, 23 May 2016 15:23:36 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, May 21, 2016 at 12:04:51PM -0400, Brian Gerst wrote: > --- a/arch/x86/entry/entry_64.S > +++ b/arch/x86/entry/entry_64.S > @@ -405,37 +405,29 @@ END(__switch_to_asm) > * A newly forked process directly context switches into this address. > * > * rax: prev task we switched from > + * rbx: kernel thread func > + * r12: kernel thread arg > */ > ENTRY(ret_from_fork) > movq %rax, %rdi > call schedule_tail /* rdi: 'prev' task parameter */ > > - testb $3, CS(%rsp) /* from kernel_thread? */ > + testq %rbx, %rbx /* from kernel_thread? */ > jnz 1f > > - /* > - * We came from kernel_thread. This code path is quite twisted, and > - * someone should clean it up. > - * > - * copy_thread_tls stashes the function pointer in RBX and the > - * parameter to be passed in RBP. The called function is permitted > - * to call do_execve and thereby jump to user mode. > - */ > - movq RBP(%rsp), %rdi > - call *RBX(%rsp) > - movq %rax, RAX(%rsp) > - > - /* > - * Fall through as though we're exiting a syscall. This makes a > - * twisted sort of sense if we just called do_execve. > - */ > - > -1: > +2: > movq %rsp, %rdi > call syscall_return_slowpath /* returns with IRQs disabled */ > TRACE_IRQS_ON /* user mode is traced as IRQS on */ > SWAPGS > jmp restore_regs_and_iret > + > +1: > + /* kernel thread */ > + movq %r12, %rdi > + call *%rbx > + movq %rax, RAX(%rsp) > + jmp 2b > END(ret_from_fork) It seems really surprising that a kernel thread would be returning to user space. It would probably be a good idea to preserve the existing comments about that. -- Josh