Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752872AbcD2Via (ORCPT ); Fri, 29 Apr 2016 17:38:30 -0400 Received: from mail-oi0-f50.google.com ([209.85.218.50]:36579 "EHLO mail-oi0-f50.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751710AbcD2Vi2 (ORCPT ); Fri, 29 Apr 2016 17:38:28 -0400 MIME-Version: 1.0 In-Reply-To: <20160429205047.t6k7rsskx7mhvwxg@treble> References: <9268772b31cc7bc4dc40c617e3baf45e07322145.1461875890.git.jpoimboe@redhat.com> <20160429205047.t6k7rsskx7mhvwxg@treble> From: Andy Lutomirski Date: Fri, 29 Apr 2016 14:38:02 -0700 Message-ID: Subject: Re: [RFC PATCH v2 03/18] x86/asm/head: standardize the bottom of the stack for idle tasks To: Josh Poimboeuf Cc: Andy Lutomirski , Jessica Yu , Jiri Kosina , Miroslav Benes , Ingo Molnar , Peter Zijlstra , Michael Ellerman , Heiko Carstens , live-patching@vger.kernel.org, "linux-kernel@vger.kernel.org" , X86 ML , linuxppc-dev@lists.ozlabs.org, "linux-s390@vger.kernel.org" , Vojtech Pavlik , Jiri Slaby , Petr Mladek , Chris J Arges Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2845 Lines: 70 On Fri, Apr 29, 2016 at 1:50 PM, Josh Poimboeuf wrote: > On Fri, Apr 29, 2016 at 12:39:16PM -0700, Andy Lutomirski wrote: >> On Thu, Apr 28, 2016 at 1:44 PM, Josh Poimboeuf wrote: >> > Thanks to all the recent x86 entry code refactoring, most tasks' kernel >> > stacks start at the same offset right above their saved pt_regs, >> > regardless of which syscall was used to enter the kernel. That creates >> > a nice convention which makes it straightforward to identify the >> > "bottom" of the stack, which can be useful for stack walking code which >> > needs to verify the stack is sane. >> > >> > However there are still a few types of tasks which don't yet follow that >> > convention: >> > >> > 1) CPU idle tasks, aka the "swapper" tasks >> > >> > 2) freshly forked TIF_FORK tasks which don't have a stack at all >> > >> > Make the idle tasks conform to the new stack bottom convention by >> > starting their stack at a sizeof(pt_regs) offset from the end of the >> > stack page. >> > >> > Signed-off-by: Josh Poimboeuf >> > --- >> > arch/x86/kernel/head_64.S | 7 ++++--- >> > 1 file changed, 4 insertions(+), 3 deletions(-) >> > >> > diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S >> > index 6dbd2c0..0b12311 100644 >> > --- a/arch/x86/kernel/head_64.S >> > +++ b/arch/x86/kernel/head_64.S >> > @@ -296,8 +296,9 @@ ENTRY(start_cpu) >> > * REX.W + FF /5 JMP m16:64 Jump far, absolute indirect, >> > * address given in m16:64. >> > */ >> > - movq initial_code(%rip),%rax >> > - pushq $0 # fake return address to stop unwinder >> > + call 1f # put return address on stack for unwinder >> > +1: xorq %rbp, %rbp # clear frame pointer >> > + movq initial_code(%rip), %rax >> > pushq $__KERNEL_CS # set correct cs >> > pushq %rax # target address in negative space >> > lretq >> > @@ -325,7 +326,7 @@ ENDPROC(start_cpu0) >> > GLOBAL(initial_gs) >> > .quad INIT_PER_CPU_VAR(irq_stack_union) >> > GLOBAL(initial_stack) >> > - .quad init_thread_union+THREAD_SIZE-8 >> > + .quad init_thread_union + THREAD_SIZE - SIZEOF_PTREGS >> >> As long as you're doing this, could you also set orig_ax to -1? I >> remember running into some oddities resulting from orig_ax containing >> garbage at some point. > > I assume you mean to initialize the orig_rax value in the pt_regs at the > bottom of the stack of the idle task? > > How could that cause a problem? Since the idle task never returns from > a system call, I'd assume that memory never gets accessed? > Look at collect_syscall in lib/syscall.c > -- > Josh -- Andy Lutomirski AMA Capital Management, LLC