Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752863AbcD3AK2 (ORCPT ); Fri, 29 Apr 2016 20:10:28 -0400 Received: from mail-ob0-f177.google.com ([209.85.214.177]:36692 "EHLO mail-ob0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752786AbcD3AKZ (ORCPT ); Fri, 29 Apr 2016 20:10:25 -0400 MIME-Version: 1.0 In-Reply-To: <20160429232746.3lp6qlwojdmm52fb@treble> References: <9268772b31cc7bc4dc40c617e3baf45e07322145.1461875890.git.jpoimboe@redhat.com> <20160429205047.t6k7rsskx7mhvwxg@treble> <20160429232746.3lp6qlwojdmm52fb@treble> From: Andy Lutomirski Date: Fri, 29 Apr 2016 17:10:05 -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: Jiri Kosina , Ingo Molnar , X86 ML , Heiko Carstens , "linux-s390@vger.kernel.org" , live-patching@vger.kernel.org, Michael Ellerman , Chris J Arges , linuxppc-dev@lists.ozlabs.org, Jessica Yu , Petr Mladek , Jiri Slaby , Vojtech Pavlik , "linux-kernel@vger.kernel.org" , Miroslav Benes , Peter Zijlstra 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: 3429 Lines: 76 On Apr 29, 2016 4:27 PM, "Josh Poimboeuf" wrote: > > On Fri, Apr 29, 2016 at 02:38:02PM -0700, Andy Lutomirski wrote: > > 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 > > I don't see how collect_syscall() can be called for the per-cpu idle > "swapper" tasks (which is what the above code affects). They don't have > pids or /proc entries so you can't do /proc//syscall on them. If so, then never mind. --Andy > > -- > Josh