From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on archive.lwn.net X-Spam-Level: X-Spam-Status: No, score=-5.8 required=5.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI autolearn=unavailable autolearn_force=no version=3.4.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by archive.lwn.net (Postfix) with ESMTP id 11A287D08E for ; Sat, 13 Oct 2018 09:34:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726870AbeJMRKs (ORCPT ); Sat, 13 Oct 2018 13:10:48 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:32966 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726287AbeJMRKr (ORCPT ); Sat, 13 Oct 2018 13:10:47 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 8C2C9ED1; Sat, 13 Oct 2018 02:34:20 -0700 (PDT) Received: from mbp (usa-sjc-mx-foss1.foss.arm.com [217.140.101.70]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 80E513F5D3; Sat, 13 Oct 2018 02:34:14 -0700 (PDT) Date: Sat, 13 Oct 2018 10:34:11 +0100 From: Catalin Marinas To: Eugene Syromiatnikov Cc: linux-doc@vger.kernel.org, Szabolcs Nagy , Palmer Dabbelt , Heiko Carstens , Yury Norov , Pavel Machek , Philipp Tomsich , Joseph Myers , linux-arch@vger.kernel.org, Steve Ellcey , Prasun Kapoor , Andreas Schwab , Alexander Graf , Bamvor Zhangjian , Geert Uytterhoeven , Dave Martin , Adam Borowski , Manuel Montezelo , James Hogan , Chris Metcalf , Arnd Bergmann , Andrew Pinski , Lin Yongting , Alexey Klimov , Wookey , Mark Brown , linux-arm-kernel@lists.infradead.org, Maxim Kuvyrkov , Florian Weimer , linux-api@vger.kernel.org, Nathan_Lynch , linux-kernel@vger.kernel.org, James Morse , Ramana Radhakrishnan , Martin Schwidefsky , "David S . Miller" , Christoph Muellner Subject: Re: [PATCH v9 00/24] ILP32 for ARM64 Message-ID: <20181013093411.o3id6yzkspsxr5jt@mbp> References: <20180516081910.10067-1-ynorov@caviumnetworks.com> <20180724173957.GA22106@yury-thinkpad> <20181010141017.GA2881@asgard.redhat.com> <20181010153655.GA212880@arrakis.emea.arm.com> <20181013021416.GE21972@asgard.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181013021416.GE21972@asgard.redhat.com> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On Sat, Oct 13, 2018 at 04:14:16AM +0200, Eugene Syromiatnikov wrote: > On Wed, Oct 10, 2018 at 04:36:56PM +0100, Catalin Marinas wrote: > > On Wed, Oct 10, 2018 at 04:10:21PM +0200, Eugene Syromiatnikov wrote: > > > I have some questions regarding AArch64 ILP32 implementation for which I > > > failed to find an answer myself: > > > * How ptrace() tracer is supposed to distinguish between ILP32 and LP64 > > > tracees? For MIPS N32 and x32 this is possible based on syscall > > > number, but for AArch64 ILP32 I do not see such a sign. There's also > > > ARM_ip is employed for signalling entering/exiting, I wonder whether > > > it's possible to employ it also for signalling tracee's personality. > > > > With the current implementation, I don't think you can distinguish. From > > the kernel perspective, the register set is the same. What is the > > use-case for this? > > Err, a ptrace()-based tracer trying to trace a process, for example? I first thought it wouldn't matter for ptrace-based tracers since the syscall numbers are (mostly) the same. But the arguments layout in register is indeed different, so I see your point now about having to distinguish. > > We could add a new regset to expose the ILP32 state (NT_ARM_..., I can't > > think of a name now but probably not PER* as this implies PER_LINUX_... > > which is independent from TIF_32BIT_*). > > So that would require an additional ptrace() call on each syscall stop, > is that correct? The ILP32 state does not change at run-time, so it could only do a ptrace() call once and save the information. No need to re-read it on each syscall stop. We could set a high bit in the syscall number reported to the ptrace caller (though not changing the syscall ABI) but I haven't thought of other consequences. For example, can the ptrace caller change the syscall number? > > > * What's the reasoning behind capping syscall arguments to 32 bit? x32 > > > and MIPS N32 do not have such a restriction (and do not need special > > > wrappers for syscalls that pass 64-bit values as a result, except > > > when they do, as it is the case for preadv2 on x32); moreover, that > > > would lead to insurmountable difficulties for AArch64 ILP32 tracers > > > that try to trace LP64 tracees, as it would be impossible to pass > > > 64-bit addresses to process_vm_{read,write} or ptrace PEEK/POKE. > > > > We've attempted in earlier versions to allow a mix of 32 and 64-bit > > register values from ILP32 but it got pretty complicated. The entry code > > would need to know which registers need zeroing of the top 32-bit > > If kernel specifies 64-bit wide registers for syscalls, then it's the > caller's (libc's) responsibility to properly sign-extend arguments when > needed, and glibc, for example, already has proper type definitions that > aimed to handle this. We tried, see my other reply. -- Catalin