From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 76A74C433E0 for ; Tue, 19 May 2020 08:15:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 44F8E207D8 for ; Tue, 19 May 2020 08:15:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589876158; bh=yJ7fQn1NM95tpi8PU4m61fRGPoc8lwpzDb11CKAj0so=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=vPqW1YSsIEHwpzSqwdwLNa7OVe8rSVMBD+1htAmOjWh+LZpkEo03S41B+fzZ03iA7 Ar5JJq7XcQrHj2H4FDTm088mS0G2Fs0C5YOEV/ExsFNl6kdhUJn+MlxxQWfIAOKX+r useaiI5CxapTFBOdS603eesMzUaZPPbEEDjs50/U= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726595AbgESIP5 (ORCPT ); Tue, 19 May 2020 04:15:57 -0400 Received: from mail.kernel.org ([198.145.29.99]:57078 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726318AbgESIP5 (ORCPT ); Tue, 19 May 2020 04:15:57 -0400 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 504B420709; Tue, 19 May 2020 08:15:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589876156; bh=yJ7fQn1NM95tpi8PU4m61fRGPoc8lwpzDb11CKAj0so=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=DQG3fiAM/dNkN3TrVzzD/YLGD7Yp+f9/SSBXWXOp71gI4M9jQba3W7vhqiHk0PNu0 WgvlAah532itEJXBqUtN54gGvhJZu+xpfFrQwxtygVx3L+C3LSu0Y1xIH68nbfgSf1 F7HCHXgWNOuwJKy0joGSnNyhY/KIOi/q3jmKHOb0= Date: Tue, 19 May 2020 09:15:51 +0100 From: Will Deacon To: Keno Fischer Cc: linux-arm-kernel@lists.infradead.org, Catalin Marinas , Oleg Nesterov , Linux Kernel Mailing List , Kyle Huey Subject: Re: arm64: Register modification during syscall entry/exit stop Message-ID: <20200519081551.GA9980@willie-the-truck> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Keno, On Mon, May 18, 2020 at 09:05:30PM -0400, Keno Fischer wrote: > Continuing my theme of "weird things I encounter > while trying to use ptrace on arm64", I ran into the > effect of the following code in the syscall entry/exit > reporting: > > ``` > /* > * A scratch register (ip(r12) on AArch32, x7 on AArch64) is > * used to denote syscall entry/exit: > */ > regno = (is_compat_task() ? 12 : 7); > saved_reg = regs->regs[regno]; > regs->regs[regno] = dir; > ``` > > This seems very weird to me. I can't think of any > other architecture that does something similar > (other than unicore32 apparently, but the ptrace > support there seems like it might have just been > copied from ARM). I'm able to work around this > in my application, but it adds another stumbling block. Yes, we inherited this from ARM and I think strace relies on it. In hindsight, it is a little odd, although x7 is a parameter register in the PCS and so it won't be live on entry to a system call. > Some examples of things that happen: > - Writes to x7 during syscall exit stops are ignored, so > if the ptracer tries to emulate a setjmp-type thing, it > might miss this register (ptracers sometimes like to do > this to manually serialize execution between different > threads, puppeteering a single thread of execution > between different register states). > - Reads from x7 are incorrect, so if the ptracer saves > a register state and later tries to set it back to the task, > it may get x7 incorrect, but user space may be expecting > the register to be preserved (when might this happen? - > consider a ptracer that wants to modify some syscall > arguments, it modifies the arguments, restarts the syscall > but then incurs a signal, so it tries to restore the original > registers to let userspace deal with the signal without > being confused - expect signal traps don't ignore x7 > modifications, so x7 may have been unexpectedly > modified). > - We now have seccomp traps, which kind of look and > act like syscall-entry traps, but don't have this behavior, > so it's not particularly reliable for ptracers to use. > > Furthermore, it seems unnecessary to me on modern > kernels. We now have PTRACE_GET_SYSCALL_INFO, > which exposes this information without lying to the ptracer > about the tracee's registers. > > I understand, we can't just change this, since people may > be relying on it, but I would like to propose adding a ptrace > option (PTRACE_O_ARM_REGSGOOD?) that turns this > behavior off. Now, I don't think we currently have any other > arch-specific ptrace options, so maybe there is a different > option that would be preferable (e.g. could be a different > regset), but I do think it would be good to have a way to > operate on the real x7 value. As I said, I can work around it, > but hopefully I will be able to save a future implementer > some headache. I'm not opposed to extending ptrace so that we can try to wean people off this interface, but I think we need some concrete situations where the current behaviour actually causes a problem. Although the examples you've listed above are interesting, I don't see why x7 is important in any of them (and we only support up to 6 system call arguments). Will