From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 35136 invoked by alias); 19 Oct 2017 11:09:04 -0000 Mailing-List: contact gdb-patches-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-patches-owner@sourceware.org Received: (qmail 35125 invoked by uid 89); 19 Oct 2017 11:09:03 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-1.9 required=5.0 tests=BAYES_00,RP_MATCHES_RCVD,SPF_HELO_PASS autolearn=ham version=3.3.2 spammy=H*M:1bd6 X-HELO: mx1.redhat.com Received: from mx1.redhat.com (HELO mx1.redhat.com) (209.132.183.28) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Thu, 19 Oct 2017 11:09:02 +0000 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 6E3D17F7A9; Thu, 19 Oct 2017 11:09:01 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com 6E3D17F7A9 Authentication-Results: ext-mx04.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx04.extmail.prod.ext.phx2.redhat.com; spf=fail smtp.mailfrom=palves@redhat.com Received: from [127.0.0.1] (ovpn04.gateway.prod.ext.ams2.redhat.com [10.39.146.4]) by smtp.corp.redhat.com (Postfix) with ESMTP id A7DF6AFD79; Thu, 19 Oct 2017 11:09:00 +0000 (UTC) Subject: Re: [PATCH] [AArch64 Linux] Get rid of top byte from tagged address To: Yao Qi References: <1508400527-20718-1-git-send-email-yao.qi@linaro.org> <561ea277-4b4c-ae82-01e1-1cde96cb54f2@redhat.com> <86po9jv29n.fsf@gmail.com> Cc: gdb-patches@sourceware.org From: Pedro Alves Message-ID: <98b855a4-77df-1bd6-d20d-5b2611cc8f83@redhat.com> Date: Thu, 19 Oct 2017 11:09:00 -0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 MIME-Version: 1.0 In-Reply-To: <86po9jv29n.fsf@gmail.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-SW-Source: 2017-10/txt/msg00602.txt.bz2 > Pedro Alves writes: > >> I'm fine with doing this if it's what arm/linaro folks want, >> though personally (with absolutely no experience in this) I have >> reservations about whether stripping the top byte in the special >> case of memory accesses is a good idea, since it may puzzle folks >> when they pass such pointers/addresses in registers/structures and >> things don't magically work then (and then gdb masks the problem when >> folks try to diagnose it, as in "but I can access the object >> via "p *s->ptr", why isn't this working??? bad gdb."). >> >> So I think this should be documented in the manual somewhere. > > I don't understand how does GDB affect the program. ARMv8 architecture > supports tagged address for data values, and top byte of virtual address > is ignored in some cases, I didn't say that GDB affects the program. From the kernel document: ~~~ All interpretation of userspace memory addresses by the kernel assumes an address tag of 0x00. This includes, but is not limited to, addresses found in: - pointer arguments to system calls, including pointers in structures passed to system calls, ~~~ This means with something like: #define tagptr(PTR) \ ((typeof (PTR)) ((uintptr_t) (PTR) | 0xf000000000000000ULL)) strcat (buf, "hello\n"); char *ptr = tagptr(buf); // assume this is hidden from view. write (1, ptr, 6); // kernel rejects this. and then the user might be puzzled because stepping through that code: (gdb) print ptr (gdb) print ptr[0] etc. works without error. Same with iovec/readv, ioctl, etc., any system call that takes a pointer argument. Thanks, Pedro Alves