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=-6.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED 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 BCCABC388F9 for ; Mon, 16 Nov 2020 22:08:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7FFE3223C7 for ; Mon, 16 Nov 2020 22:08:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726287AbgKPWIk (ORCPT ); Mon, 16 Nov 2020 17:08:40 -0500 Received: from mail.kernel.org ([198.145.29.99]:46870 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726016AbgKPWIk (ORCPT ); Mon, 16 Nov 2020 17:08:40 -0500 Received: from trantor (unknown [2.26.170.190]) (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 06EEA223BF; Mon, 16 Nov 2020 22:08:36 +0000 (UTC) Date: Mon, 16 Nov 2020 22:08:34 +0000 From: Catalin Marinas To: "Eric W. Biederman" Cc: Peter Collingbourne , Evgenii Stepanov , Kostya Serebryany , Vincenzo Frascino , Dave Martin , Will Deacon , Oleg Nesterov , "James E.J. Bottomley" , Linux ARM , Kevin Brodsky , Andrey Konovalov , linux-api@vger.kernel.org, Helge Deller , David Spickett Subject: Re: [PATCH v16 6/6] arm64: expose FAR_EL1 tag bits in siginfo Message-ID: References: <81e1307108ca8ea67aa1060f6f47b34a507410f1.1605235762.git.pcc@google.com> <87d00dge6e.fsf@x220.int.ebiederm.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87d00dge6e.fsf@x220.int.ebiederm.org> Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org On Mon, Nov 16, 2020 at 03:55:05PM -0600, Eric W. Biederman wrote: > Catalin Marinas writes: > > On Thu, Nov 12, 2020 at 06:53:36PM -0800, Peter Collingbourne wrote: > >> diff --git a/Documentation/arm64/tagged-pointers.rst b/Documentation/arm64/tagged-pointers.rst > >> index eab4323609b9..19d284b70384 100644 > >> --- a/Documentation/arm64/tagged-pointers.rst > >> +++ b/Documentation/arm64/tagged-pointers.rst > >> @@ -53,12 +53,25 @@ visibility. > >> Preserving tags > >> --------------- > >> > >> -Non-zero tags are not preserved when delivering signals. This means that > >> -signal handlers in applications making use of tags cannot rely on the > >> -tag information for user virtual addresses being maintained for fields > >> -inside siginfo_t. One exception to this rule is for signals raised in > >> -response to watchpoint debug exceptions, where the tag information will > >> -be preserved. > >> +When delivering signals, non-zero tags are not preserved in > >> +siginfo.si_addr unless the flag SA_EXPOSE_TAGBITS was set in > >> +sigaction.sa_flags when the signal handler was installed. This means > >> +that signal handlers in applications making use of tags cannot rely > >> +on the tag information for user virtual addresses being maintained > >> +in these fields unless the flag was set. > >> + > >> +Due to architecture limitations, bits 63:60 of the fault address > >> +are not preserved in response to synchronous tag check faults > >> +(SEGV_MTESERR) even if SA_EXPOSE_TAGBITS was set. Applications should > >> +treat the values of these bits as undefined in order to accommodate > >> +future architecture revisions which may preserve the bits. > > > > If future architecture versions will preserve these bits, most likely > > we'll add a new HWCAP bit so that the user knows what's going on. But > > the user shouldn't rely on them being 0, just in case. > > > >> +For signals raised in response to watchpoint debug exceptions, the > >> +tag information will be preserved regardless of the SA_EXPOSE_TAGBITS > >> +flag setting. > >> + > >> +Non-zero tags are never preserved in sigcontext.fault_address > >> +regardless of the SA_EXPOSE_TAGBITS flag setting. > > > > We could've done it the other way around (fault_address tagged, si_addr > > untagged) but that would be specific to arm64, so I think we should > > solve it for other architectures that implement (or plan to) tagging. > > The fault_address in the arm64 sigcontext was an oversight, we should > > have removed it but when we realised it was already ABI. > > > > Anyway, I'm fine with the arm64 changes here: > > > > Reviewed-by: Catalin Marinas > > > > With Eric's ack, I'm happy to take the series through the arm64 tree, > > otherwise Eric's tree is fine as well. > > In general I am fine with the last two patches. > > I want to understand where the value for SA_UNSUPPORTED comes from, and > while I have good answers I am still digesting the question of if > SA_EXPOSE_TAGBITS should be implemented in the arch specific header or > in a generic header. I quite agree it should have a generic > definition/implementation. I just don't know if it makes sense to make > the value available to userspace if the architecture does not have > tagbits. Mostly my concern is about bit consumption as we only have > 30ish sigaction bits. An alternative would be to make this opt-in per process (or thread) based on a prctl() call. We already have one for PR_TAGGED_ADDR_ENABLE to allow tagged addresses from user at the syscall ABI level. Another bit in there would allow si_addr to be tagged. The disadvantage is that this is quite coarse control affecting other signal handlers. > I will follow with my acks when I have resolved those issues. Thanks. -- Catalin