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=-9.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 3AAAEC63699 for ; Sat, 14 Nov 2020 13:53:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 081DF22261 for ; Sat, 14 Nov 2020 13:53:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726977AbgKNNxx (ORCPT ); Sat, 14 Nov 2020 08:53:53 -0500 Received: from out03.mta.xmission.com ([166.70.13.233]:43080 "EHLO out03.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726964AbgKNNxt (ORCPT ); Sat, 14 Nov 2020 08:53:49 -0500 Received: from in01.mta.xmission.com ([166.70.13.51]) by out03.mta.xmission.com with esmtps (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (Exim 4.93) (envelope-from ) id 1kdvzq-00BdDW-Pe; Sat, 14 Nov 2020 06:53:42 -0700 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95] helo=x220.xmission.com) by in01.mta.xmission.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.87) (envelope-from ) id 1kdvzq-0001Zp-0u; Sat, 14 Nov 2020 06:53:42 -0700 From: ebiederm@xmission.com (Eric W. Biederman) To: Peter Collingbourne Cc: Catalin Marinas , 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 References: <3581410892be6851d804bbbb84fccf06073f1262.1605235762.git.pcc@google.com> Date: Sat, 14 Nov 2020 07:53:28 -0600 In-Reply-To: <3581410892be6851d804bbbb84fccf06073f1262.1605235762.git.pcc@google.com> (Peter Collingbourne's message of "Thu, 12 Nov 2020 18:53:35 -0800") Message-ID: <878sb4nixz.fsf@x220.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1kdvzq-0001Zp-0u;;;mid=<878sb4nixz.fsf@x220.int.ebiederm.org>;;;hst=in01.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX1/FYdJI0tFmQTLSF0nxOUdOl1juCDlK/nA= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH v16 5/6] signal: define the SA_UNSUPPORTED bit in sa_flags X-SA-Exim-Version: 4.2.1 (built Thu, 05 May 2016 13:38:54 -0600) X-SA-Exim-Scanned: Yes (on in01.mta.xmission.com) Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org Peter Collingbourne writes: > Define a sa_flags bit, SA_UNSUPPORTED, which will never be supported > in the uapi. The purpose of this flag bit is to allow userspace to > distinguish an old kernel that does not clear unknown sa_flags bits > from a kernel that supports every flag bit. > > In other words, if userspace does something like: > > act.sa_flags |= SA_UNSUPPORTED; > sigaction(SIGSEGV, &act, 0); > sigaction(SIGSEGV, 0, &oldact); > > and finds that SA_UNSUPPORTED remains set in oldact.sa_flags, it means > that the kernel cannot be trusted to have cleared unknown flag bits > from sa_flags, so no assumptions about flag bit support can be made. > > Signed-off-by: Peter Collingbourne > Reviewed-by: Dave Martin > Link: https://linux-review.googlesource.com/id/Ic2501ad150a3a79c1cf27fb8c99be342e9dffbcb > --- > v11: > - clarify the commit message > > include/uapi/asm-generic/signal-defs.h | 7 +++++++ > kernel/signal.c | 6 ++++++ > 2 files changed, 13 insertions(+) > > diff --git a/include/uapi/asm-generic/signal-defs.h b/include/uapi/asm-generic/signal-defs.h > index 493953fe319b..0126ebda4d31 100644 > --- a/include/uapi/asm-generic/signal-defs.h > +++ b/include/uapi/asm-generic/signal-defs.h > @@ -14,6 +14,12 @@ > * SA_RESTART flag to get restarting signals (which were the default long ago) > * SA_NODEFER prevents the current signal from being masked in the handler. > * SA_RESETHAND clears the handler when the signal is delivered. > + * SA_UNSUPPORTED is a flag bit that will never be supported. Kernels from > + * before the introduction of SA_UNSUPPORTED did not clear unknown bits from > + * sa_flags when read using the oldact argument to sigaction and rt_sigaction, > + * so this bit allows flag bit support to be detected from userspace while > + * allowing an old kernel to be distinguished from a kernel that supports every > + * flag bit. > * > * SA_ONESHOT and SA_NOMASK are the historical Linux names for the Single > * Unix names RESETHAND and NODEFER respectively. > @@ -42,6 +48,7 @@ > #ifndef SA_RESETHAND > #define SA_RESETHAND 0x80000000 > #endif > +#define SA_UNSUPPORTED 0x00000400 Why this value and why not in numerical order with the other flags? At the very least not being in order with the other bits makes it a little easier to overlook it and define something at that position. Eric > #define SA_NOMASK SA_NODEFER > #define SA_ONESHOT SA_RESETHAND > diff --git a/kernel/signal.c b/kernel/signal.c > index 8f5bd12ee41b..8f34819e80de 100644 > --- a/kernel/signal.c > +++ b/kernel/signal.c > @@ -3985,6 +3985,12 @@ int do_sigaction(int sig, struct k_sigaction *act, struct k_sigaction *oact) > if (oact) > *oact = *k; > > + /* > + * Make sure that we never accidentally claim to support SA_UNSUPPORTED, > + * e.g. by having an architecture use the bit in their uapi. > + */ > + BUILD_BUG_ON(UAPI_SA_FLAGS & SA_UNSUPPORTED); > + > /* > * Clear unknown flag bits in order to allow userspace to detect missing > * support for flag bits and to allow the kernel to use non-uapi bits