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=-11.2 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,USER_AGENT_SANE_1 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 4E363C433DF for ; Wed, 19 Aug 2020 14:51:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 176B72083B for ; Wed, 19 Aug 2020 14:51:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728477AbgHSOv2 (ORCPT ); Wed, 19 Aug 2020 10:51:28 -0400 Received: from foss.arm.com ([217.140.110.172]:38902 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728370AbgHSOvS (ORCPT ); Wed, 19 Aug 2020 10:51:18 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id AEEDB1045; Wed, 19 Aug 2020 07:51:16 -0700 (PDT) Received: from arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id CD2273F71F; Wed, 19 Aug 2020 07:51:14 -0700 (PDT) Date: Wed, 19 Aug 2020 15:51:12 +0100 From: Dave Martin To: Peter Collingbourne Cc: Catalin Marinas , Evgenii Stepanov , Kostya Serebryany , Vincenzo Frascino , Will Deacon , Oleg Nesterov , "Eric W. Biederman" , "James E.J. Bottomley" , linux-parisc@vger.kernel.org, Andrey Konovalov , Kevin Brodsky , David Spickett , Linux ARM , Richard Henderson Subject: Re: [PATCH v9 4/6] signal: define the SA_UNSUPPORTED bit in sa_flags Message-ID: <20200819145112.GG6642@arm.com> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-parisc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-parisc@vger.kernel.org On Mon, Aug 17, 2020 at 08:33:49PM -0700, Peter Collingbourne wrote: > This bit 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 finds that this bit 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 > --- > View this change in Gerrit: https://linux-review.googlesource.com/q/Ic2501ad150a3a79c1cf27fb8c99be342e9dffbcb > > 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 91000b6b97e0..c30a9c1a77b2 100644 > --- a/include/uapi/asm-generic/signal-defs.h > +++ b/include/uapi/asm-generic/signal-defs.h > @@ -13,6 +13,12 @@ > * SA_RESETHAND clears the handler when the signal is delivered. > * SA_NOCLDWAIT flag on SIGCHLD to inhibit zombies. > * SA_NODEFER prevents the current signal from being masked in the handler. > + * 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 @@ > * The following bits are used in architecture-specific SA_* definitions and > * should be avoided for new generic flags: 3, 4, 5, 6, 7, 8, 9, 16, 24, 25, 26. > */ > +#define SA_UNSUPPORTED 0x00000400 This concept confused me a bit initially, since in a sense this flag is supported, just with a rather peculiar meaning. Since the main (only) purpose of this bit will be to check whether SA_XFLAGS is actually supported, I wonder whether it makes sense to weld the two together, say: #define SA_REQUEST_XFLAGS 0x00000c00 #define SA_XFLAGS_MASK 0x00000c00 #define SA_HAVE_XFLAGS 0x00000800 This is a departure from the current style of definitions though. sa.sa_flags |= SA_REQUEST_XFLAGS; sigaction(..., &sa, &sa); if ((sa.sa_flags & SA_XFLAGS_MASK) == SA_HAVE_XFLAGS) /* xflags available */ This would require some juggling of the way SA_UAPI_FLAGS works though. Maybe not worth it, so long as the semantics get clearly documented. [...] Cheers ---Dave