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=-11.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,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 80F40C433DF for ; Mon, 24 Aug 2020 13:42:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 65DAE204FD for ; Mon, 24 Aug 2020 13:42:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727104AbgHXNmJ (ORCPT ); Mon, 24 Aug 2020 09:42:09 -0400 Received: from foss.arm.com ([217.140.110.172]:34246 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728000AbgHXNlw (ORCPT ); Mon, 24 Aug 2020 09:41:52 -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 600BF1FB; Mon, 24 Aug 2020 06:41:51 -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 801CB3F66B; Mon, 24 Aug 2020 06:41:49 -0700 (PDT) Date: Mon, 24 Aug 2020 14:41:47 +0100 From: Dave Martin To: Peter Collingbourne Cc: Linux ARM , Parisc List , Catalin Marinas , Kevin Brodsky , Oleg Nesterov , "James E.J. Bottomley" , Kostya Serebryany , "Eric W. Biederman" , Andrey Konovalov , David Spickett , Vincenzo Frascino , Will Deacon , Evgenii Stepanov , Richard Henderson Subject: Re: [PATCH v9 4/6] signal: define the SA_UNSUPPORTED bit in sa_flags Message-ID: <20200824134146.GL6642@arm.com> References: <20200819145112.GG6642@arm.com> 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 Wed, Aug 19, 2020 at 05:23:25PM -0700, Peter Collingbourne wrote: > On Wed, Aug 19, 2020 at 7:51 AM Dave Martin wrote: > > > > 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. > > Hmm. Maybe it should be named "SA_UNKNOWN" to mean that the bit will > always be "unknown" to the kernel in the sense that it shall be > treated in the same way as any other "unknown" bit. Then we can define > the kernel's behavior in terms of what happens if a bit is "known". I > don't know if this is just shuffling terms around though. At any rate, > this seems like a problem to be solved with documentation. > > > Since the main (only) purpose of this bit will be to check whether > > I wouldn't necessarily say that it is the only purpose. If another new > sa_flags bit were to be introduced in the future, SA_UN(whatever) > could be used to detect kernel support for that bit in the same way as > SA_XFLAGS. > > > 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. > > I'm not sure about this. I personally think that it would be clearer > to keep the flags orthogonal. Fair enough. I didn't think my approach was a whole lot better tbh. Cheers ---Dave