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=-12.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 D3F51C433DF for ; Mon, 24 Aug 2020 13:43:34 +0000 (UTC) Received: from merlin.infradead.org (merlin.infradead.org [205.233.59.134]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id A2E93204FD for ; Mon, 24 Aug 2020 13:43:34 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="vSHkWc3w" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A2E93204FD Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=merlin.20170209; h=Sender:Content-Transfer-Encoding: Content-Type:Cc:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References:Message-ID: Subject:To:From:Date:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=LBPlDsIpvgR99mj092InDmcCS14MztFhxA5YEq8MLMM=; b=vSHkWc3wLJKo3xUsNM8dGRstx Vmkkynn27xgEvMYYSQ4NkJW1QQqOnMi/eFbVhhjUMNAzRpkNQl0qLwG/m4ryK5+GmsbTFg0fI5QN9 LGlfQyOaPgmSJGLQxFDrihH0t1Ueo4Ru4fjtJ3VCJIeVggbayOcyzcGsYWosHXqqE8rCLxQSVzNkc aWkq+yNKi2Z+c0zjp1XCJKwMpQ14st58jt3AUEP8m6BxwSwIO4kED1QWji8BTgYtd3khJ+BUSaFs4 5mw6X+RnIvIQ2yinYD4WDgLKxod39BLiuiFVqe+IAMEQaWSWGE29TvXQQaU80OLIbUd0Uhc0Mmypb t6CnDBeuw==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kACjX-0004MH-G5; Mon, 24 Aug 2020 13:41:59 +0000 Received: from foss.arm.com ([217.140.110.172]) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kACjV-0004Lo-2n for linux-arm-kernel@lists.infradead.org; Mon, 24 Aug 2020 13:41:58 +0000 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 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-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200824_094157_220716_4DB73026 X-CRM114-Status: GOOD ( 41.09 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Parisc List , Catalin Marinas , Kevin Brodsky , Oleg Nesterov , Evgenii Stepanov , "James E.J. Bottomley" , Kostya Serebryany , "Eric W. Biederman" , Andrey Konovalov , David Spickett , Vincenzo Frascino , Will Deacon , Linux ARM , Richard Henderson Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel