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=-17.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_ADSP_CUSTOM_MED,DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_GIT 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 AA6F2C07E9A for ; Tue, 13 Jul 2021 01:41:01 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (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 6DBEB6023D for ; Tue, 13 Jul 2021 01:41:01 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 6DBEB6023D Authentication-Results: mail.kernel.org; dmarc=fail (p=reject dis=none) header.from=google.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=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:Cc:To:From:Subject:Mime-Version: Message-Id:Date:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:In-Reply-To: References:List-Owner; bh=n4s3VDTLKM+c9NzR7+f7BrVMOT6Bdc+MiAR7mktFpw0=; b=kO5 2b+QurF6gnb9/9YRF0CjCh2iqR5Wxzs60UypIyXDkC0ZquqmcA4LGKaWukVu0s5MqQkR4a+mWpQhB SCFqmwJJIX0Ai84N3um+7L3pytFg5zGL1VcT3XH9luklrbYgf11btVN8QstS7kVe3i8I3DRpwGKvI A/J8BXOgrNeLTDDul5w0+EGmtWcBXXFhYnZZYW8HAYP57NemeBqXl5VMf2uUFC73DIkAoy5MzGVbP FO18tBZxMrgWSiL85dktLYYrNqGFcLZkwFcmlXDXQZtokBXT5Z988A/FV+kLz/OBVU1VLTL437pLg 4x8ahJyyLGJz6ZefT1GSUiu+sVrhH/w==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1m37O8-008sOm-43; Tue, 13 Jul 2021 01:39:08 +0000 Received: from mail-qt1-x84a.google.com ([2607:f8b0:4864:20::84a]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1m37O4-008sNx-H6 for linux-arm-kernel@lists.infradead.org; Tue, 13 Jul 2021 01:39:06 +0000 Received: by mail-qt1-x84a.google.com with SMTP id c18-20020a05622a0592b0290251fb198592so12588402qtb.1 for ; Mon, 12 Jul 2021 18:39:02 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:message-id:mime-version:subject:from:to:cc; bh=S3Wii2PyRZQO2OtmxcTVnGt9js4+EH2ry2mkpjbEdlE=; b=nzQUAZ1GYvknrVwxrh0lgikVtRIXfhnrORO3nXyMwP0F7lB3NbGmszB3786OS4x6qf dnlbI9meyD7JO59yJNuyvMH3JuOWU1EDjFwo2T8wi+BK/9T+H6ALE0UPSvKsRp6LB3Tb AceWkGjDgT9uzEv1amInfcrUPUUW002B/xFdloCbRWzZ+2qwdLnumT9uBOe503s/Yst0 iwpVPrPpE9E7HkShQZIifHmLjU/ltgpgypSe4qNk+BKkmuVmeXAT2mwaZNRKeXqRTsce uDBUbnezlmSjSqNSU7XIiZWYPka6VbiYwamweHZPzXBKUlJRx3gBLO7i/zngqaRjGFm7 NfmA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:message-id:mime-version:subject:from:to:cc; bh=S3Wii2PyRZQO2OtmxcTVnGt9js4+EH2ry2mkpjbEdlE=; b=qUmxfuyXExHUIp7QrAbL6l4TsU2gVTUctK6GaiVVQcEO1xnIzAyvOnQQ7obzf0IM84 wRvWmVm5FGMrnOIn1oJ1ofitdWHVE1Z6eJQb8P5vUsAH2FRrxGZr7gnoWxM8g1uaFP9p wo2IsTALrsep8Oql0bRmCXRb8QvxEsp5LFDn6QOb8wuxlSKlS56IlXOIVQdh9EGzWWzr 9tQc0zL7aSqcgHPW8y330nQjPHlimd2JELZdo7ZTQhtQOYFh93klx5htWT+J69Tr7l3l fDDov4bHdFxMfCTwm9hwlZiJWo/9rF1uzrJTEVkn+hHaWHNQsWcUgfUfoqPYrcg1zTnd eFYg== X-Gm-Message-State: AOAM532AJjX/4n67IAHtH0kr7EiTmhjxjfoGnUZU7g4pjoRFXfG8zCyF mK+GQpUIgJfVGjzQGFKZvozavO0= X-Google-Smtp-Source: ABdhPJw1p94roLuMlaAO8me3nT8WYuLNraxllNXIec9KgctyxfyVbU1pJGp2xX1xFCgddw9Fcj6PJVg= X-Received: from pcc-desktop.svl.corp.google.com ([2620:15c:2ce:200:5ef:57d6:e87d:2958]) (user=pcc job=sendgmr) by 2002:a05:6214:68d:: with SMTP id r13mr2186600qvz.36.1626140340164; Mon, 12 Jul 2021 18:39:00 -0700 (PDT) Date: Mon, 12 Jul 2021 18:38:57 -0700 Message-Id: <20210713013857.3237634-1-pcc@google.com> Mime-Version: 1.0 X-Mailer: git-send-email 2.32.0.93.g670b81a890-goog Subject: [PATCH v3] sigaction.2: Document SA_EXPOSE_TAGBITS and the flag support detection protocol From: Peter Collingbourne To: Catalin Marinas , Evgenii Stepanov , Kostya Serebryany , Vincenzo Frascino , Dave Martin , Will Deacon , Oleg Nesterov , "Eric W. Biederman" , "James E.J. Bottomley" , Michael Kerrisk , Alejandro Colomar Cc: Peter Collingbourne , Linux ARM , Kevin Brodsky , Andrey Konovalov , linux-api@vger.kernel.org, Helge Deller , David Spickett , linux-man@vger.kernel.org X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210712_183904_633133_2DCF1826 X-CRM114-Status: GOOD ( 20.41 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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 Signed-off-by: Peter Collingbourne --- This feature landed back in 5.11, but the manpage update seems to have fallen through the cracks. Here's a v3 with the introducing version specified and with one formatting nit fixed. v3: - s/5.x/5.11/g - s/.IR/.I/ in one location v2: - fix formatting - address feedback from Dave man2/sigaction.2 | 125 +++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 125 insertions(+) diff --git a/man2/sigaction.2 b/man2/sigaction.2 index 57ad6418c..6b90982d4 100644 --- a/man2/sigaction.2 +++ b/man2/sigaction.2 @@ -261,6 +261,44 @@ This flag is meaningful only when establishing a signal handler. .\" .I sa_sigaction .\" field was added in Linux 2.1.86.) .\" +.TP +.BR SA_UNSUPPORTED +Used to dynamically probe for flag bit support. +.IP +If an attempt to register a handler succeeds with this flag set in +.I act->sa_flags +alongside other flags that are potentially unsupported by the kernel, +and an immediately subsequent +.BR sigaction () +call specifying the same signal number n and with non-NULL +.I oldact +yields +.B SA_UNSUPPORTED +.I clear +in +.IR oldact->sa_flags , +then +.I oldact->sa_flags +may be used as a bitmask +describing which of the potentially unsupported flags are, +in fact, supported. +See the section "Dynamically probing for flag bit support" +below for more details. +.TP +.BR SA_EXPOSE_TAGBITS " (since Linux 5.11)" +Normally, when delivering a signal, +an architecture-specific set of tag bits are cleared from the +.I si_addr +field of +.IR siginfo_t . +If this flag is set, +an architecture-specific subset of the tag bits will be preserved in +.IR si_addr . +.IP +Programs that need to be compatible with Linux versions older than 5.11 +must use +.B SA_UNSUPPORTED +to probe for support. .SS The siginfo_t argument to a SA_SIGINFO handler When the .B SA_SIGINFO @@ -846,6 +884,93 @@ Triggered by a .BR seccomp (2) filter rule. .RE +.SS Dynamically probing for flag bit support +The +.BR sigaction () +call on Linux accepts unknown bits set in +.I act->sa_flags +without error. +The behavior of the kernel starting with Linux 5.11 is that a second +.BR sigaction () +will clear unknown bits from +.IR oldact->sa_flags . +However, historically, a second +.BR sigaction () +call would typically leave those bits set in +.IR oldact->sa_flags . +.PP +This means that support for new flags cannot be detected +simply by testing for a flag in +.IR sa_flags , +and a program must test that +.B SA_UNSUPPORTED +has been cleared before relying on the contents of +.IR sa_flags . +.PP +Since the behavior of the signal handler cannot be guaranteed +unless the check passes, +it is wise to either block the affected signal +while registering the handler and performing the check in this case, +or where this is not possible, +for example if the signal is synchronous, to issue the second +.BR sigaction () +in the signal handler itself. +.PP +In kernels that do not support a specific flag, +the kernel's behavior is as if the flag was not set, +even if the flag was set in +.IR act->sa_flags . +.PP +The flags +.BR SA_NOCLDSTOP , +.BR SA_NOCLDWAIT , +.BR SA_SIGINFO , +.BR SA_ONSTACK , +.BR SA_RESTART , +.BR SA_NODEFER , +.BR SA_RESETHAND , +and, if defined by the architecture, +.B SA_RESTORER +may not be reliably probed for using this mechanism, +because they were introduced before Linux 5.11. +However, in general, programs may assume that these flags are supported, +since they have all been supported since Linux 2.6, +which was released in the year 2003. +.PP +The following example program exits with status 0 if +.B SA_EXPOSE_TAGBITS +is determined to be supported, and 1 otherwise. +.PP +.EX +#include +#include +#include + +void handler(int signo, siginfo_t *info, void *context) { + struct sigaction oldact; + if (sigaction(SIGSEGV, 0, &oldact) == 0 && + !(oldact.sa_flags & SA_UNSUPPORTED) && + (oldact.sa_flags & SA_EXPOSE_TAGBITS)) { + _exit(0); + } else { + _exit(1); + } +} + +int main(void) { + struct sigaction act = {}; + act.sa_flags = SA_SIGINFO | SA_UNSUPPORTED | SA_EXPOSE_TAGBITS; + act.sa_sigaction = handler; + if (sigaction(SIGSEGV, &act, 0) != 0) { + perror("sigaction"); + return 1; + } + + /* Force a SIGSEGV. */ + *(volatile int *)0 = 0; + return 1; +} +.EE .SH RETURN VALUE .BR sigaction () returns 0 on success; on error, \-1 is returned, and -- 2.32.0.93.g670b81a890-goog _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel