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=-14.2 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,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 98238C4361B for ; Fri, 18 Dec 2020 10:28:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6533B23A53 for ; Fri, 18 Dec 2020 10:28:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728246AbgLRK2T (ORCPT ); Fri, 18 Dec 2020 05:28:19 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51088 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728213AbgLRK2S (ORCPT ); Fri, 18 Dec 2020 05:28:18 -0500 Received: from mail-wr1-x42c.google.com (mail-wr1-x42c.google.com [IPv6:2a00:1450:4864:20::42c]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4A74AC0617A7; Fri, 18 Dec 2020 02:27:38 -0800 (PST) Received: by mail-wr1-x42c.google.com with SMTP id t16so1581734wra.3; Fri, 18 Dec 2020 02:27:38 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=subject:from:to:cc:references:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=YeOkHtjJ5a3Y5Is8ONz9fKVqPTCLKULaY+CYSDx9Kmw=; b=vA+865dUOnr8X7XvM4apqfr6yFLU4KWxGn8x/ZJoFDiBETRWrUAzPHLkTV3kyTeCvu yNDmaGZ6K1BHlSXankXt8/jvBG0Q/b9A8ES+fi4pMQ6x5xrTxHRaWJA9F4sGfvaHXWcs dVbwRJ6TuC5ZDLDrufT7uTkb8Zu8wMoihMZFjEcQgU99l43UCDUF8KHAMtTmyaD1aOu0 uP57QJdbDgLnVh9XSPaRw3nffiq1X/DZwXhPzSq/Ju7v+z9lp18OA5VeVREMwKmUZRtN vJ0IyPE8dM1silw/RFDbV9i9ZES8i6USkJ/CYBaZdZSJ4GJysTSKgrgaKKRUjJ0tksqp yxUw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:from:to:cc:references:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=YeOkHtjJ5a3Y5Is8ONz9fKVqPTCLKULaY+CYSDx9Kmw=; b=hlwSrtt98l636m3L16qANYf+sdQ05vTHHWdbTsLwNzMQ9+uisZnIdFqFA3BhUwDc6f qgX7N7GGEPAHnsBP0bYqmW2fwGhm0F1wy8unxi6oH8YhEUx1JreBbvTJeJW1HaTVIWVT Vghl3W2CwHEttTSrpC7Qed4UjeX01xIRm6rGu64BJoVeyAwBVfBP60YpRqQrOizRa2Wp dGmhNW4lqSYAnoN+VT8TrMeyByrswXUCwRjrj7mp240lYdbzY6cs9R1+vBUgFZ47nq1P iWMHT8SkC1akPzy/mOsfalxNFgi/d3AIWgX1MonRSgFpWun4mknu+UjiL1nIFUFbKN6m gHEQ== X-Gm-Message-State: AOAM530Ix+hzfrJD06A6YA23FvjZsS72xI+ENeWCAX3f6UQI918+L1+H p/kTcOPGOyQZkJjsRbj8IIQ= X-Google-Smtp-Source: ABdhPJxLpMHT2cmJRwIp38GmwY4uYLQHIAo+x943oys7LhK5R1zdXNI+7jRYzi9LEu8RQvxrORorBg== X-Received: by 2002:a5d:6944:: with SMTP id r4mr3643103wrw.134.1608287257039; Fri, 18 Dec 2020 02:27:37 -0800 (PST) Received: from [192.168.1.143] ([170.253.51.130]) by smtp.gmail.com with ESMTPSA id s6sm14019602wro.79.2020.12.18.02.27.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 18 Dec 2020 02:27:36 -0800 (PST) Subject: Re: [PATCH v2] sigaction.2: Document SA_EXPOSE_TAGBITS and the flag support detection protocol From: "Alejandro Colomar (man-pages)" To: Peter Collingbourne Cc: Kostya Serebryany , "James E.J. Bottomley" , Michael Kerrisk , Catalin Marinas , "Eric W. Biederman" , Linux ARM , Evgenii Stepanov , Dave Martin , linux-api@vger.kernel.org, linux-man@vger.kernel.org, Vincenzo Frascino , Oleg Nesterov , Will Deacon References: <20201117235447.816252-1-pcc@google.com> <5b99f5af-de0e-508a-f6e8-133fb290ae03@gmail.com> Message-ID: Date: Fri, 18 Dec 2020 11:27:35 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.1 MIME-Version: 1.0 In-Reply-To: <5b99f5af-de0e-508a-f6e8-133fb290ae03@gmail.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-man@vger.kernel.org Hi Peter, Linux 5.10 has been recently released. Was this merged to 5.10 or 5.11? Do you have any updates for this patch? Thanks, Alex On 11/18/20 3:04 PM, Alejandro Colomar (man-pages) wrote: > > > On 11/18/20 12:42 PM, Alejandro Colomar (man-pages) wrote: >> [[ Removed some CCs because gmail >> didn't allow me to send it with so many CCs: >> Kevin B., Andrey K., Helge D., David S. >> ]] >> >> On 11/18/20 12:54 AM, Peter Collingbourne wrote: >>> Signed-off-by: Peter Collingbourne >>> --- >>> These features are implemented in this patch series: >>> >> https://lore.kernel.org/linux-arm-kernel/cover.1605235762.git.pcc@google.com/ >>> which is still under review, so the patch should not be applied >>> yet. >>> >>> Alejandro, thanks for the review. Since the patch was almost >>> rewritten I didn't base this on your patch, instead I tried to >>> use the correct formatting in this patch. >> >> Hi Peter, >> >> Fine. >> See below a small fix. > > D'oh! > Fixing the CCs I forgot to add the fix to the code. > See below now. > >> >> Cheers, >> >> Alex >> >>> >>> 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 6a8142324..0e4236a43 100644 >>> --- a/man2/sigaction.2 >>> +++ b/man2/sigaction.2 >>> @@ -250,6 +250,44 @@ 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 >>> +.IR oldact->sa_flags > > s/.IR/.I/ > > There's no roman part there. > >>> +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.x)" >>> +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.x >>> +must use >>> +.B SA_UNSUPPORTED >>> +to probe for support. >>> .SS The siginfo_t argument to a SA_SIGINFO handler >>> When the >>> .B SA_SIGINFO >>> @@ -833,6 +871,93 @@ 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.x 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.x. >>> +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 >>> -- Alejandro Colomar Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/ http://www.alejandro-colomar.es/