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=-26.3 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_GIT,USER_IN_DEF_DKIM_WL 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 6C541C64E7B for ; Tue, 17 Nov 2020 23:55:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 18ED022266 for ; Tue, 17 Nov 2020 23:55:00 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="Md/nmo4u" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727216AbgKQXy5 (ORCPT ); Tue, 17 Nov 2020 18:54:57 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45296 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727153AbgKQXy5 (ORCPT ); Tue, 17 Nov 2020 18:54:57 -0500 Received: from mail-pf1-x449.google.com (mail-pf1-x449.google.com [IPv6:2607:f8b0:4864:20::449]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D19D8C0613CF for ; Tue, 17 Nov 2020 15:54:55 -0800 (PST) Received: by mail-pf1-x449.google.com with SMTP id f5so13148pfa.18 for ; Tue, 17 Nov 2020 15:54:55 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=sender:date:message-id:mime-version:subject:from:to:cc; bh=F67sbUoPJ71YXDG7sdUK0EmBqROtcgMmtrjF66jycJ4=; b=Md/nmo4uiAVyWtqr7sCUzbTACIHP4eOwwozR73nab59SpdXPs8GapTu+xjzW6JH2aE GhtfmU+lugSfD10mpukhmiFFxA6gkp/sbPB8yqpft1lJ3WHXzK4lqwNG9k10dQv1JEND 31erme1g+RM0BrFqmT+73UMO86i2vdv+rBA5xBkG/V5kagU8zEPZTS9/QaRq162rltRZ LXZunBwniZET8NRFdhhIyl3LQEiF5YI+25bLj61nDddPxmAFCR8drF+PNifaIWTRt9Fx pVuRy67Ap5XQxuZZDpiHe/ggdaMv0sMklDkbZTNnIPBqMY/yW8DwLHCJdrW0XzkvFkDt iLRA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:date:message-id:mime-version:subject:from :to:cc; bh=F67sbUoPJ71YXDG7sdUK0EmBqROtcgMmtrjF66jycJ4=; b=YXTBjFIUL8/Mtly22nBQHKW88pwSTEonIcJjbwdYVHLAZoHcX7mPkIkZPNXRfzSxEy Mv5dmi7PRkpPAGlmInh4emqKjLDVLZVGgIh9R/oQ8C6lVd+Hus2WMgN7KFU3rSHFJw18 hr3f0y5BVS/VpUKq20PA5slrp/j7/3J8hO8sYHe0TRQzTq/RLrJANTo6aTZpRR7fYU2W cT+8Txz3osisqKPwd4uR0DVWXwgITnpyM/chYAZf8Ry0/JfyJ3R5NjVoSciFvZ7Jbmvt dybuLTfotnl44sOZdImgqll29lLoKds3lJ6srVOn/Vwc0NJqcd5SgWPrwYwfpli2xwQ5 vy4Q== X-Gm-Message-State: AOAM533RhRrJ+43kxBxLJhMSiUQFvWUMN8ePayQJNWViPXhZJQbCidg3 H1fRamtznMWMQkfCcc58aeChOWM= X-Google-Smtp-Source: ABdhPJwlmN1K/jlt1uoHIVnW31y/VSYJ7CSIVvxjavwBUeC71cH00smjXcq31zUDviTB4xHMlEgLniE= Sender: "pcc via sendgmr" X-Received: from pcc-desktop.svl.corp.google.com ([2620:15c:2ce:0:7220:84ff:fe09:385a]) (user=pcc job=sendgmr) by 2002:a62:17c8:0:b029:18b:5a97:a8d1 with SMTP id 191-20020a6217c80000b029018b5a97a8d1mr1823895pfx.15.1605657295349; Tue, 17 Nov 2020 15:54:55 -0800 (PST) Date: Tue, 17 Nov 2020 15:54:47 -0800 Message-Id: <20201117235447.816252-1-pcc@google.com> Mime-Version: 1.0 X-Mailer: git-send-email 2.29.2.299.gdc1121823c-goog Subject: [PATCH v2] 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 Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-man@vger.kernel.org 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. 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 +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 -- 2.29.2.299.gdc1121823c-goog