From owner-freebsd-current@freebsd.org Sat Oct 21 20:39:31 2017 Return-Path: Delivered-To: freebsd-current@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id 2CBCDE39ACF for ; Sat, 21 Oct 2017 20:39:31 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from kib.kiev.ua (kib.kiev.ua [IPv6:2001:470:d5e7:1::1]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id AE3166B132; Sat, 21 Oct 2017 20:39:30 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from tom.home (kib@localhost [127.0.0.1]) by kib.kiev.ua (8.15.2/8.15.2) with ESMTPS id v9LKdLXG008023 (version=TLSv1.2 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=NO); Sat, 21 Oct 2017 23:39:21 +0300 (EEST) (envelope-from kostikbel@gmail.com) DKIM-Filter: OpenDKIM Filter v2.10.3 kib.kiev.ua v9LKdLXG008023 Received: (from kostik@localhost) by tom.home (8.15.2/8.15.2/Submit) id v9LKdLKp008022; Sat, 21 Oct 2017 23:39:21 +0300 (EEST) (envelope-from kostikbel@gmail.com) X-Authentication-Warning: tom.home: kostik set sender to kostikbel@gmail.com using -f Date: Sat, 21 Oct 2017 23:39:21 +0300 From: Konstantin Belousov To: Andreas Tobler Cc: Tijl Coosemans , freebsd-current@FreeBSD.org, gerald@FreeBSD.org Subject: Re: Segfault in _Unwind_* code called from pthread_exit Message-ID: <20171021203921.GD2473@kib.kiev.ua> References: <20170823163707.096f93ab@kalimero.tijl.coosemans.org> <20170824154235.GD1700@kib.kiev.ua> <20170824180830.199885b0@kalimero.tijl.coosemans.org> <20170825173851.09116ddc@kalimero.tijl.coosemans.org> <20170825234442.GO1700@kib.kiev.ua> <20170826202813.1240a1ef@kalimero.tijl.coosemans.org> <20170826184034.GR1700@kib.kiev.ua> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.1 (2017-09-22) X-Spam-Status: No, score=-2.0 required=5.0 tests=ALL_TRUSTED,BAYES_00, DKIM_ADSP_CUSTOM_MED,FREEMAIL_FROM,NML_ADSP_CUSTOM_MED autolearn=no autolearn_force=no version=3.4.1 X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on tom.home X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 21 Oct 2017 20:39:31 -0000 On Sat, Oct 21, 2017 at 10:02:38PM +0200, Andreas Tobler wrote: > On 26.08.17 20:40, Konstantin Belousov wrote: > > On Sat, Aug 26, 2017 at 08:28:13PM +0200, Tijl Coosemans wrote: > >> On Sat, 26 Aug 2017 02:44:42 +0300 Konstantin Belousov wrote: > >>> How does llvm unwinder detects that the return address is a garbage ? > >> > >> It just stops unwinding when it can't find frame information (stored in > >> .eh_frame sections). GCC unwinder doesn't give up yet and checks if the > >> return address points to the signal trampoline (which means the current > >> frame is that of a signal handler). It has built-in knowledge of how to > >> unwind to the signal trampoline frame. > > So llvm just gives up on signal frames ? > > > >> A noreturn attribute isn't enough. You can still unwind such functions. > >> They are allowed to throw exceptions for example. > > Ok. > > > >> I did consider using > >> a CFI directive (see patch below) and it works, but it's architecture > >> specific and it's inserted after the function prologue so there's still > >> a window of a few instructions where a stack unwinder will try to use > >> the return address. > >> > >> Index: lib/libthr/thread/thr_create.c > >> =================================================================== > >> --- lib/libthr/thread/thr_create.c (revision 322802) > >> +++ lib/libthr/thread/thr_create.c (working copy) > >> @@ -251,6 +251,7 @@ create_stack(struct pthread_attr *pattr) > >> static void > >> thread_start(struct pthread *curthread) > >> { > >> + __asm(".cfi_undefined %rip"); > >> sigset_t set; > >> > >> if (curthread->attr.suspend == THR_CREATE_SUSPENDED) > > > > I like this approach much more than the previous patch. What can be > > done is to provide asm trampoline which calls thread_start(). There you > > can add the .cfi_undefined right at the entry. > > > > It is somewhat more work than just setting the return address on the > > kernel-constructed pseudo stack frame, but I believe this is ultimately > > correct way. You still can do it only on some arches, if you do not > > have incentive to code asm for all of them. > > > > Also crt1 probably should get the same treatment, despite we already set > > %rbp to zero AFAIR. > > Did some commit result out of this discussion or is this subject still > under investigation? Nothing was done AFAIK. > > Curious because I got this gcc PR: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82635 > > Tia, > Andreas