From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/14483 Path: news.gmane.org!.POSTED.blaine.gmane.org!not-for-mail From: Rich Felker Newsgroups: gmane.linux.lib.musl.general Subject: Re: vdso clock_gettime and time64 Date: Wed, 31 Jul 2019 13:45:21 -0400 Message-ID: <20190731174521.GC9017@brightrain.aerifal.cx> References: <20190731051313.GA27476@brightrain.aerifal.cx> <87d0hqsj19.fsf@oldenburg2.str.redhat.com> <20190731150725.GB9017@brightrain.aerifal.cx> <87o91am8mr.fsf@oldenburg2.str.redhat.com> Reply-To: musl@lists.openwall.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Injection-Info: blaine.gmane.org; posting-host="blaine.gmane.org:195.159.176.226"; logging-data="227972"; mail-complaints-to="usenet@blaine.gmane.org" User-Agent: Mutt/1.5.21 (2010-09-15) To: musl@lists.openwall.com Original-X-From: musl-return-14499-gllmg-musl=m.gmane.org@lists.openwall.com Wed Jul 31 19:45:38 2019 Return-path: Envelope-to: gllmg-musl@m.gmane.org Original-Received: from mother.openwall.net ([195.42.179.200]) by blaine.gmane.org with smtp (Exim 4.89) (envelope-from ) id 1hssfR-000xET-Pp for gllmg-musl@m.gmane.org; Wed, 31 Jul 2019 19:45:37 +0200 Original-Received: (qmail 26102 invoked by uid 550); 31 Jul 2019 17:45:35 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Original-Received: (qmail 26080 invoked from network); 31 Jul 2019 17:45:34 -0000 Content-Disposition: inline In-Reply-To: <87o91am8mr.fsf@oldenburg2.str.redhat.com> Original-Sender: Rich Felker Xref: news.gmane.org gmane.linux.lib.musl.general:14483 Archived-At: On Wed, Jul 31, 2019 at 07:11:40PM +0200, Florian Weimer wrote: > * Rich Felker: > > > On Wed, Jul 31, 2019 at 10:30:26AM +0200, Florian Weimer wrote: > >> * Rich Felker: > >> > >> > One looming thing that folks probably aren't going to like about > >> > switching to 64-bit time_t is losing the vdso clock_gettime on old > >> > kernels. Instead of a function call in userspace, you get *two* > >> > syscalls, the first (time64) one failing, every time you call > >> > clock_gettime. Of course the problem goes away immediately if you have > >> > a time64-capable kernel providing the time64 vdso function. > >> > > >> > Is this a problem, and if so, what can be done about it? > >> > >> Some users notice fairly quickly if the vDSO fast path is gone and file > >> bug reports. (This can happen for various reasons, e.g. buggy kernels > >> detecting CPU cycle counter drift when there is actually none.) I don't > >> know to what extent this matters to legacy architectures. > > > > These are good points. A lot of these archs actually don't even have > > vdso clock_gettime (only mips, arm, and i386 seem to). > > > > I wonder if it would make sense to support use of 32-bit vdso for now, > > possibly with logic to drop it if it ever returns a negative tv_sec, > > and consider removing it after the last kernel without time64 is > > EOL'd, so that it's gone well before 2038. > > In glibc, we perform vDSO lookup early. I will push for a solution that > does a probing system call during startup if it cannot find the *_time64 > vDSO entry, to determine if it should use the real *_time64 system call > or the 32-bit system call (or vDSO). That should help to keep the > complexity at bay, at the cost of increased startup time, but which will > reduce with future completion of the interfaces. > > I do not think resuming a process on a kernel with a different system > call set is supportable. Not using vdso, it's definitely supportable; musl's fallbacks for unsupported syscalls are entirely stateless. Doing it statefully without data race UB all over the place is painful. For vdso clock_gettime now, we do it on the first call and use a relaxed atomic. It wouldn't be a big deal to do it at startup conditional on linking of clock_gettime (with a weak init symbol) if that helps. Note that changing vdso is orthogonal to different syscall set. You can be resuming on a kernel with the same syscall set, but where vdso changed due to bugfixes or different hardware or whatever. Rich