From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755813Ab1HWRdr (ORCPT ); Tue, 23 Aug 2011 13:33:47 -0400 Received: from zeniv.linux.org.uk ([195.92.253.2]:35882 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755786Ab1HWRdm (ORCPT ); Tue, 23 Aug 2011 13:33:42 -0400 Date: Tue, 23 Aug 2011 18:33:17 +0100 From: Al Viro To: Linus Torvalds Cc: Andrew Lutomirski , "H. Peter Anvin" , Borislav Petkov , Ingo Molnar , "user-mode-linux-devel@lists.sourceforge.net" , Richard Weinberger , "linux-kernel@vger.kernel.org" , "mingo@redhat.com" Subject: Re: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re: [RFC] weird crap with vdso on uml/i386) Message-ID: <20110823173317.GI2203@ZenIV.linux.org.uk> References: <20110823000314.GW2203@ZenIV.linux.org.uk> <4E52EF2A.8060608@zytor.com> <20110823010146.GY2203@ZenIV.linux.org.uk> <20110823011312.GZ2203@ZenIV.linux.org.uk> <20110823021717.GA2203@ZenIV.linux.org.uk> <20110823061531.GC2203@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 23, 2011 at 09:20:12AM -0700, Linus Torvalds wrote: > It's EMULATING A SYSTEM CALL. That original "getregs" value is not > some "user space state". It's the *system call* state that you got > after the system call trapped. Setting it back is an insane operation, > but it would happen to work - if you make no changes. > > But UML *does* make changes. It takes that system call state, and then > EMULATES THE SYSTEM CALL INCORRECTLY. > > If you see it that way (which is the correct way), then it's clearly > an UML problem, and it's not at all "madness" that your > getregs/setregs pairing doesn't work. > > See? Buggy system call emulation. It's really that simple. Of course, > "simple" in this case is "really really subtle differences in how the > kernel treats syscall/sysenter/int80", so the *details* are certainly > not simple, but the concept is. It's a bit more than that (ptrace changes to syscall arguments *are* lost on syscall restart), but... as far as I'm concerned, the situation is simple now: * SYSCALL is not terminally broken wrt restarts. My apologies for misreading what was going on. * SYSENTER with Linus' patch does work just fine wrt restarts + ptrace * SYSCALL is losing ptrace-made changes to arguments when it restarts. Might or might not be a problem for somebody. * UML should not touch SYSCALL for 32bit. Not without serious changes in UML and I'm not convinced that it won't be worse than what we probably ought to do there: check if __kernel_vsyscall() does SYSCALL (recognizable by interaction with POKEUSER) and don't tell about vdso to guest processes. Anything well-behaving won't step on SYSCALL and the things that do deserve the subtle bugs they get. * asm glue is subtle, evil and doesn't have anywhere near enough documentation ;-/