From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Sun, 21 Aug 2011 15:43:52 +0100 From: Al Viro Message-ID: <20110821144352.GJ2203@ZenIV.linux.org.uk> References: <4E4E2427.9080602@nod.at> <20110820011845.GC2203@ZenIV.linux.org.uk> <4E4FD12F.70508@nod.at> <20110820201406.GF2203@ZenIV.linux.org.uk> <4E501F51.9060905@nod.at> <20110821063443.GH2203@ZenIV.linux.org.uk> <20110821084230.GI2203@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org Subject: Re: SYSCALL, ptrace and syscall restart breakages (Re: [RFC] weird crap with vdso on uml/i386) To: Andrew Lutomirski Cc: Linus Torvalds , "H. Peter Anvin" , mingo@redhat.com, Richard Weinberger , user-mode-linux-devel@lists.sourceforge.net, linux-kernel@vger.kernel.org List-ID: On Sun, Aug 21, 2011 at 07:24:35AM -0400, Andrew Lutomirski wrote: > I don't see the point of all this hackery at all. sysenter/sysexit > indeed screws up some registers, but we can return on the iret path in > the case of restart. We *do* return on iret path in case of restart, TYVM. > So why do we lie to ptrace (and iret!) at all? Why not just fill in > pt_regs with the registers as they were (at least the > non-clobbered-by-sysenter ones), set the actual C parameters correctly > to contain the six arguments (in rdi, rsi, etc.), do the syscall, and > return back to userspace without any funny business? Is there some > ABI reason that, once we've started lying to tracers, we have to keep > doing so? We do not lie to ptrace and iret. At all. We do just what you have described. And fuck up when restart returns us to the SYSCALL / SYSENTER instruction again, which expects the different calling conventions, so the values arranged in registers in the way int 0x80 would expect do us no good.