Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965672AbcCJLQw (ORCPT ); Thu, 10 Mar 2016 06:16:52 -0500 Received: from mail-wm0-f67.google.com ([74.125.82.67]:33021 "EHLO mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932895AbcCJLQu (ORCPT ); Thu, 10 Mar 2016 06:16:50 -0500 Date: Thu, 10 Mar 2016 12:16:46 +0100 From: Ingo Molnar To: Rich Felker Cc: Linus Torvalds , Andy Lutomirski , the arch/x86 maintainers , Linux Kernel Mailing List , Borislav Petkov , "musl@lists.openwall.com" , Andrew Morton , Thomas Gleixner , Peter Zijlstra Subject: Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers Message-ID: <20160310111646.GA13102@gmail.com> References: <06079088639eddd756e2092b735ce4a682081308.1457486598.git.luto@kernel.org> <20160309085631.GA3247@gmail.com> <20160309113449.GZ29662@port70.net> <20160310033446.GL9349@brightrain.aerifal.cx> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160310033446.GL9349@brightrain.aerifal.cx> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2260 Lines: 49 * Rich Felker wrote: > [...] > > I believe a new kernel cancellation API with a sticky cancellation flag (rather > than a signal), and a flag or'd onto the syscall number to make it cancellable > at the call point, could work, but then userspace needs to support fairly > different old and new kernel APIs in order to be able to run on old kernels > while also taking advantage of new ones, and it's not clear to me that it would > actually be worthwhile to do so. I could see doing it for a completely new > syscall API, but as a second syscall API for a system that already has one it > seems gratuitous. From my perspective the existing approach (checking program > counter from signal handler) is very clean and simple. After all it made enough > sense that I was able to convince the glibc folks to adopt it. I concur with your overall analysis, but things get a bit messy once we consider AT_SYSINFO which is a non-atomic mix of user-space and kernel-space code. Trying to hand cancellation status through that results in extra complexity: arch/x86/entry/vdso/Makefile | 3 +- arch/x86/entry/vdso/vdso32/cancellation_helpers.c | 116 ++++++++++++++++++++++ arch/x86/entry/vdso/vdso32/vdso32.lds.S | 2 + tools/testing/selftests/x86/unwind_vdso.c | 57 +++++++++-- 4 files changed, 171 insertions(+), 7 deletions(-) So instead of a sticky cancellation flag, we could introduce a sticky cancellation signal. A 'sticky signal' is not cleared from signal_pending() when the signal handler executes, but it's automatically blocked so no signal handler recursion occurs. (A sticky signal could still be cleared via a separate mechanism, by the cancellation cleanup code.) Such a 'sticky cancellation signal' would, in the racy situation, cause new blocking system calls to immediately return with -EINTR. Non-blocking syscalls could still be used. (So the cancellation signal handler itself would still have access to various fundamental system calls.) I think this would avoid messy coupling between the kernel's increasingly more varied system call entry code and C libraries. Sticky signals could be requested via a new SA_ flag. What do you think? Thanks, Ingo