From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 64833 invoked by alias); 9 Oct 2019 11:56:20 -0000 Mailing-List: contact libc-alpha-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: libc-alpha-owner@sourceware.org Received: (qmail 64824 invoked by uid 89); 9 Oct 2019 11:56:20 -0000 Authentication-Results: sourceware.org; auth=none X-Spam-SWARE-Status: No, score=-6.0 required=5.0 tests=AWL,BAYES_00,SPF_HELO_PASS,SPF_PASS autolearn=ham version=3.3.1 spammy=mistakes X-HELO: mx1.redhat.com From: Florian Weimer To: Christian Brauner Cc: Adhemerval Zanella , libc-alpha@sourceware.org Subject: Re: [PATCH] [GLIBC RFC] clone3: add CLONE3_RESET_SIGHAND References: <20191008134417.16113-1-christian.brauner@ubuntu.com> <5cab2cbb-e72b-4eb0-5271-1a90c4e8de95@linaro.org> <20191009104830.w2fkr4m3lrkfowxq@wittgenstein> <87bluq18po.fsf@oldenburg2.str.redhat.com> <20191009111200.rfqlk5lgityhi6rl@wittgenstein> Date: Wed, 09 Oct 2019 11:56:00 -0000 In-Reply-To: <20191009111200.rfqlk5lgityhi6rl@wittgenstein> (Christian Brauner's message of "Wed, 9 Oct 2019 13:12:01 +0200") Message-ID: <87zhiayvxc.fsf@oldenburg2.str.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.2 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-SW-Source: 2019-10/txt/msg00246.txt.bz2 * Christian Brauner: > On Wed, Oct 09, 2019 at 01:04:03PM +0200, Florian Weimer wrote: >> * Christian Brauner: >> >> > I've been thinking about two things how to do this: >> > - mask the flags that the kernel does not support >> >> That doesn't look fully backwards-compatible to me. The argument isn't >> currently read/write, is it? It would work for us though. >> >> > - add another argument to struct clone_args that is "known_flags" >> > when the syscall returns it'll be set to all the flags this kernel >> > knows about >> >> This needs some sort of protocol to detect whether the argument was >> updated. I suppose we could define CLONE3_INITIALLY_SUPPORTED_FLAGS >> with all the flag bits currently supported and tell developers to >> initialize struct clone_args with: >> >> .known_flags = CLONE3_INITIALLY_SUPPORTED_FLAGS, > > That won't work. Older kernels will verify that parts of the struct that > are not known are set to 0. Good point. > I wonder, what is stopping you from > > struct clone args args = { > .known_flags = 0, > }; > > pid_t pid = clone3(&args, sizeof(args)); > if (pid < 0) > return -1; > > ######### kernel code ############ > /* on a kernel that is aware of known_flags */ > kargs->known_flags = CLONE3_SUPPORTED_FLAGS; > ################################## > > if (!args.known_flags) > /* kernel doesn't not support the known_flags extension */ > > if (args.known_flags & NEW_FLAG_I_CARE_ABOUT) > /* > * kernel does support the known_flags extension and does > * support the feature I care about > */ With this construct, the application programmer needs to remember which flags are old and new (predate and postdate known_flags). It's too easy to make mistakes there. What about this? pid_t pid = clone3 (&args, sizeof (args)); if (pid < 0) return -1; if (args.known_flags == 0) args.known_flags = CLONE3_INITIALLY_SUPPORTED_FLAGS; if (args.known_flags & NEW_FLAG_I_CARE_ABOUT) /* Kernel does support the known_flags extension and does support the feature I care about. */ We could hide this in the clone3 wrapper for glibc if we start out with a struct clone_args that has this member. Thanks, Florian