From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rich Felker Subject: Re: Can we drop upstream Linux x32 support? Date: Tue, 11 Dec 2018 18:38:05 -0500 Message-ID: <20181211233805.GO23599@brightrain.aerifal.cx> References: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Andy Lutomirski Cc: tg@mirbsd.de, Linus Torvalds , X86 ML , LKML , Linux API , "H. Peter Anvin" , Peter Zijlstra , Borislav Petkov , Florian Weimer , Mike Frysinger , "H. J. Lu" , x32@buildd.debian.org, Arnd Bergmann , Will Deacon , Catalin Marinas List-Id: linux-api@vger.kernel.org On Tue, Dec 11, 2018 at 03:22:43PM -0800, Andy Lutomirski wrote: > On Tue, Dec 11, 2018 at 2:14 PM Thorsten Glaser wrote: > > I was also considering investing a nontrivial amount of > > work into porting klibc to x32, since hpa does not wish > > to do it himself. Thankfully I have only done a bit yet. > > > > Furthermore, x32 was the first of the many *64ilp32 > > architectures; I know I’ve seen amd64ilp32 and at least > > one other I don’t recall. It will have prototyped many > > of the problems users of these will run in, and I’d prefer > > to keep it (completely selfish because I don’t wish to > > have to crossgrade a whole system yet again). > > it kind of seems like arm64's lesson is "don't do it like x32". > > There's some effort going on right now to make it possible to add > syscalls without having to muck with every single architecture. I > don't really want x32 to derail that effort. I suppose we could say > that x32 stays but that it simply gets no new syscalls, but that seems > a bit lame. Unfortunately, on x86, x32 really is a third ABI that is > not compatible in a structure-memory-layout sense with the other two. > What happens if someone adds a struct like: > > struct nasty_on_x32 { > __kernel_long_t a; > void * __user b; > }; > > On x86_64, that's two 8-byte fields. On x86_32, it's two four-byte > fields. On x32, it's an 8-byte field and a 4-byte field. Now what? > > I'm sure we could have some magic gcc plugin or other nifty tool that gives us: > > copy_from_user(struct struct_name, kernel_ptr, user_ptr); > > where it automatically generates code for all possible ABIs to copy > over the struct and dispatches dynamically based on the current > syscall ABI, but I have trouble imagining anyone volunteering to > actually do this work. Instead we get ad hoc fixes for each syscall, > along the lines of preadv64v2(), which get done when somebody notices > a problem. For what it's worth, this is exactly what I plan to do if/when I ever get to doing my kernel with multiple/universal userspace ABI support: pure-data markup for the data structures (and syscall argument arrangements) userspace is speaking, with generic marshalling code. But I don't think it's easy to retrofit to Linux, and probably wouldn't be welcome there. Rich