Re: Can we drop upstream Linux x32 support?

From: Christian Brauner
Date: Tue Dec 11 2018 - 00:46:24 EST

On Mon, Dec 10, 2018 at 05:23:39PM -0800, Andy Lutomirski wrote:
> Hi all-
> I'm seriously considering sending a patch to remove x32 support from
> upstream Linux. Here are some problems with it:
> 1. It's not entirely clear that it has users. As far as I know, it's
> supported on Gentoo and Debian, and the Debian popcon graph for x32
> has been falling off dramatically. I don't think that any enterprise
> distro has ever supported x32.
> 2. The way that system calls work is very strange. Most syscalls on
> x32 enter through their *native* (i.e. not COMPAT_SYSCALL_DEFINE)
> entry point, and this is intentional. For example, adjtimex() uses
> the native entry, not the compat entry, because x32's struct timex
> matches the x86_64 layout. But a handful of syscalls have separate
> entry points -- these are the syscalls starting at 512. These enter
> through the COMPAT_SYSCALL_DEFINE entry points.
> The x32 syscalls that are *not* in the 512 range violate all semblance
> of kernel syscall convention. In the syscall handlers,
> in_compat_syscall() returns true, but the COMPAT_SYSCALL_DEFINE entry
> is not invoked. This is nutty and risks breaking things when people
> refactor their syscall implementations. And no one tests these
> things. Similarly, if someone calls any of the syscalls below 512 but
> sets bit 31 in RAX, then the native entry will be called with
> in_compat_set().
> Conversely, if you call a syscall in the 512 range with bit 31
> *clear*, then the compat entry is set with in_compat_syscall()
> *clear*. This is also nutty.
> Finally, the kernel has a weird distinction between CONFIG_X86_X32_ABI
> and and CONFIG_X86_X32, which I suspect results in incorrect builds if
> the host doesn't have an x32 toolchain installed.
> I propose that we make CONFIG_X86_X32 depend on BROKEN for a release
> or two and then remove all the code if no one complains. If anyone

Based on the discussion we had at the beginning of the pidfd_send_signal
syscall patchset I think this is a good idea. For once, the complex
compat handling can make adding new syscalls that need to rely on compat
types because of precedent established by older syscalls icky.

> wants to re-add it, IMO they're welcome to do so, but they need to do
> it in a way that is maintainable.
> --Andy