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
