linux-kernel - Re: Can we drop upstream Linux x32 support?

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Date:   Fri, 14 Dec 2018 13:27:40 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Rich Felker <dalias@...c.org>, Florian Weimer <fweimer@...hat.com>,
        Bernd Petrovitsch <bernd@...rovitsch.priv.at>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        Andrew Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Mike Frysinger <vapier@...too.org>,
        "H. J. Lu" <hjl.tools@...il.com>, x32@...ldd.debian.org,
        Arnd Bergmann <arnd@...db.de>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: Can we drop upstream Linux x32 support?

On Fri, Dec 14, 2018 at 12:13 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Fri, Dec 14, 2018 at 10:58 AM Andy Lutomirski <luto@...capital.net> wrote:
> >
> > Does anyone know *why* Linux’s x32 has __kernel_long_t defined as long long?
>
> It *needs* to be long long, since the headers are used for builds in
> user mode using ILP32.
>
> Since __kernel_long_t is a 64-bit (the _kernel_ is not ILP32), you
> need to use "long long" when building in ILP32.
>
> Obviously, it could be something like
>
>    #ifdef __KERNEL__
>     typedef long __kernel_long_t;
>    #else
>     typedef long long __kernel_long_t;
>    #endif
>
> or similar to make it more obvious what's going on.
>
> Or we could encourage all the uapi header files to always just use
> explicit sizing like __u64, but some of the structures really end up
> being "kernel long size" for sad historical reasons. Not lovely, but
> there we are..
>

This is probably water under the bridge, but I disagree with you here.
Or rather, I agree with you in principle but I really don't like the
way it turned out.

For legacy uapi structs (and probably some new ones too, sigh), as a
practical matter, user code is going to shove them at the C compiler,
and the C compiler is going to interpret them in the usual way, and
either we need a usermode translation layer or the kernel needs to
deal with the result.

It's a nice thought that, by convincing an x32 compiler that
__kernel_long_t is 64 bits, we end up with the x32 struct being
compatible with the native Linux struct, but it only works for structs
where the only ABI-dependent type is long.  But the real structs in
uapi aren't all like this.  We have struct iovec:

struct iovec
{
        void __user *iov_base;  /* BSD uses caddr_t (1003.1g requires void *) */
        __kernel_size_t iov_len; /* Must be size_t (1003.1g) */
};

Whoops, this one looks the same on x32 and i386, but neither one of
them match x86_64, and, just randomly grepping around a bit, I see:

struct snd_hwdep_dsp_image {
        unsigned int index;             /* W: DSP index */
        unsigned char name[64];         /* W: ID (e.g. file name) */
        unsigned char __user *image;    /* W: binary image */
        size_t length;                  /* W: size of image in bytes */
        unsigned long driver_data;      /* W: driver-specific data */
};

struct __sysctl_args {
        int __user *name;
        int nlen;
        void __user *oldval;
        size_t __user *oldlenp;
        void __user *newval;
        size_t newlen;
        unsigned long __unused[4];
};

If these had been switched from "unsigned long" to __kernel_ulong_t,
they would have had three different layouts on i386, x32, and x86_64.

So now we have a situation where, if we were to make x32 work 100%,
the whole kernel would need to recognize that there are three possible
ABIs, not two.  And this sucks.

So I think it would have been a better choice to let long be 32-bit on
x32 and to therefore make x32 match the x86_32 "compat" layout as much
as possible.  Sure, this would make x32 be more of a second-class
citizen, but I think it would have worked better, had fewer bugs, and
been more maintainable.

--Andy