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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Fri, 15 Jan 2021 23:17:09 +0100
Message-ID: <CAK8P3a1gqt-gBCPTdNeY+8SaG8eUGN4zkCrNKSjA=aEL-TkaUQ@mail.gmail.com>
Subject: Re: [PATCH] Adds a new ioctl32 syscall for backwards compatibility layers
To:     David Laight <David.Laight@aculab.com>
Cc:     "sonicadvance1@gmail.com" <sonicadvance1@gmail.com>,
        Richard Henderson <rth@twiddle.net>,
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On Fri, Jan 15, 2021 at 9:01 PM David Laight <David.Laight@aculab.com> wrote:
>
> From: sonicadvance1@gmail.com
> > Sent: 15 January 2021 07:03
> > Problem presented:
> > A backwards compatibility layer that allows running x86-64 and x86
> > processes inside of an AArch64 process.
> >   - CPU is emulated
> >   - Syscall interface is mostly passthrough
> >   - Some syscalls require patching or emulation depending on behaviour
> >   - Not viable from the emulator design to use an AArch32 host process
> >
>
> You are going to need to add all the x86 compatibility code into
> your arm64 kernel.
> This is likely to be different from the 32bit arm compatibility
> because 64bit items are only aligned on 32bit boundaries.
> The x86 x32 compatibility will be more like the 32bit arm 'compat'
> code - I'm pretty sure arm32 64bit aligned 64bit data.

All other architectures that have both 32-bit and 64-bit variants
use the same alignment for all types, except for x86.

There are additional differences though, especially if one
were to try to generalize the interface to all architectures.
A subset of the issues includes

- x32 has 64-bit types in places of some types that are
  32 bit everywhere else (time_t, ino_t, off_t, clock_t, ...)

- m68k aligns struct members to at most 16 bits

- uid_t/gid_t/ino_t/dev_t/... are

> You'll then need to remember how the process entered the kernel
> to work out which compatibility code to invoke.
> This is what x86 does.
> It allows a single process to do all three types of system call.
>
> Trying to 'patch up' structures outside the kernel, or in the
> syscall interface code will always cause grief somewhere.
> The only sane place is in the code that uses the structures.
> Which, for ioctls, means inside the driver that parses them.

He's already doing the system call emulation for all the system
calls other than ioctl in user space though. In my experience,
there are actually fairly few ioctl commands that are different
between architectures -- most of them have no misaligned
or architecture-defined struct members at all.

Once you have conversion functions to deal with the 32/64-bit
interface differences and architecture specifics of sockets,
sysvipc, signals, stat, and input_event, handling the
x86-32 specific ioctl commands is comparably easy.

         Arnd