Re: [PATCH] Adds a new ioctl32 syscall for backwards compatibility layers

Rich Felker
2021-01-15T22:41:46Z

On Fri, Jan 15, 2021 at 11:17:09PM +0100, Arnd Bergmann wrote:
> 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.

Indeed, all of this should just be done in userspace. Note (as you of
course know, but others on CC probably don't) that we did this in musl
libc for the sake of being able to run a time64 userspace on a
pre-time64 kernel, with translation from the new time64 ioctl
structures to the versions needed by the old ioctls and back using a
fairly simple table:

https://git.musl-libc.org/cgit/musl/tree/src/misc/ioctl.c?id=v1.2.2

I imagine there's a fair bit more to be done for 32-/64-bit mismatch
in size/long/pointer types and different alignments, but the problem
is almost certainly tractable, and much easier than what they already
have to be doing for syscalls.

Rich