Re: [PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag

[Andy Lutomirski <luto@kernel.org>]

On Sat, Sep 19, 2020 at 4:24 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> On Sat, Sep 19, 2020 at 03:53:40PM -0700, Andy Lutomirski wrote:
>
> > > It would not be a win - most of the syscalls don't give a damn
> > > about 32bit vs. 64bit...
> >
> > Any reasonable implementation would optimize it out for syscalls that don’t care.  Or it could be explicit:
> >
> > DEFINE_MULTIARCH_SYSCALL(...)
>
> 1) what would that look like?

In effect, it would work like this:

/* Arch-specific, but there's a generic case for sane architectures. */
enum syscall_arch {
  SYSCALL_NATIVE,
  SYSCALL_COMPAT,
  SYSCALL_X32,
};

DEFINE_MULTIARCH_SYSCALLn(args, arch)
{
  args are the args here, and arch is the arch.
}

> 2) have you counted the syscalls that do and do not need that?

No.

> 3) how many of those realistically *can* be unified with their
> compat counterparts?  [hint: ioctl(2) cannot]

There would be no requirement to unify anything.  The idea is that
we'd get rid of all the global state flags.

For ioctl, we'd have a new file_operation:

long ioctl(struct file *, unsigned int, unsigned long, enum syscall_arch);

I'm not saying this is easy, but I think it's possible and the result
would be more obviously correct than what we have now.