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

[Andy Lutomirski]

On Sat, Sep 19, 2020 at 7:57 PM Al Viro <viro at zeniv.linux.org.uk> wrote:
>
> On Sat, Sep 19, 2020 at 05:14:41PM -0700, Andy Lutomirski wrote:
>
> > > 2) have you counted the syscalls that do and do not need that?
> >
> > No.
>
> Might be illuminating...
>
> > > 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.
>
> _What_ global state flags?  When you have separate SYSCALL_DEFINE3(ioctl...)
> and COMPAT_SYSCALL_DEFINE3(ioctl...), there's no flags at all, global or
> local.  They only come into the play when you try to share the same function
> for both, right on the top level.

...

>
> > 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.
>
> No, it would not.  Seriously, from time to time a bit of RTFS before grand
> proposals turns out to be useful.

As one example, look at __sys_setsockopt().  It's called for the
native and compat versions, and it contains an in_compat_syscall()
check.  (This particularly check looks dubious to me, but that's
another story.)  If this were to be done with equivalent semantics
without a separate COMPAT_DEFINE_SYSCALL and without
in_compat_syscall(), there would need to be some indication as to
whether this is compat or native setsockopt.  There are other
setsockopt implementations in the net stack with more
legitimate-seeming uses of in_compat_syscall() that would need some
other mechanism if in_compat_syscall() were to go away.

setsockopt is (I hope!) out of scope for io_uring, but the situation
isn't fundamentally different from read and write.