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

[Arnd Bergmann <arnd@arndb.de>]

On Fri, Sep 18, 2020 at 3:44 PM Christoph Hellwig <hch@lst.de> wrote:
>
> On Fri, Sep 18, 2020 at 02:40:12PM +0100, Al Viro wrote:
> > >     /* Vector 0x110 is LINUX_32BIT_SYSCALL_TRAP */
> > > -   return pt_regs_trap_type(current_pt_regs()) == 0x110;
> > > +   return pt_regs_trap_type(current_pt_regs()) == 0x110 ||
> > > +           (current->flags & PF_FORCE_COMPAT);
> >
> > Can't say I like that approach ;-/  Reasoning about the behaviour is much
> > harder when it's controlled like that - witness set_fs() shite...
>
> I don't particularly like it either.  But do you have a better idea
> how to deal with io_uring vs compat tasks?

Do we need to worry about something other than the compat_iovec
struct for now? Regarding the code in io_import_iovec(), it would
seem that can easily be handled by exposing an internal helper.
Instead of

#ifdef CONFIG_COMPAT
     if (req->ctx->compat)
            return compat_import_iovec(rw, buf, sqe_len, UIO_FASTIOV,
iovec, iter);
#endif
        return import_iovec(rw, buf, sqe_len, UIO_FASTIOV, iovec, iter);

This could do

    __import_iovec(rw, buf, sqe_len, UIO_FASTIOV, iovec,
                     iter, req->ctx->compat);

With the normal import_iovec() becoming a trivial wrapper around
the same thing:

ssize_t import_iovec(int type, const struct iovec __user * uvector,
                 unsigned nr_segs, unsigned fast_segs,
                 struct iovec **iov, struct iov_iter *i)
{
     return __import_iovec(type, uvector, nr_segs, fast_segs, iov,
              i, in_compat_syscall());
}


         Arnd