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

[Al Viro]

On Fri, Sep 18, 2020 at 05:16:15PM +0200, Christoph Hellwig wrote:
> On Fri, Sep 18, 2020 at 02:58:22PM +0100, Al Viro wrote:
> > Said that, why not provide a variant that would take an explicit
> > "is it compat" argument and use it there?  And have the normal
> > one pass in_compat_syscall() to that...
> 
> That would help to not introduce a regression with this series yes.
> But it wouldn't fix existing bugs when io_uring is used to access
> read or write methods that use in_compat_syscall().  One example that
> I recently ran into is drivers/scsi/sg.c.

So screw such read/write methods - don't use them with io_uring.
That, BTW, is one of the reasons I'm sceptical about burying the
decisions deep into the callchain - we don't _want_ different
data layouts on read/write depending upon the 32bit vs. 64bit
caller, let alone the pointer-chasing garbage that is /dev/sg.