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

[Andy Lutomirski]

> On Sep 19, 2020, at 3:09 PM, Al Viro <viro at zeniv.linux.org.uk> wrote:
> 
> 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.

Well, we could remove in_compat_syscall(), etc and instead have an implicit parameter in DEFINE_SYSCALL.  Then everything would have to be explicit.  This would probably be a win, although it could be quite a bit of work.