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

[Andy Lutomirski]

> On Sep 19, 2020, at 3:41 PM, Al Viro <viro at zeniv.linux.org.uk> wrote:
> 
> On Sat, Sep 19, 2020 at 03:23:54PM -0700, Andy Lutomirski wrote:
>> 
>>>> 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.
> 
> 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(...)