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

[Andy Lutomirski]

> On Sep 19, 2020, at 2:16 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> 
> On Sat, Sep 19, 2020 at 6:21 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>>> On Fri, Sep 18, 2020 at 8:16 AM Christoph Hellwig <hch@xxxxxx> 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.
> 
> Ah, so reading /dev/input/event* would suffer from the same issue,
> and that one would in fact be broken by your patch in the hypothetical
> case that someone tried to use io_uring to read /dev/input/event on x32...
> 
> For reference, I checked the socket timestamp handling that has a
> number of corner cases with time32/time64 formats in compat mode,
> but none of those appear to be affected by the problem.
> 
>> Aside from the potentially nasty use of per-task variables, one thing
>> I don't like about PF_FORCE_COMPAT is that it's one-way.  If we're
>> going to have a generic mechanism for this, shouldn't we allow a full
>> override of the syscall arch instead of just allowing forcing compat
>> so that a compat syscall can do a non-compat operation?
> 
> The only reason it's needed here is that the caller is in a kernel
> thread rather than a system call. Are there any possible scenarios
> where one would actually need the opposite?
> 

I can certainly imagine needing to force x32 mode from a kernel thread.

As for the other direction: what exactly are the desired bitness/arch semantics of io_uring?  Is the operation bitness chosen by the io_uring creation or by the io_uring_enter() bitness?