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

On Sep 19, 2020, at 2:16 PM, Arnd Bergmann [off-list ref] wrote:

On Sat, Sep 19, 2020 at 6:21 PM Andy Lutomirski [off-list ref] wrote:

quoted

quoted

On Fri, Sep 18, 2020 at 8:16 AM Christoph Hellwig [off-list ref] wrote:
On Fri, Sep 18, 2020 at 02:58:22PM +0100, Al Viro wrote:

quoted

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.

quoted

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?