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

On Sep 19, 2020, at 3:41 PM, Al Viro [off-list ref] wrote:

On Sat, Sep 19, 2020 at 03:23:54PM -0700, Andy Lutomirski wrote:

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On Sep 19, 2020, at 3:09 PM, Al Viro [off-list ref] wrote:

On Fri, Sep 18, 2020 at 05:16:15PM +0200, Christoph Hellwig wrote:

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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(...)