Re: [PATCH 05/11] x86/fpu: set PKRU state for kernel threads

[Andy Lutomirski]

> On Oct 19, 2018, at 10:01 AM, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> wrote:
> 
> On 10/19/2018 09:59 AM, Andy Lutomirski wrote:
>>> That looks like a good API in general.  The ffs_user_copy_worker that
>>> Sebastian mentioned seems to be used by AIO, in which case of course it
>>> has to happen in a kernel thread.
>>> 
>>> But while the API is good, deciding on the desired semantics is
>>> "interesting".  The submitting thread might be changing PKRU between the
>>> time the I/O operation is submitted and the time it is completed, for
>>> example.
>> I think thereâs only one sensible answer: capture PKRU at the time of submission.
> 
> I think it's much more straightforward to just not enforce pkeys.
> Having this "phantom" value could cause a very odd, nearly undebuggable
> I/O failure.

But now we have the reverse. The IO can work if itâs truly async but, if the kernel decides to synchronously complete IO (with GUP or copy_to_user), itâll fail, right. This isnât exactly friendly either.