Re: [PATCH] Fix tst-pkey expectations on pkey_get

[Florian Weimer <fweimer@redhat.com>]

* Lucas A. M. Magalhaes:

> Quoting Florian Weimer (2020-02-07 15:22:16)
>> * Lucas A. M. Magalhaes:
>> 
>> > Florian, Your patch including pkey_set and pkey_get looks good to me.
>> > Can you merge it?  This one
>> > https://sourceware.org/ml/libc-alpha/2018-05/msg00760.html.
>> 
>> Thanks.  Is the patch really correct for 32-bit userspace?
>> 
>
> Thanks for pointing it out. Even in 32-bit mode the mtspr will effect all
> 64 bits and there is no 32 bit limitation for AMR. I will try to setup
> a 32 bit userspace machine with pkeys enabled to test this out.

Thanks.  We may have to tweak the constraints a little bit, though.

>> > With this there will be one failure on this test on powerpc machines.
>> > The test expects that during a signal handling the pkey_get returns
>> > PKEY_DISABLE_ACCESS for all keys. In my tests it returns the same
>> > permissions as before the signal. I couldn't find where this is done for
>> > x86. Is this kernel implementation?
>> 
>> POWER has read-disable and write-disable flags which work independently.
>> The kernel-defined flags cannot represent the read-disable
>> configuration.  At the time, there was no read-disable flag allocated in
>> the kernel.  Has this changed?  (See the ”Translate” comments in my
>> patch.)
>> 
>
> I think we are talking about this patch
> https://marc.info/?l=linux-api&m=154390462121345&w=2 that you were
> discussing a PKEY_DISABLE_READ flag for pkey_alloc. Unfortunately It
> was not merged.

Right.

> I agree that this work should be done. But this should not block the
> pkey_get and pkey_set implementation for power to be merged for the
> actual api.

That sounds about right.

>> On x86, the hardware has write-disable and read-write-disable flags
>> instead, which matches the original UAPI interfaces.  This is why no
>> translation is necessary.
>
> Excuse my ignorance. How this translation problem influences the signal
> handling behaviour?

The signal handling behavior is just different.  If I recall correctly,
x86 always resets PKRU to a mostly-disable value, while POWER inherits
the AMR value from the interrupted thread.  The POWER behavior seems
more useful to me, but that depends on what the programmer tries to do.

I think I have posted x86 patches for changing the behavior, too.

Thanks,
Florian