Re: [PATCH v4 18/29] arm64: add POE signal support

[Dave Martin <Dave.Martin@arm.com>]

On Tue, Aug 20, 2024 at 03:06:06PM +0100, Joey Gouly wrote:
> On Tue, Aug 20, 2024 at 02:54:50PM +0100, Dave Martin wrote:
> > On Tue, Aug 20, 2024 at 10:54:41AM +0100, Joey Gouly wrote:
> > > On Mon, Aug 19, 2024 at 06:09:06PM +0100, Catalin Marinas wrote:
> > > > On Thu, Aug 15, 2024 at 04:09:26PM +0100, Dave P Martin wrote:
> > > > > On Thu, Aug 15, 2024 at 02:18:15PM +0100, Joey Gouly wrote:
> > > > > > That's a lot of words to say, or ask, do you agree with the approach of only
> > > > > > saving POR_EL0 in the signal frame if num_allocated_pkeys() > 1?
> > > > > > 
> > > > > > Thanks,
> > > > > > Joey
> > > > > 
> > > > > ...So..., given all the above, it is perhaps best to go back to
> > > > > dumping POR_EL0 unconditionally after all, unless we have a mechanism
> > > > > to determine whether pkeys are in use at all.
> > > > 
> > > > Ah, I can see why checking for POR_EL0_INIT is useful. Only checking for
> > > > the allocated keys gets confusing with pkey 0.
> > > > 
> > > > Not sure what the deal is with pkey 0. Is it considered allocated by
> > > > default or unallocatable? If the former, it implies that pkeys are
> > > > already in use (hence the additional check for POR_EL0_INIT). In
> > > > principle the hardware allows us to use permissions where the pkeys do
> > > > not apply but we'd run out of indices and PTE bits to encode them, so I
> > > > think by default we should assume that pkey 0 is pre-allocated.
> > > > 
> > > > 
> > > 
> > > You can consider pkey 0 allocated by default. You can actually pkey_free(0), there's nothing stopping that.
> > 
> > Is that intentional?
> 
> I don't really know? It's intentional from my side in that it, I allow it,
> because it doesn't look like x86 or PPC block pkey_free(0).
> 
> I found this code that does pkey_free(0), but obviously it's a bit of a weird test case:
> 
> 	https://github.com/ColinIanKing/stress-ng/blob/master/test/test-pkey-free.c#L29

Of course, pkey 0 will still be in use for everything, and if the man
pages are to be believed, the PKRU bits for pkey 0 may no longer be
maintained after this call...

So this test is possibly a little braindead.  A clear use-case for
freeing pkey 0 would be more convincing.

In the meantime though, it makes most sense for arm64 to follow
the precedent set by other arches on this (as you did).

[...]

Cheers
---Dave