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

[Joey Gouly]

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

> 
> You're not supposed to free pkeys that are in use, and it's quasi-
> impossible to know whether pkey 0 is in use: all binaries in the
> process assume that pkey is available and use it by default for their
> pages, plus the stack will be painted with pkey 0, and the vDSO has to
> be painted with some pkey.
> 
> Actually, that's a good point, because of the vDSO I think that only
> special bits of code with a private ABI (e.g., JITted code etc.) that
> definitely don't call into the vDSO can block permissions on pkey 0...
> otherwise, stuff will break.
> 
> > 
> > > So I agree that it's probably best to save it unconditionally.
> > 
> > Alright, will leave it as is!
> 
> Ack, I think the whole discussion around this has shown that there
> isn't a _simple_ argument for conditionally dumping POR_EL0... so I'm
> prepared to admit defeat here.
> 
> We might still try to slow down the consumption of the remaining space
> with a "misc registers" record, instead of dedicating a record to
> POR_EL0.  I have some thoughts on that, but if nobody cares that much
> then this probably isn't worth pursuing.
> 
> Cheers
> ---Dave
>