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
