Re: [PATCH v4 17/29] arm64: implement PKEYS support

* Szabolcs Nagy:

quoted

However, does it matter much? That's only for the initial setup, the
user can then change the permissions directly via the sysreg. So maybe
we don't need all those combinations upfront. A PKEY_DISABLE_EXECUTE
together with the full PKEY_DISABLE_ACCESS would probably suffice.

this is ok.

a bit awkward in userspace when the register is directly
set to e.g write-only and pkey_get has to return something,
but we can handle settings outside of valid PKEY_* macros
as unspec, users who want that would use their own register
set/get code.

i would have designed the permission to use either existing
PROT_* flags or say that it is architectural and written to
the register directly and let the libc wrapper deal with
portable api, i guess it's too late now.

We can still define a portable API if we get a few more PKEY_* bits.
The last attempt stalled because the kernel does not really need them,
it would be for userspace benefit only.

For performance-critical code, pkey_get/pkey_set are already too slow,
so adding a bit more bit twiddling to it wouldn't be a proble, I think.
Applications that want to change protection key bits around a very short
code sequence will have to write the architecture-specific register.

(the signal handling behaviour should have a control and it
is possible to fix e.g. via pkey_alloc flags, but that may
not be the best solution and this can be done later.)

For glibc, the POWER behavior is much more useful.

Thanks,
Florian