Re: [PATCH] seccomp: Add pkru into seccomp_data

* Michael Sammler:

Thank you for the pointer about the POWER implementation. I am not
familiar with POWER in general and its protection key feature at
all. Would the AMR register be the correct register to expose here?

Yes, according to my notes, the register is called AMR (special purpose
register 13).

I understand your concern about exposing the number of protection keys
in the ABI. One idea would be to state, that the pkru field (which
should probably be renamed) contains an architecture specific value,
which could then be the PKRU on x86 and AMR (or another register) on
POWER. This new field should probably be extended to __u64 and the
reserved field removed.

POWER also has proper read/write bit separation, not PKEY_DISABLE_ACCESS
(disable read and write) and PKEY_DISABLE_WRITE like Intel.  It's
currently translated by the kernel, but I really need a
PKEY_DISABLE_READ bit in glibc to implement pkey_get in case the memory
is write-only.

Another idea would be to not add a field in the seccomp_data
structure, but instead provide a new BPF instruction, which reads the
value of a specified protection key.

I would prefer that if it's possible.  We should make sure that the bits
are the same as those returned from pkey_get.  I have an implementation
on POWER, but have yet to figure out the implications for 32-bit because
I do not know the AMR register size there.

Thanks,
Florian