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

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

The 06/17/2024 15:40, Florian Weimer wrote:
> >> A user can still set it by interacting with the register directly, but I guess
> >> we want something for the glibc interface..
> >> 
> >> Dave, any thoughts here?
> >
> > adding Florian too, since i found an old thread of his that tried
> > to add separate PKEY_DISABLE_READ and PKEY_DISABLE_EXECUTE, but
> > it did not seem to end up upstream. (this makes more sense to me
> > as libc api than the weird disable access semantics)
> 
> I still think it makes sense to have a full complenent of PKEY_* flags
> complementing the PROT_* flags, in a somewhat abstract fashion for
> pkey_alloc only.  The internal protection mask register encoding will
> differ from architecture to architecture, but the abstract glibc
> functions pkey_set and pkey_get could use them (if we are a bit
> careful).

to me it makes sense to have abstract

PKEY_DISABLE_READ
PKEY_DISABLE_WRITE
PKEY_DISABLE_EXECUTE
PKEY_DISABLE_ACCESS

where access is handled like

if (flags&PKEY_DISABLE_ACCESS)
	flags |= PKEY_DISABLE_READ|PKEY_DISABLE_WRITE;
disable_read = flags&PKEY_DISABLE_READ;
disable_write = flags&PKEY_DISABLE_WRITE;
disable_exec = flags&PKEY_DISABLE_EXECUTE;

if there are unsupported combinations like
disable_read&&!disable_write then those are rejected
by pkey_alloc and pkey_set.

this allows portable use of pkey apis.
(the flags could be target specific, but don't have to be)