Re: [PATCH v2 1/6] powerpc/pkeys: Enable all user-allocatable pkeys at init.

On Tue, Jun 19, 2018 at 10:39:52PM +1000, Michael Ellerman wrote:

Ram Pai [off-list ref] writes:

quoted

In a multithreaded application, a key allocated by one thread must
be activate and usable on all threads.

Currently this is not the case, because the UAMOR bits for all keys are
disabled by default. When a new key is allocated in one thread, though
the corresponding UAMOR bits for that thread get enabled, the UAMOR bits
for all other existing threads continue to have their bits disabled.
Other threads have no way to set permissions on the key, effectively
making the key useless.

This all seems a bit strongly worded to me. It's arguable whether a key
should be usable by the thread that allocated it or all threads.

You could conceivably have a design where threads are blocked from using
a key until they're given permission to do so by the thread that
allocated the key.

But we're changing the behaviour to match x86 and because we don't have
an API to grant another thread access to a key. Right?

correct. The other threads have no way to access or change the
permissions on the key.

quoted

Enable the UAMOR bits for all keys, at process creation. Since the
contents of UAMOR are inherited at fork, all threads are capable of
modifying the permissions on any key.

BTW: changing the permission on unallocated keys has no effect, till
those keys are not associated with any PTEs. The kernel will anyway
disallow to association of unallocated keys with PTEs.

This is an ABI change, which is bad, but I guess we call it a bug fix
because things didn't really work previously?

Yes its a behaviorial change for the better. There is no downside
to the change because no applications should break. Single threaded
apps will continue to just work fine. Multithreaded applications,
which were unable to consume the API/ABI, will now be able to do so.

I'll tag it:

  Fixes: cf43d3b26452 ("powerpc: Enable pkey subsystem")
  Cc: stable@vger.kernel.org # v4.16+

thanks,
RP