On Tue, May 8, 2018 at 5:40 AM Florian Weimer <fweimer@xxxxxxxxxx> wrote: > On 05/08/2018 04:49 AM, Andy Lutomirski wrote: > > On Mon, May 7, 2018 at 2:48 AM Florian Weimer <fweimer@xxxxxxxxxx> wrote: > > > >> On 05/03/2018 06:05 AM, Andy Lutomirski wrote: > >>> On Wed, May 2, 2018 at 7:11 PM Ram Pai <linuxram@xxxxxxxxxx> wrote: > >>> > >>>> On Wed, May 02, 2018 at 09:23:49PM +0000, Andy Lutomirski wrote: > >>>>> > >>>>>> If I recall correctly, the POWER maintainer did express a strong > >>> desire > >>>>>> back then for (what is, I believe) their current semantics, which my > >>>>>> PKEY_ALLOC_SIGNALINHERIT patch implements for x86, too. > >>>>> > >>>>> Ram, I really really don't like the POWER semantics. Can you give > > some > >>>>> justification for them? Does POWER at least have an atomic way for > >>>>> userspace to modify just the key it wants to modify or, even better, > >>>>> special load and store instructions to use alternate keys? > >>> > >>>> I wouldn't call it POWER semantics. The way I implemented it on power > >>>> lead to the semantics, given that nothing was explicitly stated > >>>> about how the semantics should work within a signal handler. > >>> > >>> I think that this is further evidence that we should introduce a new > >>> pkey_alloc() mode and deprecate the old. To the extent possible, this > >>> thing should work the same way on x86 and POWER. > > > >> Do you propose to change POWER or to change x86? > > > > Sorry for being slow to reply. I propose to introduce a new > > PKEY_ALLOC_something variant on x86 and POWER and to make the behavior > > match on both. > So basically implement PKEY_ALLOC_SETSIGNAL for POWER, and keep the > existing (different) behavior without the flag? > Ram, would you be okay with that? Could you give me a hand if > necessary? (I assume we have silicon in-house because it's a > long-standing feature of the POWER platform which was simply dormant on > Linux until now.) > > It should at least update the values loaded when a signal > > is delivered and it should probably also update it for new threads. > I think we should keep inheritance for new threads and fork. pkey_alloc > only has a single access rights argument, which makes it hard to reuse > this interface if there are two (three) separate sets of access rights. Hmm. I can get on board with the idea that fork() / clone() / pthread_create() are all just special cases of the idea that the thread that *calls* them should have the right pkey values, and the latter is already busted given our inability to asynchronously propagate the new mode in pkey_alloc(). So let's so PKEY_ALLOC_SETSIGNAL as a starting point. One thing we could do, though: the current initual state on process creation is all access blocked on all keys. We could change it so that half the keys are fully blocked and half are read-only. Then we could add a PKEY_ALLOC_STRICT or similar that allocates a key with the correct initial state *and* does the setsignal thing. If there are no keys left with the correct initial state, then it fails.
