Messages in this thread | | | From | David Hildenbrand <> | Subject | Re: [PATCH v17 07/10] mm: introduce memfd_secret system call to create "secret" memory areas | Date | Mon, 22 Feb 2021 11:50:02 +0100 |
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On 22.02.21 10:38, David Hildenbrand wrote: > On 17.02.21 17:19, James Bottomley wrote: >> On Tue, 2021-02-16 at 18:16 +0100, David Hildenbrand wrote: >> [...] >>>>> The discussion regarding migratability only really popped up >>>>> because this is a user-visible thing and not being able to >>>>> migrate can be a real problem (fragmentation, ZONE_MOVABLE, ...). >>>> >>>> I think the biggest use will potentially come from hardware >>>> acceleration. If it becomes simple to add say encryption to a >>>> secret page with no cost, then no flag needed. However, if we only >>>> have a limited number of keys so once we run out no more encrypted >>>> memory then it becomes a costly resource and users might want a >>>> choice of being backed by encryption or not. >>> >>> Right. But wouldn't HW support with configurable keys etc. need more >>> syscall parameters (meaning, even memefd_secret() as it is would not >>> be sufficient?). I suspect the simplistic flag approach might not >>> be sufficient. I might be wrong because I have no clue about MKTME >>> and friends. >> >> The theory I was operating under is key management is automatic and >> hidden, but key scarcity can't be, so if you flag requesting hardware >> backing then you either get success (the kernel found a key) or failure >> (the kernel is out of keys). If we actually want to specify the key >> then we need an extra argument and we *must* have a new system call. >> >>> Anyhow, I still think extending memfd_create() might just be good >>> enough - at least for now. >> >> I really think this is the wrong approach for a user space ABI. If we >> think we'll ever need to move to a separate syscall, we should begin >> with one. The pain of trying to shift userspace from memfd_create to a >> new syscall would be enormous. It's not impossible (see clone3) but >> it's a pain we should avoid if we know it's coming. > > Sorry for the late reply, there is just too much going on :) > > *If* we ever realize we need to pass more parameters we can easily have > a new syscall for that purpose. *Then*, we know how that syscall will > look like. Right now, it's just pure speculation. > > Until then, going with memfd_create() works just fine IMHO. > > The worst think that could happen is that we might not be able to create > all fancy sectremem flavors in the future via memfd_create() but only > via different, highly specialized syscall. I don't see a real problem > with that. >
Adding to that, I'll give up arguing now as I have more important things to do. It has been questioned by various people why we need a dedicate syscall and at least for me, without a satisfying answer.
Worst thing is that we end up with a syscall that could have been avoided, for example, because 1. We add existing/future memfd_create() flags to memfd_secret() as well when we need them (sealing, hugetlb., ..). 2. We decide in the future to still add MFD_SECRET support to memfd_secret().
So be it.
-- Thanks,
David / dhildenb
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