Messages in this thread | | | Date | Sat, 23 Aug 2014 01:13:14 +0400 | From | Cyrill Gorcunov <> | Subject | Re: [patch 4/4] prctl: PR_SET_MM -- Introduce PR_SET_MM_MAP operation, v3 |
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On Fri, Aug 22, 2014 at 01:46:28PM -0700, Andrew Morton wrote: > On Sat, 23 Aug 2014 00:38:09 +0400 Cyrill Gorcunov <gorcunov@gmail.com> wrote: > > > > > > > Or will we? What happens if we later decide that some additional field > > > needs to be added? Do we version the interface? Add a new prctl() > > > mode? Let's cook up a plan for that and at least add to changelog? > > > > I don't expect to change it anytime soon but we still have an option -- > > if we decide to extend or shrink it we always can use sizeof/offsetof > > helpers to check which exactly version userspace asks us to use. > > How does that work? We just have a blob of bytes coming in from > userspace.
Not just blob. We have it as a structure where all fields have a constant size. Say we have
struct prctl_mm_map { __u64 start_code; __u64 start_code; __u64 some-new-field; };
in the kernel, so its size will be 24 bytes but userspace uses old definition without @some-new-field member (16 bytes). So when we get a reguest with 16 bytes from userspace we can find the userspace have passed old definition. It's not as explicit as if we would have some @version field in struct prctl_mm_mmap, but looks fine for me. Still I can add @version into the structure if you prefer.
> > As far as I understand the mm_struct is not the structure which > > changes that frequently, right? > > We might find existing things which criu wants to access. And criu > lives forever, yes? The mm_struct is likely to change over that time > period ;)
Hopefully criu will live long enough so I would have a chance to update prctl_mm_map accordingly :) Still the good thing is that once mm_struct get changed the kernel fails to build in sys.c and the change will be noticed immediately so we update sys.c as well.
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