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SubjectRe: [PATCH 10/17] prmem: documentation
On Tue, Oct 30, 2018 at 10:58:14AM -0700, Matthew Wilcox wrote:
> On Tue, Oct 30, 2018 at 10:06:51AM -0700, Andy Lutomirski wrote:
> > > On Oct 30, 2018, at 9:37 AM, Kees Cook <keescook@chromium.org> wrote:
> > I support the addition of a rare-write mechanism to the upstream kernel.
> > And I think that there is only one sane way to implement it: using an
> > mm_struct. That mm_struct, just like any sane mm_struct, should only
> > differ from init_mm in that it has extra mappings in the *user* region.
>
> I'd like to understand this approach a little better. In a syscall path,
> we run with the user task's mm. What you're proposing is that when we
> want to modify rare data, we switch to rare_mm which contains a
> writable mapping to all the kernel data which is rare-write.
>
> So the API might look something like this:
>
> void *p = rare_alloc(...); /* writable pointer */
> p->a = x;
> q = rare_protect(p); /* read-only pointer */
>
> To subsequently modify q,
>
> p = rare_modify(q);
> q->a = y;
> rare_protect(p);

Why would you have rare_alloc() imply rare_modify() ? Would you have the
allocator meta data inside the rare section?

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