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SubjectRe: More info for DSM w/r/t sunffb on 2.6.15-rc6
On Sat, 24 Dec 2005, Linus Torvalds wrote:
> On Fri, 23 Dec 2005, David S. Miller wrote:
> >
> > + if (!(vma->vm_flags & VM_SHARED))
> > + return -EINVAL;
> > +
>
> Side note - as I explained to Nick the other week, VM_SHARED really means
> "shared _writable_" mapping, so you're now disallowing a shared read-only
> open too.
>
> Which may be fine, of course. Especially if sbusfb always ends up giving a
> writable pfn-mapping. But I wanted to check that that was what you meant
> to do.
>
> To test for MAP_SHARED, either do the is_cow_mapping() thing, or check
> the VM_MAYSHARE bit.

I wondered over that too. Concluded that at the moment it's right for
the test to be on VM_SHARED, because it's secretly giving away shared
write permission. But in future, once that's been corrected, such checks
would do better to check for VM_MAYSHARE (i.e. reject all MAP_PRIVATE).

Hugh
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