Re: [PATCH 2/2] mm: introduce MAP_VALIDATE, a mechanism for for safely defining new mmap flags

[Dan Williams <dan.j.williams@intel.com>]

On Thu, Aug 31, 2017 at 6:27 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Thu, Aug 31, 2017 at 6:01 PM, Dan Williams <dan.j.williams@intel.com> wrote:
>>
>> Ugh, nommu defeats the MAP_SHARED_VALIDATE proposal from Linus.
>>
>>         if ((flags & MAP_TYPE) != MAP_PRIVATE &&
>>             (flags & MAP_TYPE) != MAP_SHARED)
>>                 return -EINVAL;
>>
>> ...parisc strikes again.
>
> Why? That's no different from the case statement for the mmu case,
> just written differently.
>
> You *want* existing kernels to fail, since they don't test the bits
> you want to test.
>
> So you just want to rewrite these all as
>
>     switch (flags & MAP_TYPE) {
>     case MAP_SHARED_VALIDATE:
>         .. validate the other bits...
>         /* fallhtough */
>     case MAP_SHARED:
>         .. do the shared case ..
>     case MAP_PRIVATE:
>         .. do the private case ..
>     default:
>         return -EINVAL;
>     }
>
> and you're all good.
>
> I'm not seeing the problem.

Right, I went cross-eyed for a second. There is no problem.

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