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

On Thu, Aug 31, 2017 at 6:01 PM, Dan Williams [off-list ref] 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.

Of course, I also suspect that for nommu you might as well just always
return -EINVAL anyway. The only people who would ever use
MAP_SHARED_VALIDATE are the kinds of people who do things that just
aren't likely relevant on nommu, but whatever..

               Linus