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

On Thu, Aug 31, 2017 at 6:27 PM, Linus Torvalds
[off-list ref] wrote:

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

quoted

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.