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

On Thu, Aug 31, 2017 at 06:27:31PM -0700, Linus Torvalds wrote:

Why? That's no different from the case statement for the mmu case,
just written differently.

Yes.

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;
    }

Btw, at least my original idea was to make MAP_VALIDATE a flag
instead of another mapping type, that is take it out of MAP_TYPE.

That being said this version is ok with me too - the chances of
needing a new type of private mappings probably isn't too big.