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    SubjectRe: [PATCH 21/25] mm: implement new mprotect_key() system call
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    On Mon, 2015-09-28 at 12:18 -0700, Dave Hansen wrote:
    > From: Dave Hansen <dave.hansen@linux.intel.com>
    >
    > mprotect_key() is just like mprotect, except it also takes a
    > protection key as an argument. On systems that do not support
    > protection keys, it still works, but requires that key=0.

    I'm not sure how userspace is going to use the key=0 feature? ie. userspace
    will still have to detect that keys are not supported and use key 0 everywhere.
    At that point it could just as well skip the mprotect_key() syscalls entirely
    couldn't it?

    > I expect it to get used like this, if you want to guarantee that
    > any mapping you create can *never* be accessed without the right
    > protection keys set up.
    >
    > pkey_deny_access(11); // random pkey
    > int real_prot = PROT_READ|PROT_WRITE;
    > ptr = mmap(NULL, PAGE_SIZE, PROT_NONE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
    > ret = mprotect_key(ptr, PAGE_SIZE, real_prot, 11);
    >
    > This way, there is *no* window where the mapping is accessible
    > since it was always either PROT_NONE or had a protection key set.
    >
    > We settled on 'unsigned long' for the type of the key here. We
    > only need 4 bits on x86 today, but I figured that other
    > architectures might need some more space.

    If the existing mprotect() syscall had a flags argument you could have just
    used that. So is it worth just adding mprotect2() now and using it for this? ie:

    int mprotect2(unsigned long start, size_t len, unsigned long prot, unsigned long flags) ..

    And then you define bit zero of flags to say you're passing a pkey, and it's in
    bits 1-63?

    That way if other arches need to do something different you at least have the
    flags available?

    cheers




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