Re: [Qemu-devel] [PATCH 10/23] userfaultfd: add new syscall to provide memory externalization

[Andrea Arcangeli]

Hi Dave,

On Tue, Jun 23, 2015 at 12:00:19PM -0700, Dave Hansen wrote:
> Down in userfaultfd_wake_function(), it looks like you intended for a
> len=0 to mean "wake all".  But the validate_range() that we do from
> userspace has a !len check in it, which keeps us from passing a len=0 in
> from userspace.
> Was that "wake all" for some internal use, or is the check too strict?

It's for internal use or userfaultfd_release that has to wake them all
(after setting ctx->released) if the uffd is closed. It avoids to
enlarge the structure by depending on the invariant that userland
cannot pass len=0.

If we'd accept len=0 from userland as valid, I'd be safer if it does
nothing like in madvise, I doubt we want to expose this non standard
kernel internal behavior to userland.

> I was trying to use the wake ioctl after an madvise() (as opposed to
> filling things in using a userfd copy).

madvise will return 0 if len=0, mremap would return -EINVAL if new_len
is zero, mmap also returns -EINVAL if len is 0, not all MM syscalls
are as permissive as madvise. Can't you pass the same len you pass to
madvise to UFFDIO_WAKE (or just skip the call if the madvise len is
zero)?

Thanks,
Andrea