On Fri, 18 Feb 2022 04:10:03 +0000 Nadav Amit <nadav.amit@xxxxxxxxx> wrote:
> From: Nadav Amit <namit@xxxxxxxxxx>
>
> Userfaultfd is supposed to provide the full address (i.e., unmasked) of
> the faulting access back to userspace. However, that is not the case for
> quite some time.
>
> Even running "userfaultfd_demo" from the userfaultfd man page provides
> the wrong output (and contradicts the man page). Notice that
> "UFFD_EVENT_PAGEFAULT event" shows the masked address (7fc5e30b3000)
> and not the first read address (0x7fc5e30b300f).
Well damn.
> Address returned by mmap() = 0x7fc5e30b3000
>
> fault_handler_thread():
> poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
> UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fc5e30b3000
> (uffdio_copy.copy returned 4096)
> Read address 0x7fc5e30b300f in main(): A
> Read address 0x7fc5e30b340f in main(): A
> Read address 0x7fc5e30b380f in main(): A
> Read address 0x7fc5e30b3c0f in main(): A
>
> The exact address is useful for various reasons and specifically for
> prefetching decisions. If it is known that the memory is populated by
> certain objects whose size is not page-aligned, then based on the
> faulting address, the uffd-monitor can decide whether to prefetch and
> prefault the adjacent page.
>
> This bug has been for quite some time in the kernel: since commit
> 1a29d85eb0f1 ("mm: use vmf->address instead of of vmf->virtual_address")
> vmf->virtual_address"), which dates back to 2016. A concern has been
> raised that existing userspace application might rely on the old/wrong
> behavior in which the address is masked. Therefore, it was suggested to
> provide the masked address unless the user explicitly asks for the exact
> address.
>
> Add a new userfaultfd feature UFFD_FEATURE_EXACT_ADDRESS to direct
> userfaultfd to provide the exact address. Add a new "real_address" field
> to vmf to hold the unmasked address. Provide the address to userspace
> accordingly.
Is a manpage update planned?
