On Sat 26-02-22 02:26:55, Nadav Amit 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).
>
> 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.
>
> Initialize real_address in various code-paths to be consistent with
> address, even when it is not used, to be on the safe side.
>
> Acked-by: Peter Xu <peterx@xxxxxxxxxx>
> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
>
> ---
>
> v2->v3:
> * Initialize real_address on all code paths [Jan]
>
> v1->v2:
> * Add uffd feature to selectively enable [David, Andrea]
I've just noticed one typo below. Otherwise the patch looks good to me.
Feel free to add:
Reviewed-by: Jan Kara <jack@xxxxxxx>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 213cc569b192..27df0ca0a36a 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -478,7 +478,8 @@ struct vm_fault {
> struct vm_area_struct *vma; /* Target VMA */
> gfp_t gfp_mask; /* gfp mask to be used for allocations */
> pgoff_t pgoff; /* Logical page offset based on vma */
> - unsigned long address; /* Faulting virtual address */
> + unsigned long address; /* Faulting virtual address - masked */
> + unsigned long real_address; /* Faulting virtual address - unmaked */
^^ typo here
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
