Re: [PATCH] mmap.2: describe the 5level paging hack

Kirill A. Shutemov
2019-02-12T09:41:54Z

On Mon, Feb 11, 2019 at 05:36:53PM +0100, Jann Horn wrote:
> The manpage is missing information about the compatibility hack for
> 5-level paging that went in in 4.14, around commit ee00f4a32a76 ("x86/mm:
> Allow userspace have mappings above 47-bit"). Add some information about
> that.
>
> While I don't think any hardware supporting this is shipping yet (?), I
> think it's useful to try to write a manpage for this API, partly to
> figure out how usable that API actually is, and partly because when this
> hardware does ship, it'd be nice if distro manpages had information about
> how to use it.
>
> Signed-off-by: Jann Horn <jannh@google.com>

Thanks for doing this.

> ---
> This patch goes on top of the patch "[PATCH] mmap.2: fix description of
> treatment of the hint" that I just sent, but I'm not sending them in a
> series because I want the first one to go in, and I think this one might
> be a bit more controversial.
>
> It would be nice if the architecture maintainers and mm folks could have
> a look at this and check that what I wrote is right - I only looked at
> the source for this, I haven't tried it.
>
> man2/mmap.2 | 15 +++++++++++++++
> 1 file changed, 15 insertions(+)
>
> diff --git a/man2/mmap.2 b/man2/mmap.2
> index 8556bbfeb..977782fa8 100644
> --- a/man2/mmap.2
> +++ b/man2/mmap.2
> @@ -67,6 +67,8 @@ is NULL,
> then the kernel chooses the (page-aligned) address
> at which to create the mapping;
> this is the most portable method of creating a new mapping.
> +On Linux, in this case, the kernel may limit the maximum address that can be
> +used for allocations to a legacy limit for compatibility reasons.
> If
> .I addr
> is not NULL,
> @@ -77,6 +79,19 @@ or equal to the value specified by
> and attempt to create the mapping there.
> If another mapping already exists there, the kernel picks a new
> address, independent of the hint.
> +However, if a hint above the architecture's legacy address limit is provided
> +(on x86-64: above 0x7ffffffff000, on arm64: above 0x1000000000000, on ppc64 with
> +book3s: above 0x7fffffffffff or 0x3fffffffffff, depending on page size), the
> +kernel is permitted to allocate mappings beyond the architecture's legacy
> +address limit. The availability of such addresses is hardware-dependent.
> +Therefore, if you want to be able to use the full virtual address space of
> +hardware that supports addresses beyond the legacy range, you need to specify an
> +address above that limit; however, for security reasons, you should avoid
> +specifying a fixed valid address outside the compatibility range,
> +since that would reduce the value of userspace address space layout
> +randomization. Therefore, it is recommended to specify an address
> +.I beyond
> +the end of the userspace address space.

It probably worth recommending (void *) -1 as such address.

> .\" Before Linux 2.6.24, the address was rounded up to the next page
> .\" boundary; since 2.6.24, it is rounded down!
> The address of the new mapping is returned as the result of the call.
> --
> 2.20.1.791.gb4d0f1c61a-goog
>

--
Kirill A. Shutemov