[PATCH v15 05/17] arms64: untag user pointers passed to memory syscalls

Khalid Aziz khalid.aziz at oracle.com
Thu May 30 16:05:55 UTC 2019

On 5/30/19 9:11 AM, Catalin Marinas wrote:
> On Wed, May 29, 2019 at 01:16:37PM -0600, Khalid Aziz wrote:
>> mmap() can return the same tagged address but I am uneasy about kernel
>> pre-coloring the pages. Database can mmap 100's of GB of memory. That is
>> lot of work being offloaded to the kernel to pre-color the page even if
>> done in batches as pages are faulted in.
> For anonymous mmap() for example, the kernel would have to zero the
> faulted in pages anyway. We can handle the colouring at the same time in
> clear_user_page() (as I said below, we have to clear the colour anyway
> from previous uses, so it's simply extending this to support something
> other than tag/colour 0 by default with no additional overhead).

On sparc M7, clear_user_page() ends up in M7clear_user_page defined in
arch/sparc/lib/M7memset.S. M7 code use Block Init Store (BIS) to clear
the page. BIS on M7 clears the memory tags as well and no separate
instructions are needed to clear the tags. As a result when kernel
clears a page before returning it to user, the page is not only zeroed
out, its tags are also cleared to 0.

>>> Since we already need such loop in the kernel, we might as well allow
>>> user space to require a certain colour. This comes in handy for large
>>> malloc() and another advantage is that the C library won't be stuck
>>> trying to paint the whole range (think GB).
>> If kernel is going to pre-color all pages in a vma, we will need to
>> store the default tag in the vma. It will add more time to page fault
>> handling code. On sparc M7, kernel will need to execute additional 128
>> stxa instructions to set the tags on a normal page.
> As I said, since the user can retrieve an old colour using ldxa, the
> kernel should perform this operation anyway on any newly allocated page
> (unless you clear the existing colour on page freeing).>

Tags are not cleared on sparc on freeing. They get cleared when the page
is allocated again.

>>>> We can try to store tags for an entire region in vma but that is
>>>> expensive, plus on sparc tags are set in userspace with no
>>>> participation from kernel and now we need a way for userspace to
>>>> communicate the tags to kernel.
>>> We can't support finer granularity through the mmap() syscall and, as
>>> you said, the vma is not the right thing to store the individual tags.
>>> With the above extension to mmap(), we'd have to store a colour per vma
>>> and prevent merging if different colours (we could as well use the
>>> pkeys mechanism we already have in the kernel but use a colour per vma
>>> instead of a key).
>> Since tags can change on any part of mmap region on sparc at any time
>> without kernel being involved, I am not sure I see much reason for
>> kernel to enforce any tag related restrictions.
> It's not enforcing a tag, more like the default colour for a faulted in
> page. Anyway, if sparc is going with default 0/untagged, that's fine as
> well. We may add this mmap() option to arm64 only.
>>>> From sparc point of view, making kernel responsible for assigning tags
>>>> to a page on page fault is full of pitfalls.
>>> This could be just some arm64-specific but if you plan to deploy it more
>>> generically for sparc (at the C library level), you may find this
>>> useful.
>> Common semantics from app developer point of view will be very useful to
>> maintain. If arm64 says mmap with MAP_FIXED and a tagged address will
>> return a pre-colored page, I would rather have it be the same on any
>> architecture. Is there a use case that justifies kernel doing this extra
>> work?
> So if a database program is doing an anonymous mmap(PROT_TBI) of 100GB,
> IIUC for sparc the faulted-in pages will have random colours (on 64-byte
> granularity). Ignoring the information leak from prior uses of such
> pages, it would be the responsibility of the db program to issue the
> stxa. On arm64, since we also want to do this via malloc(), any large
> allocation would require all pages to be faulted in so that malloc() can
> set the write colour before being handed over to the user. That's what
> we want to avoid and the user is free to repaint the memory as it likes.

On sparc, any newly allocated page is cleared along with any old tags on
it. Since clearing tag happens automatically when page is cleared on
sparc, clear_user_page() will need to execute additional stxa
instructions to set a new tag. It is doable. In a way it is done already
if page is being pre-colored with tag 0 always ;) Where would the
pre-defined tag be stored - as part of address stored in vm_start or a
new field in vm_area_struct?


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