[QUERY]: Block region to mmap

[Lad, Prabhakar]

Hi Matthew,

Thank you for the feedback.

On Thu, Jan 26, 2023 at 2:37 PM Matthew Wilcox <willy at infradead.org> wrote:
>
> On Wed, Jan 25, 2023 at 12:30:13PM +0000, Lad, Prabhakar wrote:
> > Renesas RZ/Five RISC-V SoC has Instruction local memory and Data local
> > memory (ILM & DLM) mapped between region 0x30000 - 0x4FFFF. When a
> > virtual address falls within this range, the MMU doesn't trigger a
> > page fault; it assumes the virtual address is a physical address which
> > can cause undesired behaviours.
>
> Wow.  I've never come across such broken behaviour before.
>
> > To avoid this the ILM/DLM memory regions are now added to the root
> > domain region of the PMPU with permissions set to 0x0 for S/U modes so
> > that any access to these regions gets blocked and for M-mode we grant
> > full access (R/W/X). This prevents any users from accessing these
> > regions by triggering an unhandled signal 11 in S/U modes.
>
> I have no idea what any of this means.
>
Basically we are making use of the memory protection unit (MPU) so
that only M-mode is allowed to access this region and S/U modes are
blocked.

> > This works as expected but for applications say for example when doing
> > mmap to this region would still succeed and later down the path when
> > doing a read/write to this location would cause unhandled signal 11.
> > To handle this case gracefully we might want mmap() itself to fail if
> > the addr/offset falls in this local memory region.
>
> No, that's not what you want.  You want mmap to avoid allocating address
> space in that virtual address range.  I don't know if we have a good
> way to do that at the moment; like I said I've never seen such broken
> hardware before.
>
> I'd say the right way to solve this is to add a new special kind of VMA
> to the address space that covers this range.
Do you have any pointers where I can look further into this?

> We'd want to make sure it doesn't appear in /proc/*/maps and also that
> it can't be overridden with MAP_FIXED.
Agreed.

Cheers,
Prabhakar