Re: [QUERY]: Block region to mmap

[Matthew Wilcox <willy@infradead.org>]

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.

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