[RESEND PATCH v4 8/8] arm64: Allow 64-bit tasks to invoke compat syscalls

[David Laight]

From: Steven Price
> Sent: 24 May 2021 12:21
...
> So a "generic" way of requested the kernel limit the address space for
> allocations would be potentially useful for other purposes. Adding a new
> syscall for this purpose would be sensible. We already have (at least)
> two "hacks" in mmap for controlling the address range that can be used:
> 
>  * MAP_32BIT - x86 only, and really "31 bit"
> 
>  * Providing a mmap() hint with the top bits set to opt-in to 52-bit VAs.
> 
> A well defined mechanism for controlling the valid VA range for
> allocations would be much better than adding more hacks - and bonus
> points if it works for all the different types of allocation unlike the
> above.

I'd have thought a 'MAP_BELOW' flag (cf MAP_FIXED) would suffice.
I'm sort of surprised MAP_32BIT wasn't implemented that way.
'man mmap' says MAP_32BIT was added for x64 thread stacks - I though
the requirement can from 32bit wine (windows has a 2G user/kernel boundary).

	David

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