Re: arm64 uaccess series

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Jul 09, 2024 at 09:01:58AM -0700, Linus Torvalds wrote:
> This is also available at
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git arm64-uaccess
> 
> and is three patches, although I expect that I'll only merge the first
> two in the 6.11 merge window because that final one is still a bit
> special. 
> 
> I've been running variations of this on my Altra machine for the last
> month or more, but admittedly my loads are trivial and uninteresting (ie
> mostly kernel builds).  So my test coevrage is not very wide.

I can temporarily pull this branch and 'runtime-constants' into arm64
'for-kernelci'. It's an unstable branch, it doesn't end up in -next.
It's just pointed at by various CI systems to get some wider testing. I
can even pull all four branches if you think it's useful.

> I like the bit 55 checks in that access_ok() rewrite - and they are
> actually simpler than worrying about 64-bit overflow - but they are also
> admittedly quite different from what the code does elsewhere, and
> there's the whole discussion about how the top byte ignore should really
> work. 

We are still debating this. I don't think the ABI change is that bad
but, OTOH, user programs with MTE enabled (which would relax the
access_ok()) haven't been tested much. As a kind of precaution, we could
enforce the current behaviour via the sysctl abi.tagged_addr_disabled
and wire it up via a static key. Currently this sysctl only prevents
setting of the TIF_TAGGED_ADDR flag (and implicitly enforces stricter
checks in access_ok()).

-- 
Catalin