Re: [GIT PULL] x86/mm for 6.4

[Linus Torvalds]

On Fri, 16 Jun 2023 at 01:47, Alexander Potapenko <glider@xxxxxxxxxx> wrote:
>
> Shouldn't ex_handler_copy() be fixed in the same way?

I don't think ex_handler_copy() is actually reachable any more.

The only thing ex_handler_copy() did was to set %ax to the fault type.

It was used by the strange copy_user_generic_unrolled() that had
special machine check case for "don't do the tail if we get
X86_TRAP_MC".

But that was always bogus. The MC case in question was for the
__copy_user_nocache function, and the machine check case was for the
*destination*, which wasn't in user space at all.

So instead of looking at "what was the trap number", the code should
have just noticed "trapped on the destination", and stopped for *that*
reason.

See commit 034ff37d3407 ("x86: rewrite '__copy_user_nocache'
function") and in particular, see the comment there about writes on
the destination:

  * An exception on a write means that we're
  * done, but we need to update the count
  * depending on where in the unrolled loop
  * we were.

but yeah, I never removed the actual now unused _ASM_EXTABLE_CPY case.

Honestly, I had no way of even testing the code. I doubt anybody does.
There are a couple of users:

 - rdma mis-uses it for regular kernel-to-kernel copies that don't
fault (because rdma wants the "nocache" case). So it can't fault at
all.

 - a couple of GPU drivers mis-use it similarly to rdma, but at least
with a user source in the form of __copy_from_user_inatomic_nocache()

 - _copy_from_iter_flushcache uses it for pmem and dax, because it
wants that machine check handling for non-volatile memory

So two of three users don't actually *have* the MC case at all on the
destination.

And the third case - the actual "copy using uncached accesses in order
to get errors on the destination" case - depends on hardware that I'm
not convinced really exists any more.

Oh, I think there's some odd ntb mis-use too.

I'd love for somebody to actually test the machine check case, but the
old code was completely insane with odd "we handle _one_ case of
4-byte alignment, but not actually the general case", so I wonder if
the MC case ever really worked in reality. And as mentioned, I am not
convinced the hardware is available.

                 Linus