Re: [PATCH] rwonce: fix crash by removing READ_ONCE() for unaligned read

[Linus Torvalds]

On Wed, 26 Mar 2025 at 14:24, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> I've applied this on top of the asm-generic branch, but I just sent
> the pull request with the regression to Linus an hour ago.
>
> I'll try to get a new pull request out tomorrow.

I will ignore that pull request, and wait for your updated one.

That said, this whole thing worries me. The fact that the compiler
magically makes READ_ONCE() require alignment that it normally doesn't
require seems like a bug waiting to happen somewhere else.

Because I do think that we might want READ_ONCE() on unaligned data in
general. Should said places generally use "get_unaligned()"? Sure. And
are unaligned accesses potentially non-atomic anyway because of
hardware? Also sure.

But one reason for READ_ONCE() isn't for some kind of hardware
atomicity, it is to avoid any ToCToU issues due to compilers doing bad
things.

And then this seems to be a serious issue with the whole "READ_ONCE()
now requires alignment that it didn't require before".

Put another way: I wonder what other cases may lurk around this all...

           Linus