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

[Nathan Chancellor <nathan@kernel.org>]

On Wed, Mar 26, 2025 at 03:41:34PM -0700, Linus Torvalds wrote:
> 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.

For the record, I do not think it is the compiler doing this, it is the
arm64 code after commit e35123d83ee3 ("arm64: lto: Strengthen
READ_ONCE() to acquire when CONFIG_LTO=y") back in 5.11.

> 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...

That change has caused only one issue that I know of, which was fixed by
commit d3f450533bbc ("efi: tpm: Avoid READ_ONCE() for accessing the
event log"). I have not seen any since then until this point and I do
daily boots of -next with LTO enabled on both of my arm64 test machines.

Cheers,
Nathan