Re: objtool warning "uses BP as a scratch register" with clang-9

From: Linus Torvalds
Date: Thu Aug 29 2019 - 18:26:52 EST

On Thu, Aug 29, 2019 at 1:22 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> Maybe we can just pass -fno-builtin-memcpy -fno-builtin-memset
> for clang when CONFIG_KASAN is set and hope for the best?

I really hate how that disables conversions both ways, which is kind
of pointless and wrong. It's really just "we don't want surprising
memcpy calls for single writes".

Disabling all the *good* "optimize memset/memcpy" cases is really sad.

We actually have a lot of small structures in the kernel on purpose
(often for type safety), and I bet we use memcpy on them on purpose at
times. I'd hate to see that become a function call rather than "copy
two words by hand".

Even for KASAN.

And I guess that when the compiler sees 20+ "set to zero" it's quite
reasonable to say "just turn it into a memset".

So maybe the right thing to do is to just special-case this code, and
hope for the best. If moving the sas_ss_reset() out of the try/catch
thing works, then by all means lets just do that.

(Partly because I've actually wanted to turn the try/catchj thing into
a _real_ try/catch, not a "fall through and check at the end" like it
just happens to be now - the try/catch is actually very misleading as
it is now. So if this is the only case that matters, then...).