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

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Date:   Thu, 29 Aug 2019 15:26:30 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Ilie Halip <ilie.halip@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: Re: objtool warning "uses BP as a scratch register" with clang-9

On Thu, Aug 29, 2019 at 1:22 PM Arnd Bergmann <arnd@...db.de> 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...).

                Linus