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

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Date:   Fri, 30 Aug 2019 10:02:08 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        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 03:26:30PM -0700, Linus Torvalds wrote:
> 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".

For KASAN, the Clang threshold for inserting memset() is *2* consecutive
writes instead of 17.  Isn't that likely to cause tearing-related
surprises?

I suppose people don't run KASAN in production, but we don't want
tearing-related bugs showing up there because somebody's going to end up
having to debug them regardless.

-- 
Josh