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

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Date:   Fri, 30 Aug 2019 16:12:04 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Arnd Bergmann' <arnd@...db.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>
CC:     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>
Subject: RE: objtool warning "uses BP as a scratch register" with clang-9

From: Arnd Bergmann
> Sent: 30 August 2019 16:59
...
> > Or even better, it would be great if we could get Clang to change their
> > memset() insertion heuristics, so that KASAN acts more like non-KASAN
> > code in that regard.
> 
> I suspect that's going to be harder. The clang-9 release is going to be
> soon, and that change probably wouldn't be considered a regression fix.
> 
> Maybe Nick can find what happens, but I don't actually see any reference
> to KASAN in the llvm source code related to the memset generation.
> 
> https://github.com/llvm-mirror/clang/blob/master/lib/CodeGen/CGExprAgg.cpp#L1803
> has a check for >16 bytes, but that again does not match my observation.

That looks like the code for initialising a local structure variable,
not for merging 'random' writes into a memset().

	David

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