linux-kernel - Re: [tip:x86/urgent] x86/mm/kaslr: Use the _ASM_MUL macro for multiplication to work around Clang incompatibility

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Date:   Fri, 5 May 2017 11:44:05 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, md@...gle.com,
        tglx@...utronix.de, hpa@...or.com, torvalds@...ux-foundation.org,
        grundler@...omium.org, linux-kernel@...r.kernel.org,
        ghackmann@...gle.com, keescook@...omium.org,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/urgent] x86/mm/kaslr: Use the _ASM_MUL macro for
 multiplication to work around Clang incompatibility

El Fri, May 05, 2017 at 07:50:39PM +0200 Ingo Molnar ha dit:

> 
> * Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Fri, May 05, 2017 at 01:11:47AM -0700, tip-bot for Matthias Kaehlcke wrote:
> > > Commit-ID:  121843eb02a6e2fa30aefab64bfe183c97230c75
> > > Gitweb:     http://git.kernel.org/tip/121843eb02a6e2fa30aefab64bfe183c97230c75
> > > Author:     Matthias Kaehlcke <mka@...omium.org>
> > > AuthorDate: Mon, 1 May 2017 15:47:41 -0700
> > > Committer:  Ingo Molnar <mingo@...nel.org>
> > > CommitDate: Fri, 5 May 2017 08:31:05 +0200
> > > 
> > > x86/mm/kaslr: Use the _ASM_MUL macro for multiplication to work around Clang incompatibility
> > > 
> > > The constraint "rm" allows the compiler to put mix_const into memory.
> > > When the input operand is a memory location then MUL needs an operand
> > > size suffix, since Clang can't infer the multiplication width from the
> > > operand.
> > 
> > *sigh*, this is another shining example of how LLVM is a better, faster
> > moving compiler?
> 
> Well, I don't like it - but we already have similar patterns to cover some asm 
> complications so I didn't mind. Apparently Clang is very close to being able to 
> build a working Linux kernel, right?

Indeed, I expect 4.12 (with this patch ...) to build with Clang for a
x86 defconfig (with tons of warnings). ARM64 is very close.

> In that sense it would be unfair to expect it to not have various legacies, 
> missing features and quirks - just like the kernel has dozens of GCC related 
> workarounds.

Also my understanding is that this isn't really a clang issue. In the
context of this code gcc apparently chooses to use a register for
'mix_const', for memory locations it also needs a suffix.

Actually I just tried to build this code from a single C file:

void test() {
  unsigned long raw, random;
  const unsigned long mix_const = 0x3f39e593UL;

  asm("MUL %3"
    : "=a" (random), "=d" (raw)
    : "a" (random), "rm" (mix_const));
}

gcc -c /tmp/test.c
/tmp/test.c: Assembler messages:
/tmp/test.c:6: Error: no instruction mnemonic suffix given and no
  register operands; can't size instruction

gcc version 4.9.x 20150123

Cheers

Matthias