From: Greg Hackmann <ghackmann@google.com>
To: hpa@zytor.com, Kees Cook <keescook@chromium.org>
Cc: Matthias Kaehlcke <mka@chromium.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, "x86@kernel.org" <x86@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Grant Grundler <grundler@chromium.org>,
Michael Davidson <md@google.com>
Subject: Re: [PATCH v2] x86/mm/kaslr: Use _ASM_MUL macro for multiplication
Date: Wed, 26 Apr 2017 14:29:13 -0700 [thread overview]
Message-ID: <d7bd4002-41dd-d0b1-f69e-30546b0ea41d@google.com> (raw)
In-Reply-To: <3FE94529-1D0B-4B23-8D3A-0543E08B12F2@zytor.com>
On 04/26/2017 02:24 PM, hpa@zytor.com wrote:
>>> This really feels like a "fix your compiler" issue.
>>
>> We already use the other forms, what's so bad about adding mul too?
>> And if this lets us build under clang, all the better.
>>
>> -Kees
>
> It's not bad per se, but if this doesn't eventually gets fixed in clang we'll have no end of this crap.
>
AIUI the "problem" is that clang is spilling mix_const into memory
rather than assigning it to a register. This is perfectly legal since
mix_const has a constraint of "rm". But mul needs a suffix when the
input is a memory location, since it can't infer the multiplication
width from the input operand anymore.
You get the same error message with gcc if you force it to use a memory
location, by narrowing the constraint from "rm" to "m".
next prev parent reply other threads:[~2017-04-26 21:29 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-26 20:55 Matthias Kaehlcke
2017-04-26 21:00 ` hpa
2017-04-26 21:21 ` Kees Cook
2017-04-26 21:24 ` hpa
2017-04-26 21:29 ` Greg Hackmann [this message]
2017-04-29 21:23 ` H. Peter Anvin
2017-04-26 22:06 ` Kees Cook
2017-05-01 22:47 Matthias Kaehlcke
2017-05-02 2:08 ` Kees Cook
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