From: "H. Peter Anvin" <hpa@zytor.com>
To: Greg Hackmann <ghackmann@google.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: Sat, 29 Apr 2017 14:23:22 -0700 [thread overview]
Message-ID: <6b9fcaae-044d-5f71-6064-f0df88d5f8b4@zytor.com> (raw)
In-Reply-To: <d7bd4002-41dd-d0b1-f69e-30546b0ea41d@google.com>
On 04/26/17 14:29, Greg Hackmann wrote:
> 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".
OK, that's a genuine bug. Please explain that in the comment; it has
nothing to do with clang.
-hpa
next prev parent reply other threads:[~2017-04-29 21:30 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-26 20:55 [PATCH v2] x86/mm/kaslr: Use _ASM_MUL macro for multiplication 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
2017-04-29 21:23 ` H. Peter Anvin [this message]
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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