linux-kernel - Re: [PATCH] x86/alternatives: Fix alt_max

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Date:   Thu, 5 Oct 2017 15:01:48 +0200
From:   Mathias Krause <minipli@...glemail.com>
To:     Michael Matz <matz@...e.de>
Cc:     Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86-ml <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/alternatives: Fix alt_max_short macro to really be a max()

On 5 October 2017 at 14:52, Michael Matz <matz@...e.de> wrote:
> On Thu, 5 Oct 2017, Borislav Petkov wrote:
>> On Thu, Oct 05, 2017 at 02:35:33PM +0200, Mathias Krause wrote:
>> > Note the "<"! ...comment is wrong, though the implementation works!
>>
>> I know, I realized that when I looked at alternative-asm.h. Wanted to
>> double-check it with Micha first.
>
> Yeah, for bit-twiddling the result of arithmetic would need to be
> booleanized first, or alternatively the boolean operators be used in the
> first place.  So if '<' works then that's better in this context.

Ack!

> (In a different context, or in the same one there definitely was a problem
> with using '<', but I can't remember the details, it's too long ago we
> discussed about this; maybe it even was a problem only with some binutils
> versions.  So I'd suggest using the more obvious way until problems
> reoccur, and then document why exactly using relational ops was a problem
> ;-) )

Might be because the "true" value of gas' < operator is non-obvious
with being -1. But, well, if you don't know, I don't know either ;)

Anyways, the alt_max_short() macro in alternative.h is plain wrong
while the one in alternative-asm.h works and is used in a few places,
even with varying lengths of the alternatives (e.g.
arch/x86/entry/vdso/vdso32/system_call.S,
arch/x86/lib/{clear_page_64.S,memcpy_64.S,memset_64.S}), therefore
proves to be functional.


Thanks,
Mathias