linux-kernel - Re: [PATCH 05/29] x86: Base IBT bits

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Date:   Fri, 18 Feb 2022 21:24:10 +0000
From:   Andrew Cooper <Andrew.Cooper3@...rix.com>
To:     David Laight <David.Laight@...LAB.COM>,
        Peter Zijlstra <peterz@...radead.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "joao@...rdrivepizza.com" <joao@...rdrivepizza.com>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "samitolvanen@...gle.com" <samitolvanen@...gle.com>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "alyssa.milburn@...el.com" <alyssa.milburn@...el.com>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>
Subject: Re: [PATCH 05/29] x86: Base IBT bits

On 18/02/2022 21:11, David Laight wrote:
> From: Andrew Cooper
>> Sent: 18 February 2022 20:50
>>
>> On 18/02/2022 16:49, Peter Zijlstra wrote:
>>> +/*
>>> + * A bit convoluted, but matches both endbr32 and endbr64 without
>>> + * having either as literal in the text.
>>> + */
>>> +static inline bool is_endbr(const void *addr)
>>> +{
>>> +	unsigned int val = ~*(unsigned int *)addr;
>>> +	val |= 0x01000000U;
>>> +	return val == ~0xfa1e0ff3;
>>> +}
>> At this point, I feel I've earned an "I told you so". :)
>>
>> Clang 13 sees straight through the trickery and generates:
>>
>> is_endbr:                               # @is_endbr
>>         movl    $-16777217, %eax                # imm = 0xFEFFFFFF
>>         andl    (%rdi), %eax
>>         cmpl    $-98693133, %eax                # imm = 0xFA1E0FF3
>>         sete    %al
>>         retq
> I think it is enough to add:
> 	asm("", "=r" (val));
> somewhere in the middle.

(First, you mean "+r" not "=r"), but no - the problem isn't val.  It's
`~0xfa1e0ff3` which the compiler is free to transform in several unsafe way.

~Andrew