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Subject: Re: [PATCH] x86: pad assembly functions with INT3
To:     David Laight <David.Laight@ACULAB.COM>,
        "'Alexey Dobriyan'" <adobriyan@gmail.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@redhat.com" <mingo@redhat.com>
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From:   "H. Peter Anvin" <hpa@zytor.com>
Message-ID: <5c6762fe-5cbe-42ed-ac4e-a7144b8ef7ad@zytor.com>
Date:   Fri, 11 May 2018 11:53:40 -0700
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On 05/10/18 09:39, David Laight wrote:
> From: Alexey Dobriyan
>> Sent: 07 May 2018 22:38
>>
>> Use INT3 instead of NOP. All that padding between functions is
>> an illegal area, no legitimate code should jump into it.
>>
>> I've checked x86_64 allyesconfig disassembly, all changes looks sane:
>> INT3 is only used after RET or unconditional JMP.
> 
> I thought there was a performance penalty (on at least some cpu)
> depending on the number of and the actual instructions used for padding.
> 
> I believe that is why gcc generates a small number of very long 'nop'
> instructions when padding code.
> 

There is a performance penalty for using NOP instructions *in the
fallthrough case.*  In the case where the padding is never supposed to
be executed, which is what we're talking about here, it is irrelevant.

I thought I had filed a gcc enhancement request, but I can't find it
now, so I just filed this:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85751

	-hpa