RE: [PATCH] x86: pad assembly functions with INT3

[hpa]

On May 14, 2018 2:04:38 AM PDT, David Laight <David.Laight@xxxxxxxxxx> wrote:
>From: H. Peter Anvin
>> Sent: 11 May 2018 19:54
>> 
>> 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.
>
>Not completely irrelevant, the instructions stream gets fetched and
>decoded
>beyond the jmp/ret at the end of the function.
>At some point the cpu will decode the jmp/ret and fetch/decode from the
>target address.
>I guess it won't try to speculatively execute the 'pad' instructions
>- but you can never really tell!
>
>	David

The CPU doesn't speculate down past an unconditional control transfer. Doing so would be idiotic.
-- 
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