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

From: H. Peter Anvin
Date: Fri May 11 2018 - 14:54:25 EST

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: