linux-kernel - Re: [PATCH v4 1/4] x86/ibt: factor out cfi and fineibt offset

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Message-ID: <20250304053853.GA7099@noisy.programming.kicks-ass.net>
Date: Tue, 4 Mar 2025 06:38:53 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: rostedt@...dmis.org, mark.rutland@....com, alexei.starovoitov@...il.com,
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Subject: Re: [PATCH v4 1/4] x86/ibt: factor out cfi and fineibt offset

On Tue, Mar 04, 2025 at 09:10:12AM +0800, Menglong Dong wrote:
> Hello, sorry that I forgot to add something to the changelog. In fact,
> I don't add extra 5-bytes anymore, which you can see in the 3rd patch.
> 
> The thing is that we can't add extra 5-bytes if CFI is enabled. Without
> CFI, we can make the padding space any value, such as 5-bytes, and
> the layout will be like this:
> 
> __align:
>   nop
>   nop
>   nop
>   nop
>   nop
> foo: -- __align +5
> 
> However, the CFI will always make the cfi insn 16-bytes aligned. When
> we set the FUNCTION_PADDING_BYTES to (11 + 5), the layout will be
> like this:
> 
> __cfi_foo:
>   nop (11)
>   mov $0x12345678, %reg
>   nop (16)
> foo:
> 
> and the padding space is 32-bytes actually. So, we can just select
> FUNCTION_ALIGNMENT_32B instead, which makes the padding
> space 32-bytes too, and have the following layout:
> 
> __cfi_foo:
>   mov $0x12345678, %reg
>   nop (27)
> foo:

*blink*, wtf is clang smoking.

I mean, you're right, this is what it is doing, but that is somewhat
unexpected. Let me go look at clang source, this is insane.