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

Peter Zijlstra
2025-03-04T05:39:18Z

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.