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

Menglong Dong
2025-03-04T07:49:22Z

On Tue, Mar 4, 2025 at 2:16 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Mar 04, 2025 at 06:38:53AM +0100, Peter Zijlstra wrote:
> > 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.
>
> Bah, this is because assemblers are stupid :/
>
> There is no way to tell them to have foo aligned such that there are at
> least N bytes free before it.
>
> So what kCFI ends up having to do is align the __cfi symbol to the
> function alignment, and then stuff enough nops in to make the real
> symbol meet alignment.
>
> And the end result is utter insanity.
>
> I mean, look at this:
>
> 50: 2e e9 00 00 00 00 cs jmp 56 <__traceiter_initcall_level+0x46> 52: R_X86_64_PLT32 __x86_return_thunk-0x4
> 56: 66 2e 0f 1f 84 00 00 00 00 00 cs nopw 0x0(%rax,%rax,1)
>
> 0000000000000060 <__cfi___probestub_initcall_level>:
> 60: 90 nop
> 61: 90 nop
> 62: 90 nop
> 63: 90 nop
> 64: 90 nop
> 65: 90 nop
> 66: 90 nop
> 67: 90 nop
> 68: 90 nop
> 69: 90 nop
> 6a: 90 nop
> 6b: b8 b1 fd 66 f9 mov $0xf966fdb1,%eax
>
> 0000000000000070 <__probestub_initcall_level>:
> 70: 2e e9 00 00 00 00 cs jmp 76 <__probestub_initcall_level+0x6> 72: R_X86_64_PLT32 __x86_return_thunk-0x4
>
>
> That's 21 bytes wasted, for no reason other than that asm doesn't have a
> directive to say: get me a place that is M before N alignment.
>
> Because ideally the whole above thing would look like:
>
> 50: 2e e9 00 00 00 00 cs jmp 56 <__traceiter_initcall_level+0x46> 52: R_X86_64_PLT32 __x86_return_thunk-0x4
> 56: 66 2e 0f 1f 84 cs nopw (%rax,%rax,1)
>
> 000000000000005b <__cfi___probestub_initcall_level>:
> 5b: b8 b1 fd 66 f9 mov $0xf966fdb1,%eax
>
> 0000000000000060 <__probestub_initcall_level>:
> 60: 2e e9 00 00 00 00 cs jmp 76 <__probestub_initcall_level+0x6> 72: R_X86_64_PLT32 __x86_return_thunk-0x4

Hi, peter. Thank you for the testing, which is quite helpful
to understand the whole thing.

I was surprised at this too. Without CALL_PADDING, the cfi is
nop(11) + mov; with CALL_PADDING, the cfi is mov + nop(11),
which is weird, as it seems that we can select CALL_PADDING if
CFI_CLANG to make things consistent. And I thought that it is
designed to be this for some reasons :/

Hmm......so what should we do now? Accept and bear it,
or do something different?

I'm good at clang, so the solution that I can think of is how to
bear it :/

According to my testing, the text size will increase:

~2.2% if we make FUNCTION_PADDING_BYTES 27 and select
FUNCTION_ALIGNMENT_16B.

~3.5% if we make FUNCTION_PADDING_BYTES 27 and select
FUNCTION_ALIGNMENT_32B.

We don't have to select FUNCTION_ALIGNMENT_32B, so the
worst case is to increase ~2.2%.

What do you think?

Thanks!
Menglong Dong

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