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

Peter Zijlstra
2025-03-04T06:16:55Z

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