On Tue, Mar 4, 2025 at 2:16 PM Peter Zijlstra [off-list ref] wrote:

On Tue, Mar 04, 2025 at 06:38:53AM +0100, Peter Zijlstra wrote:

quoted

On Tue, Mar 04, 2025 at 09:10:12AM +0800, Menglong Dong wrote:

quoted

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