Re: [PATCH bpf-next v2] add function metadata support

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Feb 28, 2025 at 05:53:07PM +0800, Menglong Dong wrote:

> I tested it a little by enabling CFI_CLANG and the extra 5-bytes
> padding. It works fine, as mostly CFI_CLANG use
> CONFIG_FUNCTION_PADDING_BYTES to find the tags. I'll
> do more testing on CFI_CLANG to make sure everything goes
> well.

I don't think you understand; please read:

arch/x86/kernel/alternative.c:__apply_fineibt() 

and all the code involved with patching FineIBT. I think you'll find it
very broken if you change anything here.

Can you post an actual function preamble from a kernel with
CONFIG_FINEIBT=y with your changes on?

Ex.

$ objdump -wdr build/kernel/futex/core.o

Disassembly of section .text:

0000000000000000 <__cfi_futex_hash>:
       0:       b9 93 0c f9 ad          mov    $0xadf90c93,%ecx

0000000000000005 <.Ltmp0>:
       5:       90                      nop
       6:       90                      nop
       7:       90                      nop
       8:       90                      nop
       9:       90                      nop
       a:       90                      nop
       b:       90                      nop
       c:       90                      nop
       d:       90                      nop
       e:       90                      nop
       f:       90                      nop

0000000000000010 <futex_hash>:
      10:       f3 0f 1e fa             endbr64
      14:       e8 00 00 00 00          call   19 <futex_hash+0x9>      15: R_X86_64_PLT32      __fentry__-0x4
      19:       8b 47 10                mov    0x10(%rdi),%eax


Any change to the layout here *WILL* break the FineIBT code.


If you want to test, make sure your build has FINEIBT=y and boot on an
Intel CPU that has CET-IBT (alderlake and later).