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

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Feb 28, 2025 at 11:26:46AM +0100, Peter Zijlstra wrote:
> 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).

Oh, wait, not true, tigerlake also has IBT.