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

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Message-ID: <CADxym3aECb5xOm0+YMycsx8kD6ijuEfMx6xrR9UKrH-FFn=KBw@mail.gmail.com>
Date: Fri, 28 Feb 2025 21:01:16 +0800
From: Menglong Dong <menglong8.dong@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: rostedt@...dmis.org, mark.rutland@....com, alexei.starovoitov@...il.com, 
	catalin.marinas@....com, will@...nel.org, mhiramat@...nel.org, 
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, 
	mathieu.desnoyers@...icios.com, nathan@...nel.org, ndesaulniers@...gle.com, 
	morbo@...gle.com, justinstitt@...gle.com, dongml2@...natelecom.cn, 
	akpm@...ux-foundation.org, rppt@...nel.org, graf@...zon.com, 
	dan.j.williams@...el.com, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
	bpf@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH bpf-next v2] add function metadata support

On Fri, Feb 28, 2025 at 6:26 PM Peter Zijlstra <peterz@...radead.org> 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).

Yeah, I understand now. As I see the definition of CFI_PRE_PADDING, I
know that things is not as simple as I thought, as the layout can be different
with different config. I'll dig it deeper on this part.

Thanks a lot for clearing that up for me. I'll test the CFI_CLANG and
FINEIBT together with this function later.

Thanks!
Menglong Dong