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

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Message-ID: <20250228102646.GW11590@noisy.programming.kicks-ass.net>
Date: Fri, 28 Feb 2025 11:26:46 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Menglong Dong <menglong8.dong@...il.com>
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 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).