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

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Message-ID: <20250228102935.GM31462@noisy.programming.kicks-ass.net>
Date: Fri, 28 Feb 2025 11:29:35 +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 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.