Re: [tip:x86/core 16/17] vmlinux.o: warning: objtool: do_jit+0x276: relocation to !ENDBR: .noinstr.text+0x6a60

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Mar 03, 2025 at 07:47:57AM +0800, kernel test robot wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git x86/core
> head:   dfebe7362f6f461d771cdb9ac2c5172a4721f064
> commit: 0c92385dc05ee9637c04372ea95a11bbf6e010ff [16/17] x86/ibt: Implement FineIBT-BHI mitigation
> config: x86_64-randconfig-071-20250303 (https://download.01.org/0day-ci/archive/20250303/202503030704.H9KFysNS-lkp@intel.com/config)
> compiler: clang version 19.1.7 (https://github.com/llvm/llvm-project cd708029e0b2869e80abe31ddb175f7c35361f90)
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250303/202503030704.H9KFysNS-lkp@intel.com/reproduce)
> 
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202503030704.H9KFysNS-lkp@intel.com/
> 
> All warnings (new ones prefixed by >>):
> 
> >> vmlinux.o: warning: objtool: do_jit+0x276: relocation to !ENDBR: .noinstr.text+0x6a60

Thanks, below seems to cure it for me.

---
Subject: x86/ibt: Make cfi_bhi a constant for FINEIBT_BHI=n
From: Peter Zijlstra <peterz@infradead.org>
Date: Mon Mar 3 10:21:47 CET 2025

Robot yielded a .config that tripped:

  vmlinux.o: warning: objtool: do_jit+0x276: relocation to !ENDBR: .noinstr.text+0x6a60

This is the result of using __bhi_args[1] in unreachable code; make
sure the compiler is able to determine this is unreachable and trigger
DCE.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202503030704.H9KFysNS-lkp@intel.com/
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/include/asm/cfi.h    |    5 +++++
 arch/x86/kernel/alternative.c |    3 +++
 2 files changed, 8 insertions(+)

--- a/arch/x86/include/asm/cfi.h
+++ b/arch/x86/include/asm/cfi.h
@@ -100,7 +100,12 @@ enum cfi_mode {
 };
 
 extern enum cfi_mode cfi_mode;
+
+#ifdef CONFIG_FINEIBT_BHI
 extern bool cfi_bhi;
+#else
+#define cfi_bhi (0)
+#endif
 
 typedef u8 bhi_thunk[32];
 extern bhi_thunk __bhi_args[];
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -936,7 +936,10 @@ void __init_or_module apply_seal_endbr(s
 #endif
 
 enum cfi_mode cfi_mode __ro_after_init = __CFI_DEFAULT;
+
+#ifdef CONFIG_FINEIBT_BHI
 bool cfi_bhi __ro_after_init = false;
+#endif
 
 #ifdef CONFIG_CFI_CLANG
 struct bpf_insn;