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

[Peter Zijlstra]

On Mon, Mar 03, 2025 at 10:24:59AM +0100, Peter Zijlstra wrote:
> 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@xxxxxxxxx/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@xxxxxxxxx/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@xxxxxxxxx>
> > | Closes: https://lore.kernel.org/oe-kbuild-all/202503030704.H9KFysNS-lkp@xxxxxxxxx/
> > 
> > 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@xxxxxxxxxxxxx>
> 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@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-kbuild-all/202503030704.H9KFysNS-lkp@xxxxxxxxx/
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---

diff --git a/arch/x86/include/asm/cfi.h b/arch/x86/include/asm/cfi.h
index 2f6a01f098b5..3e51ba459154 100644
--- 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[];
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 32e4b801db99..bf82c6f7d690 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -936,7 +936,10 @@ void __init_or_module apply_seal_endbr(s32 *start, s32 *end) { }
 #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;
@@ -1070,11 +1073,15 @@ static __init int cfi_parse_cmdline(char *str)
 				pr_err("Ignoring paranoid; depends on fineibt.\n");
 			}
 		} else if (!strcmp(str, "bhi")) {
+#ifdef CONFIG_FINEIBT_BHI
 			if (cfi_mode == CFI_FINEIBT) {
 				cfi_bhi = true;
 			} else {
 				pr_err("Ignoring bhi; depends on fineibt.\n");
 			}
+#else
+			pr_err("Ignoring bhi; depends on FINEIBT_BHI=y.\n");
+#endif
 		} else {
 			pr_err("Ignoring unknown cfi option (%s).", str);
 		}