[tip: x86/core] x86/ibt: Make cfi_bhi a constant for FINEIBT_BHI=n

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the x86/core branch of tip:

Commit-ID:     73e8079be9e7ae5ed197d074e0ba6c43674c52f7
Gitweb:        https://git.kernel.org/tip/73e8079be9e7ae5ed197d074e0ba6c43674c52f7
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Mon, 03 Mar 2025 10:21:47 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 03 Mar 2025 10:54:11 +01:00

x86/ibt: Make cfi_bhi a constant for FINEIBT_BHI=n

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.

Closes: https://lore.kernel.org/oe-kbuild-all/202503030704.H9KFysNS-lkp@xxxxxxxxx/
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20250303094911.GL5880@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 arch/x86/include/asm/cfi.h    | 5 +++++
 arch/x86/kernel/alternative.c | 7 +++++++
 2 files changed, 12 insertions(+)

diff --git a/arch/x86/include/asm/cfi.h b/arch/x86/include/asm/cfi.h
index 2f6a01f..3e51ba4 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 32e4b80..bf82c6f 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);
 		}