[PATCH v3 4/7] arm64: cfi: Use CONFIG_ARM64_FUNCTION_PREFIX_NOPS for the type hash offset

["Jose Fernandez (Anthropic)" <jose.fernandez@linux.dev>]

Make cfi_get_offset(), SYM_TYPED_FUNC_START and the BPF JIT use
CONFIG_ARM64_FUNCTION_PREFIX_NOPS instead of hardcoding four bytes. kCFI
kernels with CALL_OPS need three NOPs, or five with ThinLTO and BTI.

CONFIG_ARM64_FUNCTION_PREFIX_NOPS is 0 on every CFI kernel today, so no
functional change.

Reviewed-by: Ben Cressey <ben@cressey.dev>
Reviewed-by: Florent Revest (Anthropic) <florent.revest@linux.dev>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Assisted-by: LLM
Signed-off-by: Jose Fernandez (Anthropic) <jose.fernandez@linux.dev>
---
 arch/arm64/include/asm/cfi.h     | 11 +++++++++++
 arch/arm64/include/asm/linkage.h | 11 +++++++++++
 arch/arm64/net/bpf_jit_comp.c    |  8 ++++++--
 3 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/cfi.h b/arch/arm64/include/asm/cfi.h
index ab90f0351b7a..f3c04a2a6a95 100644
--- a/arch/arm64/include/asm/cfi.h
+++ b/arch/arm64/include/asm/cfi.h
@@ -2,6 +2,17 @@
 #ifndef _ASM_ARM64_CFI_H
 #define _ASM_ARM64_CFI_H
 
+#include <asm/insn-def.h>
+
 #define __bpfcall
 
+#ifdef CONFIG_CFI
+static inline int cfi_get_offset(void)
+{
+	/* The 4-byte type hash sits before the prefix NOPs. */
+	return 4 + CONFIG_ARM64_FUNCTION_PREFIX_NOPS * AARCH64_INSN_SIZE;
+}
+#define cfi_get_offset cfi_get_offset
+#endif /* CONFIG_CFI */
+
 #endif /* _ASM_ARM64_CFI_H */
diff --git a/arch/arm64/include/asm/linkage.h b/arch/arm64/include/asm/linkage.h
index d1f7a16729d2..14bc3c67c38b 100644
--- a/arch/arm64/include/asm/linkage.h
+++ b/arch/arm64/include/asm/linkage.h
@@ -39,6 +39,17 @@
 	SYM_START(name, SYM_L_WEAK, SYM_A_NONE)		\
 	bti c ;
 
+/*
+ * The compiler emits CONFIG_ARM64_FUNCTION_PREFIX_NOPS NOPs between a C
+ * function's kCFI type hash and its entry point. Callers check the hash
+ * at that offset.
+ */
+#define __CFI_TYPE(name)				\
+	.4byte __kcfi_typeid_##name ASM_NL		\
+	.rept CONFIG_ARM64_FUNCTION_PREFIX_NOPS ASM_NL	\
+	nop ASM_NL					\
+	.endr
+
 #define SYM_TYPED_FUNC_START(name)				\
 	SYM_TYPED_START(name, SYM_L_GLOBAL, SYM_A_ALIGN)	\
 	bti c ;
diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
index aaa34c739004..42cb2d446680 100644
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -185,8 +185,12 @@ static inline void emit_bti(u32 insn, struct jit_ctx *ctx)
 
 static inline void emit_kcfi(u32 hash, struct jit_ctx *ctx)
 {
-	if (IS_ENABLED(CONFIG_CFI))
-		emit_u32_data(hash, ctx);
+	if (!IS_ENABLED(CONFIG_CFI))
+		return;
+
+	emit_u32_data(hash, ctx);
+	for (int i = 0; i < CONFIG_ARM64_FUNCTION_PREFIX_NOPS; i++)
+		emit(A64_NOP, ctx);
 }
 
 /*

-- 
2.54.0