[PATCH v2 6/9] arm64: cfi: Use CONFIG_ARM64_FUNCTION_PREFIX_NOPS for the type hash offset
From: Jose Fernandez (Anthropic) <hidden>
Date: 2026-09-24 22:37:24
Also in:
bpf, linux-efi, linux-kbuild, live-patching, lkml, llvm, rust-for-linux
Subsystem:
arm64 port (aarch64 architecture), bpf jit for arm64, bpf [general] (safe dynamic programs and tools), the rest · Maintainers:
Catalin Marinas, Will Deacon, Daniel Borkmann, Alexei Starovoitov, Puranjay Mohan, Andrii Nakryiko, Eduard Zingerman, Kumar Kartikeya Dwivedi, Linus Torvalds
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 <redacted> Reviewed-by: Florent Revest (Anthropic) <redacted> Assisted-by: LLM Signed-off-by: Jose Fernandez (Anthropic) <redacted> --- arch/arm64/include/asm/cfi.h | 8 ++++++++ arch/arm64/include/asm/linkage.h | 11 +++++++++++ arch/arm64/net/bpf_jit_comp.c | 8 ++++++-- 3 files changed, 25 insertions(+), 2 deletions(-)
diff --git a/arch/arm64/include/asm/cfi.h b/arch/arm64/include/asm/cfi.h
index ab90f0351b7ae..03f02d4614c23 100644
--- a/arch/arm64/include/asm/cfi.h
+++ b/arch/arm64/include/asm/cfi.h@@ -4,4 +4,12 @@ #define __bpfcall +#ifdef CONFIG_CFI +static inline int cfi_get_offset(void) +{ + return 4 + 4 * CONFIG_ARM64_FUNCTION_PREFIX_NOPS; +} +#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 d1f7a16729d25..14bc3c67c38b3 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 c18e005a41dbe..7ad14d9847138 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); } /*
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2.52.0