linux-hardening - [PATCH 2/5] x86/traps: Clarify KCFI instruction layout

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Message-Id: <20250825142603.1907143-2-kees@kernel.org>
Date: Mon, 25 Aug 2025 07:25:49 -0700
From: Kees Cook <kees@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Kees Cook <kees@...nel.org>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Mark Rutland <mark.rutland@....com>,
	Puranjay Mohan <puranjay@...nel.org>,
	Jonathan Corbet <corbet@....net>,
	Nathan Chancellor <nathan@...nel.org>,
	x86@...nel.org,
	linux-doc@...r.kernel.org,
	linux-kbuild@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-riscv@...ts.infradead.org,
	llvm@...ts.linux.dev,
	linux-hardening@...r.kernel.org
Subject: [PATCH 2/5] x86/traps: Clarify KCFI instruction layout

Just a nit-picky change to the KCFI indirect call check instruction
documentation. The addl offset isn't always -4 (it depends on patchable
function entry configuration).

Signed-off-by: Kees Cook <kees@...nel.org>
---
 arch/x86/kernel/cfi.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cfi.c b/arch/x86/kernel/cfi.c
index 77086cf565ec..638eb5c933e0 100644
--- a/arch/x86/kernel/cfi.c
+++ b/arch/x86/kernel/cfi.c
@@ -27,7 +27,7 @@ static bool decode_cfi_insn(struct pt_regs *regs, unsigned long *target,
 	 * for indirect call checks:
 	 *
 	 *   movl    -<id>, %r10d       ; 6 bytes
-	 *   addl    -4(%reg), %r10d    ; 4 bytes
+	 *   addl    -<pos>(%reg), %r10d; 4 bytes
 	 *   je      .Ltmp1             ; 2 bytes
 	 *   ud2                        ; <- regs->ip
 	 *   .Ltmp1:
-- 
2.34.1