x86/traps: Clarify KCFI instruction layout

kees · Peter Zijlstra · commit 628a15e0536a · 2025-09-04T21:59:07.000+02:00
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@kernel.org> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lore.kernel.org/r/20250904034656.3670313-2-kees@kernel.org
diff --git 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:

Original file line number	Diff line number	Diff line change
`@@ -27,7 +27,7 @@ static bool decode_cfi_insn(struct pt_regs regs, unsigned long target,`
`27`	`27`	`* for indirect call checks:`
`28`	`28`	`*`
`29`	`29`	`* movl -<id>, %r10d ; 6 bytes`
`30`		`- * addl -4(%reg), %r10d ; 4 bytes`
	`30`	`+ * addl -<pos>(%reg), %r10d; 4 bytes`
`31`	`31`	`* je .Ltmp1 ; 2 bytes`
`32`	`32`	`* ud2 ; <- regs->ip`
`33`	`33`	`* .Ltmp1:`