[RFC PATCH v2 07/21] cfi: Add type helper macros

Sami Tolvanen
2022-05-13T20:24:17Z

With CONFIG_CFI_CLANG, assembly functions called indirectly
from C code must be annotated with type identifiers to pass CFI
checking. The compiler emits a __kcfi_typeid_<function> symbol for
each address-taken function declaration in C, which contains the
expected type identifier. Add typed versions of SYM_FUNC_START and
SYM_FUNC_START_ALIAS, which emit the type identifier before the
function.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
include/linux/cfi_types.h | 57 +++++++++++++++++++++++++++++++++++++++
1 file changed, 57 insertions(+)
create mode 100644 include/linux/cfi_types.h

diff --git a/include/linux/cfi_types.h b/include/linux/cfi_types.h
new file mode 100644
index 000000000000..dd16e755a197
--- /dev/null
+++ b/include/linux/cfi_types.h
@@ -0,0 +1,57 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Clang Control Flow Integrity (CFI) type definitions.
+ */
+#ifndef _LINUX_CFI_TYPES_H
+#define _LINUX_CFI_TYPES_H
+
+#ifdef CONFIG_CFI_CLANG
+#include <linux/linkage.h>
+
+#ifdef __ASSEMBLY__
+/*
+ * Use the __kcfi_typeid_<function> type identifier symbol to
+ * annotate indirectly called assembly functions. The compiler emits
+ * these symbols for all address-taken function declarations in C
+ * code.
+ */
+#ifndef __CFI_TYPE
+#define __CFI_TYPE(name) \
+ .4byte __kcfi_typeid_##name
+#endif
+
+#define SYM_TYPED_ENTRY(name, fname, linkage, align...) \
+ linkage(name) ASM_NL \
+ align ASM_NL \
+ __CFI_TYPE(fname) ASM_NL \
+ name:
+
+#define __SYM_TYPED_FUNC_START_ALIAS(name, fname) \
+ SYM_TYPED_ENTRY(name, fname, SYM_L_GLOBAL, SYM_A_ALIGN)
+
+#define __SYM_TYPED_FUNC_START(name, fname) \
+ SYM_TYPED_ENTRY(name, fname, SYM_L_GLOBAL, SYM_A_ALIGN)
+
+#endif /* __ASSEMBLY__ */
+
+#else /* CONFIG_CFI_CLANG */
+
+#ifdef __ASSEMBLY__
+#define __SYM_TYPED_FUNC_START_ALIAS(name, fname) \
+ SYM_FUNC_START_ALIAS(name)
+
+#define __SYM_TYPED_FUNC_START(name, fname) \
+ SYM_FUNC_START(name)
+#endif /* __ASSEMBLY__ */
+
+#endif /* CONFIG_CFI_CLANG */
+
+#ifdef __ASSEMBLY__
+#define SYM_TYPED_FUNC_START_ALIAS(name) \
+ __SYM_TYPED_FUNC_START_ALIAS(name, name)
+
+#define SYM_TYPED_FUNC_START(name) \
+ __SYM_TYPED_FUNC_START(name, name)
+#endif /* __ASSEMBLY__ */
+
+#endif /* _LINUX_CFI_TYPES_H */
--
2.36.0.550.gb090851708-goog