[PATCH v4 03/15] linkage: Add DECLARE_ASM_FUNC_SYMBOL

Sami Tolvanen
2021-09-30T18:05:45Z

The kernel has several assembly functions, which are not directly
callable from C but need to be referred to from C code. This change adds
the DECLARE_ASM_FUNC_SYMBOL macro, which allows us to declare these
symbols using an opaque type, which makes misuse harder, and avoids the
need to annotate references to the functions for Clang's Control-Flow
Integrity (CFI).

Suggested-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
include/linux/linkage.h | 13 +++++++++++++
1 file changed, 13 insertions(+)

diff --git a/include/linux/linkage.h b/include/linux/linkage.h
index dbf8506decca..f1eac26b2dd6 100644
--- a/include/linux/linkage.h
+++ b/include/linux/linkage.h
@@ -48,6 +48,19 @@
#define __PAGE_ALIGNED_DATA .section ".data..page_aligned", "aw"
#define __PAGE_ALIGNED_BSS .section ".bss..page_aligned", "aw"

+/*
+ * Declares a function not callable from C using an opaque type. Defined as
+ * an array to allow the address of the symbol to be taken without '&'.
+ */
+#ifndef DECLARE_ASM_FUNC_SYMBOL
+#define DECLARE_ASM_FUNC_SYMBOL(sym) \
+ extern const u8 sym[]
+#endif
+
+#ifndef __ASSEMBLY__
+typedef const u8 *asm_func_ptr;
+#endif
+
/*
* This is used by architectures to keep arguments on the stack
* untouched by the compiler by keeping them live until the end.
--
2.33.0.800.g4c38ced690-goog