[PATCH v4 03/17] mm: add generic function_nocfi macro

[Sami Tolvanen]

With CONFIG_CFI_CLANG, the compiler replaces function addresses
in instrumented C code with jump table addresses. This means that
__pa_symbol(function) returns the physical address of the jump table
entry instead of the actual function, which may not work as the jump
table code will immediately jump to a virtual address that may not be
mapped.

To avoid this address space confusion, this change adds a generic
definition for function_nocfi(), which architectures that support CFI
can override. The typical implementation of would use inline assembly
to take the function address, which avoids compiler instrumentation.

Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 include/linux/mm.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8ba434287387..22cce9c7dd05 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -124,6 +124,16 @@ extern int mmap_rnd_compat_bits __read_mostly;
 #define lm_alias(x)	__va(__pa_symbol(x))
 #endif
 
+/*
+ * With CONFIG_CFI_CLANG, the compiler replaces function addresses in
+ * instrumented C code with jump table addresses. Architectures that
+ * support CFI can define this macro to return the actual function address
+ * when needed.
+ */
+#ifndef function_nocfi
+#define function_nocfi(x) (x)
+#endif
+
 /*
  * To prevent common memory management code establishing
  * a zero page mapping on a read fault.
-- 
2.31.0.291.g576ba9dcdaf-goog