[PATCH 03/17] mm: add generic __va_function and __pa_function macros

Sami Tolvanen
2021-03-12T00:50:20Z

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 generic
definitions for __va_function and __pa_function, which architectures
that support CFI can override. The typical implementation of the
__va_function macro would use inline assembly to take the function
address, which avoids compiler instrumentation.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
include/linux/mm.h | 8 ++++++++
1 file changed, 8 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 77e64e3eac80..1262c4c0242c 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -116,6 +116,14 @@ extern int mmap_rnd_compat_bits __read_mostly;
#define __pa_symbol(x) __pa(RELOC_HIDE((unsigned long)(x), 0))
#endif

+#ifndef __va_function
+#define __va_function(x) (x)
+#endif
+
+#ifndef __pa_function
+#define __pa_function(x) __pa_symbol(__va_function(x))
+#endif
+
#ifndef page_to_virt
#define page_to_virt(x) __va(PFN_PHYS(page_to_pfn(x)))
#endif
--
2.31.0.rc2.261.g7f71774620-goog