[PATCH] compiler, clang: handle randomizable anonymous structs

[Sandipan Das]

The GCC randomize layout plugin can randomize the member
offsets of sensitive kernel data structures. To use this
feature, certain annotations and members are added to the
structures which affect the member offsets even if this
plugin is not used.

All of these structures are completely randomized, except
for task_struct which leaves out some of its members. All
the other members are wrapped within an anonymous struct
with the __randomize_layout attribute. This is done using
the randomized_struct_fields_start and
randomized_struct_fields_end defines. When the plugin is
disabled, the behaviour of this attribute can vary based
on the GCC version. For GCC 5.1+, this attribute maps to
__designated_init otherwise it is just an empty define
but the anonymous structure is still present. For other
compilers, both randomized_struct_fields_start and
randomized_struct_fields_end default to empty defines
meaning the anonymous structure is not introduced at all.
So, if a module compiled with Clang, such as a BPF program,
needs to access task_struct fields such as pid and comm,
the offsets of these members as recognized by Clang are
different from those recognized by modules compiled with
GCC. If GCC 4.6+ is used to build the kernel, this can be
solved by introducing appropriate defines for Clang so
that the anonymous structure is seen when determining the
offsets for the members.

Signed-off-by: Sandipan Das <sandipan@xxxxxxxxxxxxxxxxxx>
---
 include/linux/compiler-clang.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
index 54dfef70a072..780b1242bf24 100644
--- a/include/linux/compiler-clang.h
+++ b/include/linux/compiler-clang.h
@@ -16,3 +16,6 @@
  * with any version that can compile the kernel
  */
 #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__)
+
+#define randomized_struct_fields_start	struct {
+#define randomized_struct_fields_end	};
-- 
2.13.6