[PATCH v5 12/18] READ_ONCE: Drop pointer qualifiers when reading from scalar types

[Will Deacon]

Passing a volatile-qualified pointer to READ_ONCE() is an absolute
trainwreck for code generation: the use of 'typeof()' to define a
temporary variable inside the macro means that the final evaluation in
macro scope ends up forcing a read back from the stack. When stack
protector is enabled (the default for arm64, at least), this causes
the compiler to vomit up all sorts of junk.

Unfortunately, dropping pointer qualifiers inside the macro poses quite
a challenge, especially since the pointed-to type is permitted to be an
aggregate, and this is relied upon by mm/ code accessing things like
'pmd_t'. Based on numerous hacks and discussions on the mailing list,
this is the best I've managed to come up with.

Introduce '__unqual_scalar_typeof()' which takes an expression and, if
the expression is an optionally qualified 8, 16, 32 or 64-bit scalar
type, evaluates to the unqualified type. Other input types, including
aggregates, remain unchanged. Hopefully READ_ONCE() on volatile aggregate
pointers isn't something we do on a fast-path.

Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Reported-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
---
 include/linux/compiler.h       |  6 +++---
 include/linux/compiler_types.h | 26 ++++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index e1b839e42563..0caced170a8a 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -204,7 +204,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
  * atomicity or dependency ordering guarantees. Note that this may result
  * in tears!
  */
-#define __READ_ONCE(x)	(*(const volatile typeof(x) *)&(x))
+#define __READ_ONCE(x)	(*(const volatile __unqual_scalar_typeof(x) *)&(x))
 
 #define __READ_ONCE_SCALAR(x)						\
 ({									\
@@ -212,10 +212,10 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 	kcsan_check_atomic_read(__xp, sizeof(*__xp));			\
 	__kcsan_disable_current();					\
 	({								\
-		typeof(x) __x = __READ_ONCE(*__xp);			\
+		__unqual_scalar_typeof(x) __x = __READ_ONCE(*__xp);	\
 		__kcsan_enable_current();				\
 		smp_read_barrier_depends();				\
-		__x;							\
+		(typeof(x))__x;						\
 	});								\
 })
 
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index e970f97a7fcb..6ed0612bc143 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -210,6 +210,32 @@ struct ftrace_likely_data {
 /* Are two types/vars the same type (ignoring qualifiers)? */
 #define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))
 
+/*
+ * __unqual_scalar_typeof(x) - Declare an unqualified scalar type, leaving
+ *			       non-scalar types unchanged.
+ *
+ * We build this out of a couple of helper macros in a vain attempt to
+ * help you keep your lunch down while reading it.
+ */
+#define __pick_scalar_type(x, type, otherwise)					\
+	__builtin_choose_expr(__same_type(x, type), (type)0, otherwise)
+
+/*
+ * 'char' is not type-compatible with either 'signed char' or 'unsigned char',
+ * so we include the naked type here as well as the signed/unsigned variants.
+ */
+#define __pick_integer_type(x, type, otherwise)					\
+	__pick_scalar_type(x, type,						\
+		__pick_scalar_type(x, unsigned type,				\
+			__pick_scalar_type(x, signed type, otherwise)))
+
+#define __unqual_scalar_typeof(x) typeof(					\
+	__pick_integer_type(x, char,						\
+		__pick_integer_type(x, short,					\
+			__pick_integer_type(x, int,				\
+				__pick_integer_type(x, long,			\
+					__pick_integer_type(x, long long, x))))))
+
 /* Is this type a native word size -- useful for atomic operations */
 #define __native_word(t) \
 	(sizeof(t) == sizeof(char) || sizeof(t) == sizeof(short) || \
-- 
2.26.2.645.ge9eca65c58-goog