[PATCH v2 1/2] rcu: Drop the address space qualifier from the dereference macros

[Ricardo Ribalda]

The RCU dereference macros end with a cast that is meant to hand back a
plain kernel pointer from a __rcu pointer.

For that it uses:
	((typeof(*p) __force __kernel *)(local))

The problem is that typeof preserves every qualifier, including the
address space qualifiers (__rcu).

Recent versions of smatch[1] care about this and throw tens of warnings
like this one:
  ./include/trace/events/vb2.h:46:1: warning: incorrect type in assignment (different address spaces)
  ./include/trace/events/vb2.h:46:1:    expected struct tracepoint_func *it_func_ptr
  ./include/trace/events/vb2.h:46:1:    got struct tracepoint_func __rcu *

Use a new macro TYPEOF_NO_ADDRESS_SPACE() for the result type. This new
macro strips all the qualifiers when running with sparse (so const and
volatile are gone). But keeps all the qualifiers when running with the
compiler, caring about const/volatile mismatch.

[1] https://github.com/error27/smatch/commit/e53027a4e816a772403baafa83c09e4a94c1cb8f

Suggested-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20260825-unqual-v1-1-7024fb81b4f9@xxxxxxxxxxxx
Signed-off-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
---
 include/linux/compiler.h | 17 +++++++++++++++++
 include/linux/rcupdate.h | 10 +++++-----
 2 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index cb2f6050bdf7..70cb31d60538 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -239,6 +239,23 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 # define TYPEOF_UNQUAL(exp) __typeof__(exp)
 #endif
 
+/*
+ * TYPEOF_NO_ADDRESS_SPACE() - typeof() without the address space qualifiers
+ *
+ * No operator strips only the address space qualifiers: typeof() keeps every
+ * qualifier and TYPEOF_UNQUAL() drops every qualifier, const and volatile
+ * included.
+ *
+ * Approximate one by dropping the qualifiers for sparse only, as it is the
+ * only one that knows about address spaces. The compiler keeps seeing the fully
+ * qualified type, so a missing const or volatile will still throw a warning.
+ */
+#ifdef __CHECKER__
+# define TYPEOF_NO_ADDRESS_SPACE(exp) TYPEOF_UNQUAL(exp)
+#else
+# define TYPEOF_NO_ADDRESS_SPACE(exp) __typeof__(exp)
+#endif
+
 #endif /* __KERNEL__ */
 
 #if defined(CONFIG_CFI) && !defined(__DISABLE_EXPORTS) && !defined(BUILD_VDSO)
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 44c07a66edff..3f74ae6d6e1f 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -488,7 +488,7 @@ static __always_inline bool lockdep_assert_rcu_helper(bool c, const struct __ctx
 context_unsafe(								\
 	typeof(*p) *local = (typeof(*p) *__force)(p);			\
 	rcu_check_sparse(p, __rcu);					\
-	((typeof(*p) __force __kernel *)(local))			\
+	((TYPEOF_NO_ADDRESS_SPACE(*p) __force __kernel *)(local))	\
 )
 /**
  * unrcu_pointer - mark a pointer as not being RCU protected
@@ -503,7 +503,7 @@ context_unsafe(								\
 ({ \
 	typeof(*p) *local = (typeof(*p) *__force)READ_ONCE(p); \
 	rcu_check_sparse(p, space); \
-	((typeof(*p) __force __kernel *)(local)); \
+	((TYPEOF_NO_ADDRESS_SPACE(*p) __force __kernel *)(local)); \
 }) )
 #define __rcu_dereference_check(p, local, c, space) \
 ({ \
@@ -511,19 +511,19 @@ context_unsafe(								\
 	typeof(*p) *local = (typeof(*p) *__force)READ_ONCE(p); \
 	RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_check() usage"); \
 	rcu_check_sparse(p, space); \
-	((typeof(*p) __force __kernel *)(local)); \
+	((TYPEOF_NO_ADDRESS_SPACE(*p) __force __kernel *)(local)); \
 })
 #define __rcu_dereference_protected(p, local, c, space) \
 ({ \
 	RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_protected() usage"); \
 	rcu_check_sparse(p, space); \
-	((typeof(*p) __force __kernel *)(p)); \
+	((TYPEOF_NO_ADDRESS_SPACE(*p) __force __kernel *)(p)); \
 })
 #define __rcu_dereference_raw(p, local) \
 ({ \
 	/* Dependency order vs. p above. */ \
 	typeof(p) local = READ_ONCE(p); \
-	((typeof(*p) __force __kernel *)(local)); \
+	((TYPEOF_NO_ADDRESS_SPACE(*p) __force __kernel *)(local)); \
 })
 #define rcu_dereference_raw(p) __rcu_dereference_raw(p, __UNIQUE_ID(rcu))
 

-- 
2.55.0.1007.g17ff1f9808-goog