Re: [PATCH v3 2/6] compiler.h: Introduce TYPEOF_UNQUAL() macro

[Uros Bizjak]

On Mon, Dec 9, 2024 at 12:30 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Sun, Dec 08, 2024 at 09:45:17PM +0100, Uros Bizjak wrote:
> > Define TYPEOF_UNQUAL() to use __typeof_unqual__() as typeof operator
> > when available, to return unqualified type of the expression.
> >
> > Current version of sparse doesn't know anything about __typeof_unqual__()
> > operator. Avoid the usage of __typeof_unqual__() when sparse checking
> > is active to prevent sparse errors with unknowing keyword.
>
> Ooooh, new toys.
>
> I suppose __unqual_scalar_typeof() wants to be using this when
> available?

Not only that, the new toy enables clang to check kernel's address
spaces in a generic way using address_space attribute.

Please find attached a follow-up patch that enables __percpu checks
for all targets, supported by clang. Clang is a little bit pickier
than gcc about named address space declarations (it warns for use of
duplicated address space attribute), so the patch in addition to the
obvious

+#  define __percpu_qual        __attribute__((address_space(3)))

also fixes a couple of macros that could result in a duplicated
address space attribute.

The patch, applied as a follow-up to the series, survives allyesconfig
compilation with clang-19 and produces a bootable kernel. The patch
was tested only for x86_64 target, for other targets a couple of
trivial fixes would be necessary (a cast or a substitution of typeof()
with TYPEOF_UNQUAL()).

AFAICS, the same approach using clang's address_space attribute can be
implemented to also check other address spaces: __user, __iommu  and
__rcu.

Uros.
diff --git a/include/asm-generic/percpu.h b/include/asm-generic/percpu.h
index 02aeca21479a..4109d828a564 100644
--- a/include/asm-generic/percpu.h
+++ b/include/asm-generic/percpu.h
@@ -16,7 +16,12 @@
  * space qualifier).
  */
 #ifndef __percpu_qual
-# define __percpu_qual
+# if __has_attribute(address_space) && \
+     defined(CONFIG_CC_HAS_TYPEOF_UNQUAL) && !defined(__CHECKER__)
+#  define __percpu_qual		__attribute__((address_space(3)))
+# else
+#  define __percpu_qual
+# endif
 #endif
 
 #ifdef CONFIG_SMP
diff --git a/include/linux/device.h b/include/linux/device.h
index 667cb6db9019..1d6a55d5250a 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -431,9 +431,9 @@ static inline int __devm_add_action_or_reset(struct device *dev, void (*action)(
  * RETURNS:
  * Pointer to allocated memory on success, NULL on failure.
  */
-#define devm_alloc_percpu(dev, type)      \
-	((typeof(type) __percpu *)__devm_alloc_percpu((dev), sizeof(type), \
-						      __alignof__(type)))
+#define devm_alloc_percpu(dev, type)				    \
+	((TYPEOF_UNQUAL(type) __percpu *)__devm_alloc_percpu((dev), \
+				sizeof(type), __alignof__(type)))
 
 void __percpu *__devm_alloc_percpu(struct device *dev, size_t size,
 				   size_t align);
diff --git a/include/linux/percpu.h b/include/linux/percpu.h
index 52b5ea663b9f..c3bf040aba66 100644
--- a/include/linux/percpu.h
+++ b/include/linux/percpu.h
@@ -148,13 +148,13 @@ extern void __percpu *pcpu_alloc_noprof(size_t size, size_t align, bool reserved
 	alloc_hooks(pcpu_alloc_noprof(_size, _align, true, GFP_KERNEL))
 
 #define alloc_percpu_gfp(type, gfp)					\
-	(typeof(type) __percpu *)__alloc_percpu_gfp(sizeof(type),	\
+	(TYPEOF_UNQUAL(type) __percpu *)__alloc_percpu_gfp(sizeof(type), \
 						__alignof__(type), gfp)
 #define alloc_percpu(type)						\
-	(typeof(type) __percpu *)__alloc_percpu(sizeof(type),		\
+	(TYPEOF_UNQUAL(type) __percpu *)__alloc_percpu(sizeof(type),	\
 						__alignof__(type))
 #define alloc_percpu_noprof(type)					\
-	((typeof(type) __percpu *)pcpu_alloc_noprof(sizeof(type),	\
+	((TYPEOF_UNQUAL(type) __percpu *)pcpu_alloc_noprof(sizeof(type), \
 					__alignof__(type), false, GFP_KERNEL))
 
 extern void free_percpu(void __percpu *__pdata);