[PATCH] compiler: Use __typeof_unqual__() for __unqual_scalar_typeof()

[Thomas Gleixner <tglx@kernel.org>]

From: Peter Zijlstra <peterz@infradead.org>

The recent changes to get_unaligned() resulted in a new sparse warning:

   net/rds/ib_cm.c:96:35: sparse: sparse: incorrect type in argument 1 (different modifiers) @@     expected void * @@     got restricted __be64 const * @@
   net/rds/ib_cm.c:96:35: sparse:     expected void *
   net/rds/ib_cm.c:96:35: sparse:     got restricted __be64 const *
 
The updated get_unaligned_t() uses __unqual_scalar_typeof() to get an
unqualified type. This works correctly for the compilers, but fails for
sparse when the data type is __be64 (or any other __beNN variant).

On sparse runs (C=[12]) __beNN types are annotated with
__attribute__((bitwise)).

That annotation allows sparse to detect incompatible operations on __beNN
variables, but it also prevents sparse from evaluating the _Generic() in
__unqual_scalar_typeof() and map __beNN to a unqualified scalar type, so it
ends up with the default, i.e. the original qualified type of a 'const
__beNN' pointer. That then ends up as the first pointer argument to
builtin_memcpy(), which obviously causes the above sparse warnings.

The sparse git tree supports typeof_unqual() now, which allows to use it
instead of the _Generic() based __unqual_scalar_typeof(). With that sparse
correctly evaluates the unqualified type and keeps the __beNN logic intact.

The downside is that this requires a top of tree sparse build and an old
sparse version will emit a metric ton of incomprehensible error messages
before it dies with a segfault.

Therefore implement a sanity check which validates that the checker is
available and capable of handling typeof_unqual(). Emit a warning if not so
the user can take informed action.

[ tglx: Move the evaluation of USE_TYPEOF_UNQUAL to compiler_types.h so it is
  	set before use and implement the sanity checker ]

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Closes: https://lore.kernel.org/oe-kbuild-all/202601150001.sKSN644a-lkp@intel.com/
---
 Makefile                       |    8 ++++++++
 include/linux/compiler.h       |   10 ----------
 include/linux/compiler_types.h |   11 +++++++++++
 scripts/checker-valid.sh       |   19 +++++++++++++++++++
 4 files changed, 38 insertions(+), 10 deletions(-)

--- a/Makefile
+++ b/Makefile
@@ -1178,6 +1178,14 @@ ifdef CONFIG_CC_IS_CLANG
 KBUILD_USERLDFLAGS += --ld-path=$(LD)
 endif
 
+# Validate the checker is available and functional
+ifneq ($(KBUILD_CHECKSRC), 0)
+  ifneq ($(shell $(srctree)/scripts/checker-valid.sh $(CHECK)), 1)
+    $(warning C=$(KBUILD_CHECKSRC) specified, but $(CHECK) is not available or not up to date)
+    KBUILD_CHECKSRC = 0
+  endif
+endif
+
 # make the checker run with the right architecture
 CHECKFLAGS += --arch=$(ARCH)
 
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -231,16 +231,6 @@ void ftrace_likely_update(struct ftrace_
 				"must be non-C-string (not NUL-terminated)")
 
 /*
- * Use __typeof_unqual__() when available.
- *
- * XXX: Remove test for __CHECKER__ once
- * sparse learns about __typeof_unqual__().
- */
-#if CC_HAS_TYPEOF_UNQUAL && !defined(__CHECKER__)
-# define USE_TYPEOF_UNQUAL 1
-#endif
-
-/*
  * Define TYPEOF_UNQUAL() to use __typeof_unqual__() as typeof
  * operator when available, to return an unqualified type of the exp.
  */
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -562,6 +562,13 @@ struct ftrace_likely_data {
 #define asm_inline asm
 #endif
 
+/*
+ * Use __typeof_unqual__() when available.
+ */
+#if CC_HAS_TYPEOF_UNQUAL || defined(__CHECKER__)
+# define USE_TYPEOF_UNQUAL 1
+#endif
+
 /* Are two types/vars the same type (ignoring qualifiers)? */
 #define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))
 
@@ -569,6 +576,7 @@ struct ftrace_likely_data {
  * __unqual_scalar_typeof(x) - Declare an unqualified scalar type, leaving
  *			       non-scalar types unchanged.
  */
+#ifndef USE_TYPEOF_UNQUAL
 /*
  * Prefer C11 _Generic for better compile-times and simpler code. Note: 'char'
  * is not type-compatible with 'signed char', and we define a separate case.
@@ -586,6 +594,9 @@ struct ftrace_likely_data {
 			 __scalar_type_to_expr_cases(long),		\
 			 __scalar_type_to_expr_cases(long long),	\
 			 default: (x)))
+#else
+#define __unqual_scalar_typeof(x) __typeof_unqual__(x)
+#endif
 
 /* Is this type a native word size -- useful for atomic operations */
 #define __native_word(t) \
--- /dev/null
+++ b/scripts/checker-valid.sh
@@ -0,0 +1,19 @@
+#!/bin/sh -eu
+# SPDX-License-Identifier: GPL-2.0
+
+[ ! -x "$(command -v "$1")" ] && exit 1
+
+tmp_file=$(mktemp)
+trap "rm -f $tmp_file" EXIT
+
+cat << EOF >$tmp_file
+static inline int u(const int *q)
+{
+	__typeof_unqual__(*q) v = *q;
+	return v;
+}
+EOF
+
+# sparse happily exits with 0 on error so validate
+# there is none on stderr. Use awk as grep is a pain with sh -e
+$1 $tmp_file 2>&1 | awk -v c=1 '/error/{c=0}END{print c}'