[PATCH v2] kcsan: Silence -Wmaybe-uninitialized when calling __kcsan_check_access()

[Marco Elver <elver@google.com>]

Some subsystems enable -Wmaybe-uninitialized [1], which can trigger
false positives when KCSAN is enabled. Specifically, passing an
uninitialized variable to functions that instrument accesses (e.g.,
copy_from_user()) results in calls to __kcsan_check_access().

Because __kcsan_check_access() takes a `const volatile void *ptr`, GCC
infers that the function may only read the memory location, and thus
warns if the passed variable is uninitialized.

However, KCSAN is a dynamic analysis tool for data race detection; while
it does read the memory location to detect concurrent modifications, the
"initialized'ness" of the memory location is irrelevant for its analysis.

Use absolute_pointer() in __kcsan_check_write(), kcsan_check_write(),
and kcsan_check_atomic_write() to hide the pointer from the compiler,
preventing it from concluding that the pointer passed points to
uninitialized memory.

This fixes warnings like:

|   CC      fs/ntfs3/file.o
| In file included from include/asm-generic/rwonce.h:27,
|                  from arch/arm64/include/asm/rwonce.h:81,
|                  from include/linux/compiler.h:369,
|                  from include/linux/array_size.h:5,
|                  from include/linux/kernel.h:16,
|                  from include/linux/backing-dev.h:12,
|                  from fs/ntfs3/file.c:10:
| In function 'instrument_copy_from_user_before',
|     inlined from '_inline_copy_from_user' at include/linux/uaccess.h:184:2,
|     inlined from 'copy_from_user' at include/linux/uaccess.h:221:9,
|     inlined from 'ntfs_ioctl_fitrim' at fs/ntfs3/file.c:77:6,
|     inlined from 'ntfs_ioctl' at fs/ntfs3/file.c:164:10:
| include/linux/kcsan-checks.h:220:28: error: 'range' may be used uninitialized [-Werror=maybe-uninitialized]
|   220 | #define kcsan_check_access __kcsan_check_access
|       |                            ^
| include/linux/kcsan-checks.h:311:9: note: in expansion of macro 'kcsan_check_access'
|   311 |         kcsan_check_access(ptr, size, KCSAN_ACCESS_WRITE)
|       |         ^~~~~~~~~~~~~~~~~~
| include/linux/instrumented.h:147:9: note: in expansion of macro 'kcsan_check_write'
|   147 |         kcsan_check_write(to, n);
|       |         ^~~~~~~~~~~~~~~~~
| include/linux/kcsan-checks.h: In function 'ntfs_ioctl':
| include/linux/kcsan-checks.h:37:6: note: by argument 1 of type 'const volatile void *' to '__kcsan_check_access' declared here
|    37 | void __kcsan_check_access(const volatile void *ptr, size_t size, int type);
|       |      ^~~~~~~~~~~~~~~~~~~~
| fs/ntfs3/file.c:65:29: note: 'range' declared here
|    65 |         struct fstrim_range range;
|       |                             ^~~~~

Link: https://lore.kernel.org/all/5da10cca-875b-418d-b54e-6be3ea32c266@app.fastmail.com/ [1]
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Marco Elver <elver@google.com>
---
v2:
* Switch from __attribute__((access)) to absolute_pointer() to silence
  the warnings instead of introducing new ones.
---
 include/linux/kcsan-checks.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
index 92f3843d9ebb..e135dacaa90f 100644
--- a/include/linux/kcsan-checks.h
+++ b/include/linux/kcsan-checks.h
@@ -282,7 +282,7 @@ static inline void __kcsan_disable_current(void) { }
  * @size: size of access
  */
 #define __kcsan_check_write(ptr, size)                                         \
-	__kcsan_check_access(ptr, size, KCSAN_ACCESS_WRITE)
+	__kcsan_check_access(absolute_pointer(ptr), size, KCSAN_ACCESS_WRITE)
 
 /**
  * __kcsan_check_read_write - check regular read-write access for races
@@ -308,7 +308,7 @@ static inline void __kcsan_disable_current(void) { }
  * @size: size of access
  */
 #define kcsan_check_write(ptr, size)                                           \
-	kcsan_check_access(ptr, size, KCSAN_ACCESS_WRITE)
+	kcsan_check_access(absolute_pointer(ptr), size, KCSAN_ACCESS_WRITE)
 
 /**
  * kcsan_check_read_write - check regular read-write access for races
@@ -331,7 +331,7 @@ static inline void __kcsan_disable_current(void) { }
 #define kcsan_check_atomic_read(ptr, size)                                     \
 	kcsan_check_access(ptr, size, KCSAN_ACCESS_ATOMIC)
 #define kcsan_check_atomic_write(ptr, size)                                    \
-	kcsan_check_access(ptr, size, KCSAN_ACCESS_ATOMIC | KCSAN_ACCESS_WRITE)
+	kcsan_check_access(absolute_pointer(ptr), size, KCSAN_ACCESS_ATOMIC | KCSAN_ACCESS_WRITE)
 #define kcsan_check_atomic_read_write(ptr, size)                               \
 	kcsan_check_access(ptr, size, KCSAN_ACCESS_ATOMIC | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_COMPOUND)
 #endif
-- 
2.54.0.rc2.533.g4f5dca5207-goog