[PATCH 2/2] 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 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.

Silence the warning by annotating `__kcsan_check_access()` with
`__access(none, 1)`. This tells the compiler that the pointer is not
used to access the referenced object, avoiding the false positive.

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>
---
 include/linux/kcsan-checks.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
index 92f3843d9ebb..3a44f90ebd2b 100644
--- a/include/linux/kcsan-checks.h
+++ b/include/linux/kcsan-checks.h
@@ -34,7 +34,7 @@
  * @size: size of access
  * @type: access type modifier
  */
-void __kcsan_check_access(const volatile void *ptr, size_t size, int type);
+void __kcsan_check_access(const volatile void *ptr, size_t size, int type) __access(none, 1);
 
 /*
  * See definition of __tsan_atomic_signal_fence() in kernel/kcsan/core.c.
-- 
2.54.0.rc2.533.g4f5dca5207-goog