[PATCH] compiler_types: Introduce __nonstring_array

[PATCH] compiler_types: Introduce __nonstring_array

[Kees Cook]

GCC has expanded support of the "nonstring" attribute so that it can be
applied to arrays of character arrays[1], which is needed to identify
correct static initialization of those kinds of objects. Since this was
not supported prior to GCC 15, we need to distinguish the usage of Linux's
existing __nonstring macro for the attribute for non-multi-dimensional
char arrays. Until GCC 15 is the minimum version, use __nonstring_array to
mark arrays of non-string character arrays. (Regular non-string character
arrays can continue to use __nonstring.)

This allows for changes like this:

-static const char table_sigs[][ACPI_NAMESEG_SIZE] __initconst = {
+static const char table_sigs[][ACPI_NAMESEG_SIZE] __nonstring_array __initconst = {
        ACPI_SIG_BERT, ACPI_SIG_BGRT, ACPI_SIG_CPEP, ACPI_SIG_ECDT,

Which will silence the coming -Wunterminated-string-initialization
warnings in GCC 15:

In file included from ../include/acpi/actbl.h:371,                                                                   from ../include/acpi/acpi.h:26,                                                                     from ../include/linux/acpi.h:26,
                 from ../drivers/acpi/tables.c:19:
../include/acpi/actbl1.h:30:33: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (5 chars into 4 available) [-Wunterminated-string-initialization]
   30 | #define ACPI_SIG_BERT           "BERT"  /* Boot Error Record Table */
      |                                 ^~~~~~                                                      ../drivers/acpi/tables.c:400:9: note: in expansion of macro 'ACPI_SIG_BERT'                           400 |         ACPI_SIG_BERT, ACPI_SIG_BGRT, ACPI_SIG_CPEP, ACPI_SIG_ECDT,
      |         ^~~~~~~~~~~~~                                                                       ../include/acpi/actbl1.h:31:33: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (5 chars into 4 available) [-Wunterminated-string-initialization]
   31 | #define ACPI_SIG_BGRT           "BGRT"  /* Boot Graphics Resource Table */
      |                                 ^~~~~~

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117178 [1]
Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Len Brown <lenb@kernel.org>
Cc: Marco Elver <elver@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Cc: Hao Luo <haoluo@google.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: linux-acpi@vger.kernel.org
---
 include/linux/compiler_types.h | 12 ++++++++++++
 init/Kconfig                   |  3 +++
 2 files changed, 15 insertions(+)

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 981cc3d7e3aa..7ccea700b46d 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -348,6 +348,18 @@ struct ftrace_likely_data {
 # define __counted_by(member)
 #endif
 
+/*
+ * Optional: only supported since gcc >= 15
+ * Optional: not supported by Clang
+ *
+ * gcc: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117178
+ */
+#ifdef CONFIG_CC_HAS_MULTIDIMENSIONAL_NONSTRING
+# define __nonstring_array		__attribute__((__nonstring__))
+#else
+# define __nonstring_array
+#endif
+
 /*
  * Apply __counted_by() when the Endianness matches to increase test coverage.
  */
diff --git a/init/Kconfig b/init/Kconfig
index d0d021b3fa3b..723dc69507d6 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -129,6 +129,9 @@ config CC_HAS_COUNTED_BY
 	# https://github.com/llvm/llvm-project/pull/112636
 	depends on !(CC_IS_CLANG && CLANG_VERSION < 190103)
 
+config CC_HAS_MULTIDIMENSIONAL_NONSTRING
+	def_bool $(success,echo 'char tag[][4] __attribute__((__nonstring__)) = { };' | $(CC) $(CLANG_FLAGS) -x c - -c -o /dev/null -Werror)
+
 config RUSTC_HAS_COERCE_POINTEE
 	def_bool RUSTC_VERSION >= 108400
 
-- 
2.34.1

Re: [PATCH] compiler_types: Introduce __nonstring_array

[Kees Cook]

On Mon, Mar 10, 2025 at 02:42:48PM -0700, Kees Cook wrote:
> In file included from ../include/acpi/actbl.h:371,                                                                   from ../include/acpi/acpi.h:26,                                                                     from ../include/linux/acpi.h:26,
>                  from ../drivers/acpi/tables.c:19:

Ugh, this whole paste went poorly. I've fixed it locally. It should be:

In file included from ../include/acpi/actbl.h:371,
                 from ../include/acpi/acpi.h:26,
                 from ../include/linux/acpi.h:26,
                 from ../drivers/acpi/tables.c:19:
../include/acpi/actbl1.h:30:33: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (5 chars into 4 available) [-Wunterminated-string-initialization]
   30 | #define ACPI_SIG_BERT           "BERT"  /* Boot Error Record Table */
      |                                 ^~~~~~
../drivers/acpi/tables.c:400:9: note: in expansion of macro 'ACPI_SIG_BERT'
  400 |         ACPI_SIG_BERT, ACPI_SIG_BGRT, ACPI_SIG_CPEP, ACPI_SIG_ECDT,
      |         ^~~~~~~~~~~~~
../include/acpi/actbl1.h:31:33: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (5 chars into 4 available) [-Wunterminated-string-initialization]
   31 | #define ACPI_SIG_BGRT           "BGRT"  /* Boot Graphics Resource Table */
      |                                 ^~~~~~

-- 
Kees Cook

Re: [PATCH] compiler_types: Introduce __nonstring_array

[Miguel Ojeda]

On Mon, Mar 10, 2025 at 10:42 PM Kees Cook <kees@kernel.org> wrote:
>
> GCC has expanded support of the "nonstring" attribute so that it can be
> applied to arrays of character arrays[1], which is needed to identify
> correct static initialization of those kinds of objects. Since this was
> not supported prior to GCC 15, we need to distinguish the usage of Linux's
> existing __nonstring macro for the attribute for non-multi-dimensional
> char arrays. Until GCC 15 is the minimum version, use __nonstring_array to
> mark arrays of non-string character arrays. (Regular non-string character
> arrays can continue to use __nonstring.)

Sounds reasonable. So this means that, when the GCC minimum goes over
15 in some years, we can both remove it and move it to
`compiler_attributes.h`, right?

Perhaps we should mention part of this commit message (or the sentence
above) in the comment to clarify (it will also help to clarify what
that "only supported since GCC >= 15" means).

If you are going to use it in a series that has a use case:

Acked-by: Miguel Ojeda <ojeda@kernel.org>

Thanks!

Cheers,
Miguel