[PATCH v2] compiler.h: Introduce __must_be_noncstr()

[Kees Cook]

In preparation for adding more type checking to the memtostr/strtomem*()
helpers, introduce the ability to check for the "nonstring" attribute.
This is the reverse of what was added to strscpy*() in commit 559048d156ff
("string: Check for "nonstring" attribute on strscpy() arguments").

Note that __annotated() must be explicitly tested for, as GCC added
__builtin_has_attribute() after it added the "nonstring" attribute. Do
so here to avoid the !__annotated() test triggering build failures
when __builtin_has_attribute() was missing but __nonstring was defined.
(I've opted to squash this fix into this patch so we don't end up with
a possible bisection target that would leave the kernel unbuildable.)

Reported-by: Venkat Rao Bagalkote <venkat88@xxxxxxxxxxxxxxxxxx>
Reported-by: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
Reported-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
Reported-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
Closes: https://lore.kernel.org/all/adbe8dd1-a725-4811-ae7e-76fe770cf096@xxxxxxxxxxxxxxxxxx/
Signed-off-by: Kees Cook <kees@xxxxxxxxxx>
---
 include/linux/compiler.h       | 18 +++++++++++++++++-
 include/linux/compiler_types.h |  9 ++++++---
 2 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 200fd3c5bc70..d5201464c5e6 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -206,9 +206,25 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 #define __must_be_byte_array(a)	__BUILD_BUG_ON_ZERO_MSG(!__is_byte_array(a), \
 							"must be byte array")
 
+/*
+ * If the "nonstring" attribute isn't available, we have to return true
+ * so the __must_*() checks pass when "nonstring" isn't supported.
+ */
+#if __has_attribute(__nonstring__) && defined(__annotated)
+#define __is_cstr(a)		(!__annotated(a, nonstring))
+#define __is_noncstr(a)		(__annotated(a, nonstring))
+#else
+#define __is_cstr(a)		(true)
+#define __is_noncstr(a)		(true)
+#endif
+
 /* Require C Strings (i.e. NUL-terminated) lack the "nonstring" attribute. */
 #define __must_be_cstr(p) \
-	__BUILD_BUG_ON_ZERO_MSG(__annotated(p, nonstring), "must be cstr (NUL-terminated)")
+	__BUILD_BUG_ON_ZERO_MSG(!__is_cstr(p), \
+				"must be C-string (NUL-terminated)")
+#define __must_be_noncstr(p) \
+	__BUILD_BUG_ON_ZERO_MSG(!__is_noncstr(p), \
+				"must be non-C-string (not NUL-terminated)")
 
 #endif /* __KERNEL__ */
 
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 981cc3d7e3aa..f59393464ea7 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -446,11 +446,14 @@ struct ftrace_likely_data {
 #define __member_size(p)	__builtin_object_size(p, 1)
 #endif
 
-/* Determine if an attribute has been applied to a variable. */
+/*
+ * Determine if an attribute has been applied to a variable.
+ * Using __annotated needs to check for __annotated being available,
+ * or negative tests may fail when annotation cannot be checked. For
+ * example, see the definition of __is_cstr().
+ */
 #if __has_builtin(__builtin_has_attribute)
 #define __annotated(var, attr)	__builtin_has_attribute(var, attr)
-#else
-#define __annotated(var, attr)	(false)
 #endif
 
 /*
-- 
2.34.1