[PATCH 09/10] compiler.h: Introduce __must_be_noncstr()

Kees Cook
2025-02-07T01:00:23Z

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").

Signed-off-by: Kees Cook <kees@kernel.org>
---
include/linux/compiler.h | 18 +++++++++++++++++-
1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 1c0688319435..c89070a2f964 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -229,9 +229,25 @@ 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__)
+#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__ */

--
2.34.1