linux-hardening - [PATCH 09/10] compiler.h: Introduce __must_be

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Message-Id: <20250207010022.749952-9-kees@kernel.org>
Date: Thu,  6 Feb 2025 17:00:18 -0800
From: Kees Cook <kees@...nel.org>
To: Andy Shevchenko <andy@...nel.org>
Cc: Kees Cook <kees@...nel.org>,
	Sathya Prakash <sathya.prakash@...adcom.com>,
	Sreekanth Reddy <sreekanth.reddy@...adcom.com>,
	Suganath Prabu Subramani <suganath-prabu.subramani@...adcom.com>,
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	Sumit Saxena <sumit.saxena@...adcom.com>,
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	Andrew Morton <akpm@...ux-foundation.org>,
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Subject: [PATCH 09/10] compiler.h: Introduce __must_be_noncstr()

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@...nel.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