linux-hardening - [PATCH v2 2/3] compiler.h: Introduce __must_be_byte

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Message-Id: <20250206214014.3567372-2-kees@kernel.org>
Date: Thu,  6 Feb 2025 13:40:07 -0800
From: Kees Cook <kees@...nel.org>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Kees Cook <kees@...nel.org>,
	Kent Overstreet <kent.overstreet@...ux.dev>,
	Miguel Ojeda <ojeda@...nel.org>,
	kernel test robot <lkp@...el.com>,
	Andy Shevchenko <andy@...nel.org>,
	Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Philipp Reisner <philipp.reisner@...bit.com>,
	linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org,
	llvm@...ts.linux.dev
Subject: [PATCH v2 2/3] compiler.h: Introduce __must_be_byte_array()

In preparation for adding stricter type checking to the str/mem*()
helpers, provide a way to check that a variable is a byte array
via __must_be_byte_array().

Suggested-by: Kent Overstreet <kent.overstreet@...ux.dev>
Signed-off-by: Kees Cook <kees@...nel.org>
---
 include/linux/compiler.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 7af999a131cb..1c0688319435 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -221,7 +221,13 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 #endif /* __CHECKER__ */
 
 /* &a[0] degrades to a pointer: a different type from an array */
-#define __must_be_array(a)	__BUILD_BUG_ON_ZERO_MSG(__same_type((a), &(a)[0]), "must be array")
+#define __is_array(a)		(!__same_type((a), &(a)[0]))
+#define __must_be_array(a)	__BUILD_BUG_ON_ZERO_MSG(!__is_array(a), \
+							"must be array")
+
+#define __is_byte_array(a)	(__is_array(a) && sizeof((a)[0]) == 1)
+#define __must_be_byte_array(a)	__BUILD_BUG_ON_ZERO_MSG(!__is_byte_array(a), \
+							"must be byte array")
 
 /* Require C Strings (i.e. NUL-terminated) lack the "nonstring" attribute. */
 #define __must_be_cstr(p) \
-- 
2.34.1