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Date: Wed,  8 Nov 2017 23:30:17 +0100
From: Rasmus Villemoes <linux@...musvillemoes.dk>
To: kernel-hardening@...ts.openwall.com
Cc: linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: [RFC 3/6] compiler.h: add __attribute__((format_arg)) shorthand

The __format_arg attribute tells gcc that it can use a specific
argument to the annotated function as the format string for the
purpose of type-checking a surrounding __printf function call, e.g.

  sprintf(buf, fmtcheck(what->ever, "%d %lx", 0), i, m)

makes gcc check that i and m are indeed an int and a long; with

  sprintf(buf, what->ever, i, m)

the compiler cannot do any type checking.

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 include/linux/compiler-gcc.h | 1 +
 include/linux/compiler.h     | 4 ++++
 2 files changed, 5 insertions(+)

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index bb78e5bdff26..69a233f9fa26 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -128,6 +128,7 @@
 #define __maybe_unused		__attribute__((unused))
 #define __always_unused		__attribute__((unused))
 #define __mode(x)               __attribute__((mode(x)))
+#define __format_arg(n)         __attribute__((format_arg(n)))
 
 /* gcc version specific checks */
 
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 478914ad280b..2c8793f72fb0 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -631,4 +631,8 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 #define __format_template(x)
 #endif
 
+#ifndef __format_arg
+#define __format_arg(n)
+#endif
+
 #endif /* __LINUX_COMPILER_H */
-- 
2.11.0

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