x86: Add a Kconfig option to turn the copy_from_user warnings into errors

For automated testing it is useful to have the option to turn
the warnings on copy_from_user() etc checks into errors:

 In function ‘copy_from_user’,
     inlined from ‘fd_copyin’ at drivers/block/floppy.c:3080,
     inlined from ‘fd_ioctl’ at drivers/block/floppy.c:3503:
   linux/arch/x86/include/asm/uaccess_32.h:213:
  error: call to ‘copy_from_user_overflow’ declared with attribute error:
  copy_from_user buffer size is not provably correct
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
LKML-Reference: <20091002075050.4e9f7641@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/Kconfig.debug

@@ -287,4 +287,18 @@ config OPTIMIZE_INLINING
 
 	  If unsure, say N.
 
+config DEBUG_STRICT_USER_COPY_CHECKS
+	bool "Strict copy size checks"
+	depends on DEBUG_KERNEL
+	---help---
+	  Enabling this option turns a certain set of sanity checks for user
+	  copy operations into compile time failures.
+
+	  The copy_from_user() etc checks are there to help test if there
+	  are sufficient security checks on the length argument of
+	  the copy operation, by having gcc prove that the argument is
+	  within bounds.
+
+	  If unsure, or if you run an older (pre 4.4) gcc, say N.
+
 endmenu

arch/x86/include/asm/uaccess_32.h

@@ -193,7 +193,9 @@ unsigned long __must_check _copy_from_user(void *to,
 
 
 extern void copy_from_user_overflow(void)
-#ifdef CONFIG_DEBUG_STACKOVERFLOW
+#ifdef CONFIG_DEBUG_STRICT_USER_COPY_CHECKS
+	__compiletime_error("copy_from_user() buffer size is not provably correct")
+#else
 	__compiletime_warning("copy_from_user() buffer size is not provably correct")
 #endif
 ;

include/linux/compiler-gcc4.h

@@ -41,4 +41,5 @@
 #define __compiletime_object_size(obj) __builtin_object_size(obj, 0)
 #if __GNUC_MINOR__ >= 4
 #define __compiletime_warning(message) __attribute__((warning(message)))
+#define __compiletime_error(message) __attribute__((error(message)))
 #endif

include/linux/compiler.h

@@ -273,6 +273,9 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
 #ifndef __compiletime_warning
 # define __compiletime_warning(message)
 #endif
+#ifndef __compiletime_error
+# define __compiletime_error(message)
+#endif
 
 /*
  * Prevent the compiler from merging or refetching accesses.  The compiler