Hi,

[auto build test WARNING on char-misc/char-misc-testing]
[also build test WARNING on v4.7-rc2 next-20160607]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Kees-Cook/lkdtm-add-usercopy-and-rodata-fix-atomic/20160608-060444
config: x86_64-randconfig-s2-06080621 (attached as .config)
compiler: gcc-6 (Debian 6.1.1-1) 6.1.1 20160430
reproduce:
        # save the attached .config to linux build tree
        make ARCH=x86_64 

All warnings (new ones prefixed by >>):

   drivers/misc/lkdtm_core.c: In function 'do_usercopy_stack_callee':
>> drivers/misc/lkdtm_core.c:420:9: warning: function may return address of local variable [-Wreturn-local-addr]
     return ptr;
            ^~~
   drivers/misc/lkdtm_core.c:405:16: note: declared here
     unsigned char buf[32];
                   ^~~

vim +420 drivers/misc/lkdtm_core.c

   404	{
   405		unsigned char buf[32];
   406		unsigned char *ptr = NULL;
   407		int i;
   408	
   409		/* Exercise stack to avoid everything living in registers. */
   410		for (i = 0; i < sizeof(buf); i++) {
   411			/*
   412			 * Hack to trick gcc into letting us return a reference
   413			 * to a local stack frame.
   414			 */
   415			if (i == 0)
   416				ptr = buf;
   417			buf[i] = value & 0xff;
   418		}
   419	
 > 420		return ptr;
   421	}
   422	
   423	static noinline void do_usercopy_stack(bool to_user)
   424	{
   425		unsigned long user_addr;
   426		unsigned char good_stack[32];
   427		unsigned char *bad_stack;
   428		int i;

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