build: improve stack protector check

Testing a toolchain for proper -fstack-protector must go beyond ensuring
the compiler and linker accept the option.
If the test C program does nothing with the stack then guards aren't
inserted and/or are optimized away giving the false impression that it
works when in fact the libc might not support it.

Update the check to a program that uses the stack, hence making a link
fail if proper support isn't available, for example in non-ssp enabled
uclibc toolchains like this:

test.c:(.text.startup+0x64): undefined reference to `__stack_chk_fail'

Signed-off-by: Gustavo Zacarias <[email protected]>
Reviewed-by: Jeremy Allison <[email protected]>
Reviewed-by: Ralph Böhme <[email protected]>

Autobuild-User(master): Jeremy Allison <[email protected]>
Autobuild-Date(master): Mon Sep 21 23:29:13 CEST 2015 on sn-devel-104

buildtools/wafsamba/samba_autoconf.py

@@ -657,9 +657,23 @@ def SAMBA_CONFIG_H(conf, path=None):
     if not IN_LAUNCH_DIR(conf):
         return
 
-    if conf.CHECK_CFLAGS(['-fstack-protector']) and conf.CHECK_LDFLAGS(['-fstack-protector']):
-        conf.ADD_CFLAGS('-fstack-protector')
-        conf.ADD_LDFLAGS('-fstack-protector')
+    # we need to build real code that can't be optimized away to test
+    if conf.check(fragment='''
+        #include <stdio.h>
+
+        int main(void)
+        {
+            char t[100000];
+            while (fgets(t, sizeof(t), stdin));
+            return 0;
+        }
+        ''',
+        execute=0,
+        ccflags='-fstack-protector',
+        ldflags='-fstack-protector',
+        msg='Checking if toolchain accepts -fstack-protector'):
+            conf.ADD_CFLAGS('-fstack-protector')
+            conf.ADD_LDFLAGS('-fstack-protector')
 
     if Options.options.debug:
         conf.ADD_CFLAGS('-g', testflags=True)