x86, kaslr: Return location from decompress_kernel

This allows decompress_kernel to return a new location for the kernel to
be relocated to. Additionally, enforces CONFIG_PHYSICAL_START as the
minimum relocation position when building with CONFIG_RELOCATABLE.

With CONFIG_RANDOMIZE_BASE set, the choose_kernel_location routine
will select a new location to decompress the kernel, though here it is
presently a no-op. The kernel command line option "nokaslr" is introduced
to bypass these routines.

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: http://lkml.kernel.org/r/1381450698-28710-3-git-send-email-keescook@chromium.org
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/boot/compressed/misc.h

@@ -39,23 +39,38 @@ static inline void debug_putstr(const char *s)
 
 #endif
 
-#ifdef CONFIG_EARLY_PRINTK
-
+#if CONFIG_EARLY_PRINTK || CONFIG_RANDOMIZE_BASE
 /* cmdline.c */
 int cmdline_find_option(const char *option, char *buffer, int bufsize);
 int cmdline_find_option_bool(const char *option);
+#endif
 
+
+#if CONFIG_RANDOMIZE_BASE
+/* aslr.c */
+unsigned char *choose_kernel_location(unsigned char *input,
+				      unsigned long input_size,
+				      unsigned char *output,
+				      unsigned long output_size);
+#else
+static inline
+unsigned char *choose_kernel_location(unsigned char *input,
+				      unsigned long input_size,
+				      unsigned char *output,
+				      unsigned long output_size)
+{
+	return output;
+}
+#endif
+
+#ifdef CONFIG_EARLY_PRINTK
 /* early_serial_console.c */
 extern int early_serial_base;
 void console_init(void);
-
 #else
-
-/* early_serial_console.c */
 static const int early_serial_base;
 static inline void console_init(void)
 { }
-
 #endif
 
 #endif