of/flattree: Make unflatten_device_tree() safe to call from any arch

This patch makes unflatten_device_tree() safe to call from any arch
setup code with the following changes:
- Make sure initial_boot_params actually points to a device tree blob
  before unflattening
- Make sure the initial_boot_params->magic field is correct
- If CONFIG_OF_FLATTREE is not set, then make unflatten_device_tree()
  an empty static inline function.

This patch also adds some additional debug output to the top of
unflatten_device_tree().
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/of/fdt.c

@@ -556,6 +556,21 @@ void __init unflatten_device_tree(void)
 
 	pr_debug(" -> unflatten_device_tree()\n");
 
+	if (!initial_boot_params) {
+		pr_debug("No device tree pointer\n");
+		return;
+	}
+
+	pr_debug("Unflattening device tree:\n");
+	pr_debug("magic: %08x\n", be32_to_cpu(initial_boot_params->magic));
+	pr_debug("size: %08x\n", be32_to_cpu(initial_boot_params->totalsize));
+	pr_debug("version: %08x\n", be32_to_cpu(initial_boot_params->version));
+
+	if (be32_to_cpu(initial_boot_params->magic) != OF_DT_HEADER) {
+		pr_err("Invalid device tree blob header\n");
+		return;
+	}
+
 	/* First pass, scan for size */
 	start = ((unsigned long)initial_boot_params) +
 		be32_to_cpu(initial_boot_params->off_dt_struct);

include/linux/of_fdt.h

@@ -99,6 +99,8 @@ extern int early_init_dt_scan_root(unsigned long node, const char *uname,
 /* Other Prototypes */
 extern void unflatten_device_tree(void);
 extern void early_init_devtree(void *);
+#else /* CONFIG_OF_FLATTREE */
+static inline void unflatten_device_tree(void) {}
 #endif /* CONFIG_OF_FLATTREE */
 
 #endif /* __ASSEMBLY__ */