[PATCH v4] of/fdt: export fdt blob as /sys/firmware/fdt

[Grant Likely]

On Wed, 12 Nov 2014 13:28:28 +0100
, Ard Biesheuvel <ard.biesheuvel at linaro.org>
 wrote:
> Create a new /sys entry '/sys/firmware/fdt' to export the FDT blob
> that was passed to the kernel by the bootloader. This allows userland
> applications such as kexec to access the raw binary.
> 
> The fact that this node does not reside under /sys/firmware/device-tree
> is deliberate: FDT is also used on arm64 UEFI/ACPI systems to
> communicate just the UEFI and ACPI entry points, but the FDT is never
> unflattened and used to configure the system.
> 
> A CRC32 checksum is calculated over the entire FDT blob, and verified
> at late_initcall time. The sysfs entry is instantiated only if the
> checksum is valid, i.e., if the FDT blob has not been modified in the
> mean time. Otherwise, a warning is printed.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>

Actually, I made a couple of changes when I merged it. I removed the
older debugfs interface since it overlaps, and I added tests for
initial_boot_params to make sure it doesn't try to run on an invalid
FDT:

diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
index be16ce2ffd69..fbe8f8d418f7 100644
--- a/drivers/of/Kconfig
+++ b/drivers/of/Kconfig
@@ -23,6 +23,7 @@ config OF_FLATTREE
 	bool
 	select DTC
 	select LIBFDT
+	select CRC32
 
 config OF_EARLY_FLATTREE
 	bool
diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
index 83a8e1154602..c7997e87b4b5 100644
--- a/drivers/of/fdt.c
+++ b/drivers/of/fdt.c
@@ -9,6 +9,7 @@
  * version 2 as published by the Free Software Foundation.
  */
 
+#include <linux/crc32.h>
 #include <linux/kernel.h>
 #include <linux/initrd.h>
 #include <linux/memblock.h>
@@ -22,10 +23,20 @@
 #include <linux/libfdt.h>
 #include <linux/debugfs.h>
 #include <linux/serial_core.h>
+#include <linux/sysfs.h>
 
 #include <asm/setup.h>  /* for COMMAND_LINE_SIZE */
 #include <asm/page.h>
 
+static u32 of_fdt_crc32;
+
+static u32 of_fdt_get_crc32(void *fdt)
+{
+	static u32 const OF_FDT_CRC32_SEED = 0x04c11db7;
+
+	return crc32_be(OF_FDT_CRC32_SEED, fdt, fdt_totalsize(fdt));
+}
+
 /*
  * of_fdt_limit_memory - limit the number of regions in the /memory node
  * @limit: maximum entries
@@ -1003,6 +1014,7 @@ bool __init early_init_dt_verify(void *params)
 
 	/* Setup flat device-tree pointer */
 	initial_boot_params = params;
+	of_fdt_crc32 = of_fdt_get_crc32(initial_boot_params);
 	return true;
 }
 
@@ -1080,27 +1092,31 @@ void __init unflatten_and_copy_device_tree(void)
 	unflatten_device_tree();
 }
 
-#if defined(CONFIG_DEBUG_FS) && defined(DEBUG)
-static struct debugfs_blob_wrapper flat_dt_blob;
-
-static int __init of_flat_dt_debugfs_export_fdt(void)
+#ifdef CONFIG_SYSFS
+static ssize_t of_fdt_raw_read(struct file *filp, struct kobject *kobj,
+			       struct bin_attribute *bin_attr,
+			       char *buf, loff_t off, size_t count)
 {
-	struct dentry *d = debugfs_create_dir("device-tree", NULL);
-
-	if (!d)
-		return -ENOENT;
+	memcpy(buf, initial_boot_params + off, count);
+	return count;
+}
 
-	flat_dt_blob.data = initial_boot_params;
-	flat_dt_blob.size = fdt_totalsize(initial_boot_params);
+static int __init of_fdt_raw_init(void)
+{
+	static struct bin_attribute of_fdt_raw_attr =
+		__BIN_ATTR(fdt, S_IRUSR, of_fdt_raw_read, NULL, 0);
 
-	d = debugfs_create_blob("flat-device-tree", S_IFREG | S_IRUSR,
-				d, &flat_dt_blob);
-	if (!d)
-		return -ENOENT;
+	if (!initial_boot_params || fdt_check_header(initial_boot_params))
+		return 0;
 
-	return 0;
+	if (of_fdt_crc32 != of_fdt_get_crc32(initial_boot_params)) {
+		pr_warn("fdt: not creating '/sys/firmware/fdt': CRC check failed");
+		return 0;
+	}
+	of_fdt_raw_attr.size = fdt_totalsize(initial_boot_params);
+	return sysfs_create_bin_file(firmware_kobj, &of_fdt_raw_attr);
 }
-module_init(of_flat_dt_debugfs_export_fdt);
+late_initcall(of_fdt_raw_init);
 #endif
 
 #endif /* CONFIG_OF_EARLY_FLATTREE */


g.