CRC loop device: when you do not trust your disk subsystem

[Pavel Machek]

Hi!

Here it is. With crc loop device, you can no longer get silent data
corruption due to disk subsystem...
								Pavel
--- clean/drivers/block/loop.c	Mon Jan  8 22:49:50 2001
+++ linux/drivers/block/loop.c	Mon Jan  8 22:44:09 2001
@@ -61,6 +61,7 @@
 #include <linux/devfs_fs_kernel.h>
 
 #include <asm/uaccess.h>
+#include <asm/checksum.h>
 
 #include <linux/loop.h>		
 
@@ -118,6 +119,101 @@
 	return 0;
 }
 
+#define ID printk(KERN_ERR "crc: info about (%s, %d, %d) ", kdevname(lo->lo_device), real_block, blksize);
+
+
+static int transfer_crc(struct loop_device *lo, int cmd, char *raw_buf,
+		  char *loop_buf, int size, int real_block)
+{
+	struct buffer_head *bh;
+	int blksize = 1024, nsect;	/* Size of block on auxilary media */
+	int cksum;
+	u32 *data;
+	nsect = blksize / 4;
+
+	if (!lo->second_device) {
+		ID; printk( "reading from not-yet-setup crc device can result in armagedon. Dont try again.\n" );
+		return -1;
+	}
+	bh = getblk(lo->second_device, 1+real_block/nsect, blksize);
+	if (!bh) {
+		ID; printk( "getblk returned NULL.\n" );
+		return -1;
+	}
+	if (!buffer_uptodate(bh)) {
+		ll_rw_block(READ, 1, &bh);
+		wait_on_buffer(bh);
+		if (!buffer_uptodate(bh)) {
+			ID; printk(  "could not read block with CRC\n" );
+			goto error;
+		}
+	}
+
+	data = (u32 *) bh->b_data;
+	if (cmd == READ)
+		cksum = csum_partial_copy_nocheck(raw_buf, loop_buf, size, 0);
+	else
+		cksum = csum_partial_copy_nocheck(loop_buf, raw_buf, size, 0);
+
+	if (cmd == READ) {
+		if (le32_to_cpu(data[real_block%nsect]) != cksum) {
+			if (lo->lo_encrypt_key_size == 0) {	/* Normal mode */
+				ID; printk( "wrong checksum reading, is %x, should be %x\n", cksum, 0x1234 );
+				goto error;
+			} else { 
+				ID; printk( "wrong checksum repairing, setting to %x\n", cksum );
+				goto repair;
+			}
+		}
+	} else {
+	repair:
+		data[real_block%nsect] = cpu_to_le32(cksum);
+		mark_buffer_uptodate(bh, 1);
+		mark_buffer_dirty(bh);
+	}
+
+	brelse(bh);
+	return 0;
+error:
+	brelse(bh);
+	return -1;
+
+}
+
+static int ioctl_crc(struct loop_device *lo, int cmd, unsigned long arg)
+{
+	struct file	*file;
+	struct inode	*inode;
+	int error;
+
+	printk( "Entering ioctl_crc\n" );
+	if (cmd != LOOP_CRC_SET_FD)
+		return -EINVAL;
+
+	error = -EBADF;
+	file = fget(arg);
+	if (!file)
+		return -EINVAL;
+
+	error = -EINVAL;
+	inode = file->f_dentry->d_inode;
+	if (!inode) {
+		printk(KERN_ERR "ioctl_crc: NULL inode?!?\n");
+		goto out;
+	}
+
+	if (S_ISBLK(inode->i_mode)) {
+		error = blkdev_open(inode, file);
+		lo->second_device = inode->i_rdev;
+		printk( "loop_crc: Registered device %x\n", lo->second_device );
+		return error;
+	} else {
+	out:
+		fput(file);
+		return -EINVAL;
+	}
+}
+
 static int none_status(struct loop_device *lo, struct loop_info *info)
 {
 	return 0; 
@@ -142,10 +238,19 @@
 	init: xor_status
 }; 	
 
+struct loop_func_table crc_funcs = { 
+	number: LO_CRYPT_CRC,
+	transfer: transfer_crc,
+	init: none_status,
+	ioctl: ioctl_crc
+}; 	
+
 /* xfer_funcs[0] is special - its release function is never called */ 
 struct loop_func_table *xfer_funcs[MAX_LO_CRYPT] = {
 	&none_funcs,
-	&xor_funcs  
+	&xor_funcs,
+	NULL, NULL, NULL, NULL, NULL,
+	&crc_funcs,
 };
 
 #define MAX_DISK_SIZE 1024*1024*1024
@@ -551,6 +656,7 @@
 	lo->transfer = NULL;
 	lo->ioctl = NULL;
 	lo->lo_device = 0;
+	lo->second_device = 0;
 	lo->lo_encrypt_type = 0;
 	lo->lo_offset = 0;
 	lo->lo_encrypt_key_size = 0;
--- clean/include/linux/loop.h	Mon Nov 23 06:29:54 1998
+++ linux/include/linux/loop.h	Sat Apr 15 13:22:57 2000
@@ -22,6 +22,7 @@
 	struct dentry	*lo_dentry;
 	int		lo_refcnt;
 	kdev_t		lo_device;
+	kdev_t		second_device;
 	int		lo_offset;
 	int		lo_encrypt_type;
 	int		lo_encrypt_key_size;
@@ -94,6 +95,7 @@
 #define LO_CRYPT_BLOW     4
 #define LO_CRYPT_CAST128  5
 #define LO_CRYPT_IDEA     6
+#define LO_CRYPT_CRC	  7
 #define LO_CRYPT_DUMMY    9
 #define LO_CRYPT_SKIPJACK 10
 #define MAX_LO_CRYPT	20
@@ -126,5 +128,6 @@
 #define LOOP_CLR_FD	0x4C01
 #define LOOP_SET_STATUS	0x4C02
 #define LOOP_GET_STATUS	0x4C03
+#define LOOP_CRC_SET_FD 0x4C04
 
 #endif