[PATCH 6/7] omfs: add checksumming routines

[Bob Copeland <me@bobcopeland.com>]

OMFS checksums the metadata of all filesystem objects.  This change adds
the necessary functions to do so.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
---
 fs/omfs/checksum.c |   48 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 48 insertions(+), 0 deletions(-)
 create mode 100644 fs/omfs/checksum.c

diff --git a/fs/omfs/checksum.c b/fs/omfs/checksum.c
new file mode 100644
index 0000000..d6f1023
--- /dev/null
+++ b/fs/omfs/checksum.c
@@ -0,0 +1,48 @@
+#include <linux/fs.h>
+#include <linux/buffer_head.h>
+#include "omfs.h"
+
+#define POLY 0x1021
+
+/*
+ * crc-ccitt with MSB first (i.e., backwards), so we can't use the
+ * kernel table as-is.
+ */
+static u16 omfs_crc(u16 crc, unsigned char *buf, int count)
+{
+	int i, j;
+	for (i = 0; i < count; i++) {
+		crc ^= buf[i] << 8;
+		for (j = 0; j < 8; j++)
+			crc = (crc << 1) ^ ((crc & 0x8000) ? POLY : 0);
+	}
+	return crc;
+}
+
+/*
+ * Update the header checksums for a dirty inode based on its contents.
+ * Caller is expected to hold the buffer head underlying oi and mark it
+ * dirty.
+ */
+int omfs_update_checksums(struct omfs_inode *oi, struct super_block *sb,
+		ino_t ino)
+{
+	int ret = 0;
+	int xor, i, ofs = 0, count;
+	u16 crc = 0;
+	unsigned char *ptr = (unsigned char *) oi;
+
+	count = be32_to_cpu(oi->i_head.h_body_size);
+	ofs = sizeof(struct omfs_header);
+
+	crc = omfs_crc(crc, ptr + ofs, count);
+	oi->i_head.h_crc = cpu_to_be16(crc);
+
+	xor = ptr[0];
+	for (i = 1; i < OMFS_XOR_COUNT; i++)
+		xor ^= ptr[i];
+
+	oi->i_head.h_check_xor = xor;
+
+	return ret;
+}
-- 
1.5.4.2.182.gb3092