Added optionally crc32c calculation to dbgblock.py

This is useful for debugging checksum mismatches on disk.

And since dbgblock.py has some relatively flexible options for slicing
the disk, this can be used to find the checksum of any on-disk data
pretty easily.

scripts/dbgblock.py

@@ -73,11 +73,20 @@ def xxd(data, width=16):
                 b if b >= ' ' and b <= '~' else '.'
                 for b in map(chr, data[i:i+width])))
 
+def crc32c(data, crc=0):
+    crc ^= 0xffffffff
+    for b in data:
+        crc ^= b
+        for j in range(8):
+            crc = (crc >> 1) ^ ((crc & 1) * 0x82f63b78)
+    return 0xffffffff ^ crc
+
 def main(disk, block=None, *,
         block_size=None,
         block_count=None,
         off=None,
-        size=None):
+        size=None,
+        cksum=False):
     # is bd geometry specified?
     if isinstance(block_size, tuple):
         block_size, block_count_ = block_size
@@ -115,6 +124,7 @@ def main(disk, block=None, *,
                 else off[1] - off[0] if isinstance(off, tuple) and len(off) > 1
                 else block_size)
 
+        # print the header
         print('block %s, size %d' % (
             '0x%x.%x' % (block, off)
                 if off is not None
@@ -129,6 +139,11 @@ def main(disk, block=None, *,
         for o, line in enumerate(xxd(data)):
             print('%08x: %s' % ((off or 0) + 16*o, line))
 
+        # render the checksum if requested
+        if cksum:
+            cksum = crc32c(data)
+            print('%8s: %08x' % ('crc32c', cksum))
+
 if __name__ == "__main__":
     import argparse
     import sys
@@ -163,6 +178,10 @@ if __name__ == "__main__":
             int(x, 0) if x.strip() else None
             for x in x.split(',')),
         help="Show this many bytes, may be a range.")
+    parser.add_argument(
+        '-x', '--cksum', '--crc32c',
+        action='store_true',
+        help="Calculate and show the crc32c of the data.")
     sys.exit(main(**{k: v
         for k, v in vars(parser.parse_intermixed_args()).items()
         if v is not None}))