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-rw-r--r--Documentation/device-mapper/verity.txt35
1 files changed, 33 insertions, 2 deletions
diff --git a/Documentation/device-mapper/verity.txt b/Documentation/device-mapper/verity.txt
index e15bc1a0fb98..d602c801ff59 100644
--- a/Documentation/device-mapper/verity.txt
+++ b/Documentation/device-mapper/verity.txt
@@ -18,11 +18,11 @@ Construction Parameters
0 is the original format used in the Chromium OS.
The salt is appended when hashing, digests are stored continuously and
- the rest of the block is padded with zeros.
+ the rest of the block is padded with zeroes.
1 is the current format that should be used for new devices.
The salt is prepended when hashing and each digest is
- padded with zeros to the power of two.
+ padded with zeroes to the power of two.
<dev>
This is the device containing data, the integrity of which needs to be
@@ -79,6 +79,32 @@ restart_on_corruption
not compatible with ignore_corruption and requires user space support to
avoid restart loops.
+use_fec_from_device <fec_dev>
+ Use forward error correction (FEC) to recover from corruption if hash
+ verification fails. Use encoding data from the specified device. This
+ may be the same device where data and hash blocks reside, in which case
+ fec_start must be outside data and hash areas.
+
+ If the encoding data covers additional metadata, it must be accessible
+ on the hash device after the hash blocks.
+
+ Note: block sizes for data and hash devices must match. Also, if the
+ verity <dev> is encrypted the <fec_dev> should be too.
+
+fec_roots <num>
+ Number of generator roots. This equals to the number of parity bytes in
+ the encoding data. For example, in RS(M, N) encoding, the number of roots
+ is M-N.
+
+fec_blocks <num>
+ The number of encoding data blocks on the FEC device. The block size for
+ the FEC device is <data_block_size>.
+
+fec_start <offset>
+ This is the offset, in <data_block_size> blocks, from the start of the
+ FEC device to the beginning of the encoding data.
+
+
Theory of operation
===================
@@ -98,6 +124,11 @@ per-block basis. This allows for a lightweight hash computation on first read
into the page cache. Block hashes are stored linearly, aligned to the nearest
block size.
+If forward error correction (FEC) support is enabled any recovery of
+corrupted data will be verified using the cryptographic hash of the
+corresponding data. This is why combining error correction with
+integrity checking is essential.
+
Hash Tree
---------