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Commit a739ff3f authored by Sami Tolvanen's avatar Sami Tolvanen Committed by Mike Snitzer
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dm verity: add support for forward error correction 



Add support for correcting corrupted blocks using Reed-Solomon.

This code uses RS(255, N) interleaved across data and hash
blocks. Each error-correcting block covers N bytes evenly
distributed across the combined total data, so that each byte is a
maximum distance away from the others. This makes it possible to
recover from several consecutive corrupted blocks with relatively
small space overhead.

In addition, using verity hashes to locate erasures nearly doubles
the effectiveness of error correction. Being able to detect
corrupted blocks also improves performance, because only corrupted
blocks need to corrected.

For a 2 GiB partition, RS(255, 253) (two parity bytes for each
253-byte block) can correct up to 16 MiB of consecutive corrupted
blocks if erasures can be located, and 8 MiB if they cannot, with
16 MiB space overhead.

Signed-off-by: default avatarSami Tolvanen <samitolvanen@google.com>
Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
parent bb4d73ac

Documentation/device-mapper/verity.txt

+33 −2
Original line number Diff line number Diff line
@@ -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

---------

drivers/md/Kconfig

+12 −0
Original line number Diff line number Diff line
@@ -467,6 +467,18 @@ config DM_VERITY 


	  If unsure, say N.



config DM_VERITY_FEC

	bool "Verity forward error correction support"

	depends on DM_VERITY

	select REED_SOLOMON

	select REED_SOLOMON_DEC8

	---help---

	  Add forward error correction support to dm-verity. This option

	  makes it possible to use pre-generated error correction data to

	  recover from corrupted blocks.



	  If unsure, say N.



config DM_SWITCH

	tristate "Switch target support (EXPERIMENTAL)"

	depends on BLK_DEV_DM

drivers/md/Makefile

+4 −0
Original line number Diff line number Diff line
@@ -64,3 +64,7 @@ obj-$(CONFIG_DM_LOG_WRITES) += dm-log-writes.o 
ifeq ($(CONFIG_DM_UEVENT),y)

dm-mod-objs			+= dm-uevent.o

endif



ifeq ($(CONFIG_DM_VERITY_FEC),y)

dm-verity-objs			+= dm-verity-fec.o

endif

drivers/md/dm-verity-fec.c

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drivers/md/dm-verity-fec.h

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