[gparted] Add clearing of ZFS labels

[Curtis Gedak <gedakc src gnome org>]

commit 32b5106aa12c51d63273ab8e43f27ff6ef3b4826
Author: Mike Fleetwood <mike fleetwood googlemail com>
Date:   Sat Oct 31 12:55:07 2015 +0000

    Add clearing of ZFS labels
    
    ZFS labels were not cleared by GParted when clearing old file system
    signatures.
    
        # wget 
https://git.kernel.org/cgit/utils/util-linux/util-linux.git/plain/tests/ts/blkid/images-fs/zfs.img.bz2
        # bzip2 -dc zfs.img.bz2 > /dev/sdb1
        [In GParted format to cleared /dev/sdb1]
        # blkid /dev/sdb1
        /dev/sdb1: LABEL="tank" UUID="1782036546311300980" UUID_SUB="13179280127379850514" TYPE="zfs_member"
    
    Update to also zero all 4 ZFS labels.
    
    NOTE:
    GParted now writes a little over 1 MiB when clearing old file system
    signatures.  As this is performed in the main thread the UI is not able
    to respond during this action.  Testing this on a range of USB flash
    keys and hard drives found the slowest normal time to write this was
    0.25 seconds, with an occasional outlier up to 2.8 seconds from a USB
    flash key.  This is considered acceptable.

 src/GParted_Core.cc |   46 ++++++++++++++++++++++++++++++++++------------
 1 files changed, 34 insertions(+), 12 deletions(-)
---
diff --git a/src/GParted_Core.cc b/src/GParted_Core.cc
index 87f9578..eee3194 100644
--- a/src/GParted_Core.cc
+++ b/src/GParted_Core.cc
@@ -3623,14 +3623,24 @@ bool GParted_Core::erase_filesystem_signatures( const Partition & partition, Ope
        //  don't try to surgically overwrite just the few bytes of each signature as this
        //  code overwrites whole sectors and it embeds more knowledge that is necessary.
        //
-       //  First byte range from offset 0 of length 68 KiB covers the primary super block
-       //  of all currently supported file systems and is also likely to include future
-       //  file system super blocks too.  Only a few file systems have super blocks and
-       //  signatures located elsewhere.
+       //  First byte range from offset 0 of length 512 KiB covers the primary super
+       //  block of all currently supported file systems and is also likely to include
+       //  future file system super blocks too.  Only a few file systems have super
+       //  blocks and signatures located elsewhere.
        //
        //  Btrfs super blocks are located at: 64 KiB, 64 MiB, 256 GiB and 1 PiB.
+       //  (The super block at 64 KiB will be erased by the zeroing from offset 0.  Other
+       //  super block mirror copies need to be explicitly cleared).
        //  https://btrfs.wiki.kernel.org/index.php/On-disk_Format#Superblock
        //
+       //  ZFS labels are 256 KiB in size and 4 copies are stored on every member device,
+       //  2 at the beginning and 2 at the end of the device, aligned to 256 KiB.  (The
+       //  front two labels, L0 and L1, will be erased by the zeroing from offset 0.  The
+       //  back two labels, L2 and L3, need to be explicitly cleared).
+       //  Reference:
+       //      ZFS On-Disk Specification
+       //      http://maczfs.googlecode.com/files/ZFSOnDiskFormat.pdf
+       //
        //  Linux Software RAID metadata 0.90 stores it's super block at 64 KiB before the
        //  end of the device, aligned to 64 KiB boundary.  Length 4 KiB.
        //  Ref: mdadm/super0.c load_super0()
@@ -3646,19 +3656,31 @@ bool GParted_Core::erase_filesystem_signatures( const Partition & partition, Ope
        //  Nilfs2 secondary super block is located at the last whole 4 KiB block.
        //  Ref: nilfs-utils-2.1.4/include/nilfs2_fs.h
        //  #define NILFS_SB2_OFFSET_BYTES(devsize) ((((devsize) >> 12) - 1) << 12)
+       //
+       // NOTE:
+       // Most of the time partitions are aligned to whole MiBs so the writing of zeros
+       // at offsets -64 KiB and -8 KiB will be overwriting zeros already just written
+       // at offset -512 KiB, length 512 KiB.  This will double write 12 KiB of zeros.
+       // However partitions don't have to be MiB aligned and real disk drives generally
+       // aren't an exact multiple of 1024^2 bytes in size either.  So zeroing at offsets
+       // -64 KiB and -8 KiB with their smaller rounding / alignment requirements will
+       // write outside the -512 KiB zeroed region and needs to be kept.  Because of the
+       // small amount of double writing it is not worth the effort to suppress it when
+       // not needed.
        struct {
                Byte_Value offset;    //Negative offsets work backwards from the end of the partition
                Byte_Value rounding;  //Minimum desired rounding for offset
                Byte_Value length;
        } ranges[] = {
-               //offset          , rounding       , length
-               {   0LL           ,  1LL           , 68LL * KIBIBYTE },  // All primary super blocks
-               {  64LL * MEBIBYTE,  1LL           ,  4LL * KIBIBYTE },  // Btrfs super block mirror copy
-               { 256LL * GIBIBYTE,  1LL           ,  4LL * KIBIBYTE },  // Btrfs super block mirror copy
-               {   1LL * PEBIBYTE,  1LL           ,  4LL * KIBIBYTE },  // Btrfs super block mirror copy
-               { -64LL * KIBIBYTE, 64LL * KIBIBYTE,  4LL * KIBIBYTE },  // SWRaid metadata 0.90 super block
-               {  -8LL * KIBIBYTE,  4LL * KIBIBYTE,  8LL * KIBIBYTE }   // @-8K SWRaid metadata 1.0 super 
block
-                                                                        // and @-4K Nilfs2 secondary super 
block
+               //offset           , rounding       , length
+               {    0LL           ,   1LL           , 512LL * KIBIBYTE },  // All primary super blocks
+               {   64LL * MEBIBYTE,   1LL           ,   4LL * KIBIBYTE },  // Btrfs super block mirror copy
+               {  256LL * GIBIBYTE,   1LL           ,   4LL * KIBIBYTE },  // Btrfs super block mirror copy
+               {    1LL * PEBIBYTE,   1LL           ,   4LL * KIBIBYTE },  // Btrfs super block mirror copy
+               { -512LL * KIBIBYTE, 256LL * KIBIBYTE, 512LL * KIBIBYTE },  // ZFS labels L2 and L3
+               {  -64LL * KIBIBYTE,  64LL * KIBIBYTE,   4LL * KIBIBYTE },  // SWRaid metadata 0.90 super 
block
+               {   -8LL * KIBIBYTE,   4LL * KIBIBYTE,   8LL * KIBIBYTE }   // @-8K SWRaid metadata 1.0 super 
block
+                                                                           // and @-4K Nilfs2 secondary 
super block
        } ;
        for ( unsigned int i = 0 ; overall_success && i < sizeof( ranges ) / sizeof( ranges[0] ) ; i ++ )
        {