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    Subject[PATCH 5.10 69/71] ext4: add check to prevent attempting to resize an fs with sparse_super2
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    From: Josh Triplett <josh@joshtriplett.org>

    commit b1489186cc8391e0c1e342f9fbc3eedf6b944c61 upstream.

    The in-kernel ext4 resize code doesn't support filesystem with the
    sparse_super2 feature. It fails with errors like this and doesn't finish
    the resize:
    EXT4-fs (loop0): resizing filesystem from 16640 to 7864320 blocks
    EXT4-fs warning (device loop0): verify_reserved_gdb:760: reserved GDT 2 missing grp 1 (32770)
    EXT4-fs warning (device loop0): ext4_resize_fs:2111: error (-22) occurred during file system resize
    EXT4-fs (loop0): resized filesystem to 2097152

    To reproduce:
    mkfs.ext4 -b 4096 -I 256 -J size=32 -E resize=$((256*1024*1024)) -O sparse_super2 ext4.img 65M
    truncate -s 30G ext4.img
    mount ext4.img /mnt
    python3 -c 'import fcntl, os, struct ; fd = os.open("/mnt", os.O_RDONLY | os.O_DIRECTORY) ; fcntl.ioctl(fd, 0x40086610, struct.pack("Q", 30 * 1024 * 1024 * 1024 // 4096), False) ; os.close(fd)'
    dmesg | tail
    e2fsck ext4.img

    The userspace resize2fs tool has a check for this case: it checks if the
    filesystem has sparse_super2 set and if the kernel provides
    /sys/fs/ext4/features/sparse_super2. However, the former check requires
    manually reading and parsing the filesystem superblock.

    Detect this case in ext4_resize_begin and error out early with a clear
    error message.

    Signed-off-by: Josh Triplett <josh@joshtriplett.org>
    Link: https://lore.kernel.org/r/74b8ae78405270211943cd7393e65586c5faeed1.1623093259.git.josh@joshtriplett.org
    Signed-off-by: Theodore Ts'o <tytso@mit.edu>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    fs/ext4/resize.c | 5 +++++
    1 file changed, 5 insertions(+)

    --- a/fs/ext4/resize.c
    +++ b/fs/ext4/resize.c
    @@ -74,6 +74,11 @@ int ext4_resize_begin(struct super_block
    return -EPERM;
    }

    + if (ext4_has_feature_sparse_super2(sb)) {
    + ext4_msg(sb, KERN_ERR, "Online resizing not supported with sparse_super2");
    + return -EOPNOTSUPP;
    + }
    +
    if (test_and_set_bit_lock(EXT4_FLAGS_RESIZING,
    &EXT4_SB(sb)->s_ext4_flags))
    ret = -EBUSY;

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