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    Subject[PATCH 5.16 002/200] btrfs: fix too long loop when defragging a 1 byte file
    Date
    From: Filipe Manana <fdmanana@suse.com>

    commit 6b34cd8e175bfbf4f3f01b6d19eae18245e1a8cc upstream.

    When attempting to defrag a file with a single byte, we can end up in a
    too long loop, which is nearly infinite because at btrfs_defrag_file()
    we end up with the variable last_byte assigned with a value of
    18446744073709551615 (which is (u64)-1). The problem comes from the fact
    we end up doing:

    last_byte = round_up(last_byte, fs_info->sectorsize) - 1;

    So if last_byte was assigned 0, which is i_size - 1, we underflow and
    end up with the value 18446744073709551615.

    This is trivial to reproduce and the following script triggers it:

    $ cat test.sh
    #!/bin/bash

    DEV=/dev/sdj
    MNT=/mnt/sdj

    mkfs.btrfs -f $DEV
    mount $DEV $MNT

    echo -n "X" > $MNT/foobar

    btrfs filesystem defragment $MNT/foobar

    umount $MNT

    So fix this by not decrementing last_byte by 1 before doing the sector
    size round up. Also, to make it easier to follow, make the round up right
    after computing last_byte.

    Reported-by: Anthony Ruhier <aruhier@mailbox.org>
    Fixes: 7b508037d4cac3 ("btrfs: defrag: use defrag_one_cluster() to implement btrfs_defrag_file()")
    Link: https://lore.kernel.org/linux-btrfs/0a269612-e43f-da22-c5bc-b34b1b56ebe8@mailbox.org/
    CC: stable@vger.kernel.org # 5.16
    Reviewed-by: Qu Wenruo <wqu@suse.com>
    Signed-off-by: Filipe Manana <fdmanana@suse.com>
    Signed-off-by: David Sterba <dsterba@suse.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    fs/btrfs/ioctl.c | 12 ++++++------
    1 file changed, 6 insertions(+), 6 deletions(-)

    --- a/fs/btrfs/ioctl.c
    +++ b/fs/btrfs/ioctl.c
    @@ -1492,12 +1492,16 @@ int btrfs_defrag_file(struct inode *inod

    if (range->start + range->len > range->start) {
    /* Got a specific range */
    - last_byte = min(isize, range->start + range->len) - 1;
    + last_byte = min(isize, range->start + range->len);
    } else {
    /* Defrag until file end */
    - last_byte = isize - 1;
    + last_byte = isize;
    }

    + /* Align the range */
    + cur = round_down(range->start, fs_info->sectorsize);
    + last_byte = round_up(last_byte, fs_info->sectorsize) - 1;
    +
    /*
    * If we were not given a ra, allocate a readahead context. As
    * readahead is just an optimization, defrag will work without it so
    @@ -1510,10 +1514,6 @@ int btrfs_defrag_file(struct inode *inod
    file_ra_state_init(ra, inode->i_mapping);
    }

    - /* Align the range */
    - cur = round_down(range->start, fs_info->sectorsize);
    - last_byte = round_up(last_byte, fs_info->sectorsize) - 1;
    -
    while (cur < last_byte) {
    u64 cluster_end;


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