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    Subject[patch|rfc] block: don't mark buffers beyond end of disk as mapped
    Date
    Hi,

    We have a bug report open where a squashfs image mounted on ppc64 would
    exhibit errors due to trying to read beyond the end of the disk. It can
    easily be reproduced by doing the following:

    [root@ibm-p750e-02-lp3 ~]# ls -l install.img
    -rw-r--r-- 1 root root 142032896 Apr 30 16:46 install.img
    [root@ibm-p750e-02-lp3 ~]# mount -o loop ./install.img /mnt/test
    [root@ibm-p750e-02-lp3 ~]# dd if=/dev/loop0 of=/dev/null
    dd: reading `/dev/loop0': Input/output error
    277376+0 records in
    277376+0 records out
    142016512 bytes (142 MB) copied, 0.9465 s, 150 MB/s

    In dmesg, you'll find the following:

    squashfs: version 4.0 (2009/01/31) Phillip Lougher
    [ 43.106012] attempt to access beyond end of device
    [ 43.106029] loop0: rw=0, want=277410, limit=277408
    [ 43.106039] Buffer I/O error on device loop0, logical block 138704
    [ 43.106053] attempt to access beyond end of device
    [ 43.106057] loop0: rw=0, want=277412, limit=277408
    [ 43.106061] Buffer I/O error on device loop0, logical block 138705
    [ 43.106066] attempt to access beyond end of device
    [ 43.106070] loop0: rw=0, want=277414, limit=277408
    [ 43.106073] Buffer I/O error on device loop0, logical block 138706
    [ 43.106078] attempt to access beyond end of device
    [ 43.106081] loop0: rw=0, want=277416, limit=277408
    [ 43.106085] Buffer I/O error on device loop0, logical block 138707
    [ 43.106089] attempt to access beyond end of device
    [ 43.106093] loop0: rw=0, want=277418, limit=277408
    [ 43.106096] Buffer I/O error on device loop0, logical block 138708
    [ 43.106101] attempt to access beyond end of device
    [ 43.106104] loop0: rw=0, want=277420, limit=277408
    [ 43.106108] Buffer I/O error on device loop0, logical block 138709
    [ 43.106112] attempt to access beyond end of device
    [ 43.106116] loop0: rw=0, want=277422, limit=277408
    [ 43.106120] Buffer I/O error on device loop0, logical block 138710
    [ 43.106124] attempt to access beyond end of device
    [ 43.106128] loop0: rw=0, want=277424, limit=277408
    [ 43.106131] Buffer I/O error on device loop0, logical block 138711
    [ 43.106135] attempt to access beyond end of device
    [ 43.106139] loop0: rw=0, want=277426, limit=277408
    [ 43.106143] Buffer I/O error on device loop0, logical block 138712
    [ 43.106147] attempt to access beyond end of device
    [ 43.106151] loop0: rw=0, want=277428, limit=277408
    [ 43.106154] Buffer I/O error on device loop0, logical block 138713
    [ 43.106158] attempt to access beyond end of device
    [ 43.106162] loop0: rw=0, want=277430, limit=277408
    [ 43.106166] attempt to access beyond end of device
    [ 43.106169] loop0: rw=0, want=277432, limit=277408
    ...
    [ 43.106307] attempt to access beyond end of device
    [ 43.106311] loop0: rw=0, want=277470, limit=2774

    What's strange is that the same messages aren't output when mounting an
    ext2, 3, or 4 image of the same size. Digging into this, I found that
    squashfs manages to read in the end block(s) of the disk during the
    mount operation. That leads to block_read_full_page being called with
    buffers that are beyond end of disk, but are marked as mapped. Thus, it
    would end up submitting read I/O against them, resulting in the errors
    mentioned above. I fixed the problem by modifying init_page_buffers to
    only set the buffer mapped if it fell inside of i_size. Since I haven't
    fully explained the reason that extN does not hit this problem, I'm
    proposing this as an RFC, though it does look like a good fix to me.

    Comments would be appreciated.

    Cheers,
    Jeff

    Signed-off-by: Jeff Moyer <jmoyer@redhat.com>

    diff --git a/fs/buffer.c b/fs/buffer.c
    index 351e18e..b3d6a97 100644
    --- a/fs/buffer.c
    +++ b/fs/buffer.c
    @@ -921,6 +921,8 @@ init_page_buffers(struct page *page, struct block_device *bdev,
    struct buffer_head *head = page_buffers(page);
    struct buffer_head *bh = head;
    int uptodate = PageUptodate(page);
    + struct inode *inode = bdev->bd_inode;
    + sector_t last_block = (i_size_read(inode) - 1) >> inode->i_blkbits;

    do {
    if (!buffer_mapped(bh)) {
    @@ -929,7 +931,8 @@ init_page_buffers(struct page *page, struct block_device *bdev,
    bh->b_blocknr = block;
    if (uptodate)
    set_buffer_uptodate(bh);
    - set_buffer_mapped(bh);
    + if (block <= last_block)
    + set_buffer_mapped(bh);
    }
    block++;
    bh = bh->b_this_page;

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