block: don't mark buffers beyond end of disk as mapped

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

Squashfs manages to read in the end block(s) of the disk during the
mount operation.  Then, when dd reads the block device, it 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.

Cheers,
Jeff

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Acked-by: Nick Piggin <npiggin@kernel.dk>

--

Changes from v1->v2: re-used max_block, as suggested by Nick Piggin.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

3 files changed, 7 insertions, 4 deletions

diff --git a/fs/block_dev.c b/fs/block_dev.c
index e08f6a20a5bb..ba11c30f302d 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -70,7 +70,7 @@ static void bdev_inode_switch_bdi(struct inode *inode,
         spin_unlock(&dst->wb.list_lock);
 }
 
-static sector_t max_block(struct block_device *bdev)
+sector_t blkdev_max_block(struct block_device *bdev)
 {
         sector_t retval = ~((sector_t)0);
         loff_t sz = i_size_read(bdev->bd_inode);
@@ -163,7 +163,7 @@ static int
 blkdev_get_block(struct inode *inode, sector_t iblock,
                 struct buffer_head *bh, int create)
 {
-        if (iblock >= max_block(I_BDEV(inode))) {
+        if (iblock >= blkdev_max_block(I_BDEV(inode))) {
                 if (create)
                         return -EIO;
 
@@ -185,7 +185,7 @@ static int
 blkdev_get_blocks(struct inode *inode, sector_t iblock,
                 struct buffer_head *bh, int create)
 {
-        sector_t end_block = max_block(I_BDEV(inode));
+        sector_t end_block = blkdev_max_block(I_BDEV(inode));
         unsigned long max_blocks = bh->b_size >> inode->i_blkbits;
 
         if ((iblock + max_blocks) > end_block) {
diff --git a/fs/buffer.c b/fs/buffer.c
index 351e18ea2e53..ad5938ca357c 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -921,6 +921,7 @@ 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);
+        sector_t end_block = blkdev_max_block(I_BDEV(bdev->bd_inode));
 
         do {
                 if (!buffer_mapped(bh)) {
@@ -929,7 +930,8 @@ 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 < end_block)
+                                set_buffer_mapped(bh);
                 }
                 block++;
                 bh = bh->b_this_page;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 8de675523e46..25c40b9f848a 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2051,6 +2051,7 @@ extern void unregister_blkdev(unsigned int, const char *);
 extern struct block_device *bdget(dev_t);
 extern struct block_device *bdgrab(struct block_device *bdev);
 extern void bd_set_size(struct block_device *, loff_t size);
+extern sector_t blkdev_max_block(struct block_device *bdev);
 extern void bd_forget(struct inode *inode);
 extern void bdput(struct block_device *);
 extern void invalidate_bdev(struct block_device *);