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    Subject[PATCH v8 1/9] dax: fix NULL pointer dereference in __dax_dbg()
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    In __dax_pmd_fault() we currently assume that get_block() will always set
    bh.b_bdev and we unconditionally dereference it in __dax_dbg(). This
    assumption isn't always true - when called for reads of holes
    ext4_dax_mmap_get_block() returns a buffer head where bh->b_bdev is never
    set. I hit this BUG while testing the DAX PMD fault path.

    Instead, initialize bh.b_bdev before passing bh into get_block(). It is
    possible that the filesystem's get_block() will update bh.b_bdev, and this
    is fine - we just want to initialize bh.b_bdev to something reasonable so
    that the calls to __dax_dbg() work and print something useful.

    Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
    Cc: Dan Williams <dan.j.williams@intel.com>
    ---
    fs/dax.c | 1 +
    1 file changed, 1 insertion(+)

    diff --git a/fs/dax.c b/fs/dax.c
    index 7af8797..513bba5 100644
    --- a/fs/dax.c
    +++ b/fs/dax.c
    @@ -624,6 +624,7 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
    }

    memset(&bh, 0, sizeof(bh));
    + bh.b_bdev = inode->i_sb->s_bdev;
    block = (sector_t)pgoff << (PAGE_SHIFT - blkbits);

    bh.b_size = PMD_SIZE;
    --
    2.5.0
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