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    Subject[PATCH v7 1/9] dax: fix NULL pointer dereference in __dax_dbg()
    Date
    __dax_dbg() currently assumes that bh->b_bdev is non-NULL, passing it into
    bdevname() where is is dereferenced. 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, verify that we have a valid bh->b_bdev, else just say "unknown"
    for the block device.

    Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
    Cc: Dan Williams <dan.j.williams@intel.com>
    ---
    fs/dax.c | 7 ++++++-
    1 file changed, 6 insertions(+), 1 deletion(-)

    diff --git a/fs/dax.c b/fs/dax.c
    index 7af8797..03cc4a3 100644
    --- a/fs/dax.c
    +++ b/fs/dax.c
    @@ -563,7 +563,12 @@ static void __dax_dbg(struct buffer_head *bh, unsigned long address,
    {
    if (bh) {
    char bname[BDEVNAME_SIZE];
    - bdevname(bh->b_bdev, bname);
    +
    + if (bh->b_bdev)
    + bdevname(bh->b_bdev, bname);
    + else
    + snprintf(bname, BDEVNAME_SIZE, "unknown");
    +
    pr_debug("%s: %s addr: %lx dev %s state %lx start %lld "
    "length %zd fallback: %s\n", fn, current->comm,
    address, bname, bh->b_state, (u64)bh->b_blocknr,
    --
    2.5.0


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