[PATCH v7 1/9] dax: fix NULL pointer dereference in __dax_dbg()

[Ross Zwisler <ross.zwisler@linux.intel.com>]

__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