dax: never rely on bh.b_dev being set by get_block()

Previously in DAX we assumed that calls to get_block() would set
bh.b_bdev, and we would then use that value even in error cases for
debugging.  This caused a NULL pointer dereference in __dax_dbg() which
was fixed by a previous commit, but that commit only changed the one
place where we were hitting an error.

Instead, update dax.c so that we always initialize bh.b_bdev as best we
can based on the information that DAX has.  get_block() may or may not
update to a new value, but this at least lets us get something helpful
from bh.b_bdev for error messages and not have to worry about whether it
was set by get_block() or not.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Reported-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/fs/dax.c b/fs/dax.c
index d5f6aca5a4d7..206650f82762 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -246,6 +246,7 @@ ssize_t dax_do_io(struct kiocb *iocb, struct inode *inode,
         loff_t end = pos + iov_iter_count(iter);
 
         memset(&bh, 0, sizeof(bh));
+        bh.b_bdev = inode->i_sb->s_bdev;
 
         if ((flags & DIO_LOCKING) && iov_iter_rw(iter) == READ) {
                 struct address_space *mapping = inode->i_mapping;
@@ -607,6 +608,7 @@ int __dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
 
         memset(&bh, 0, sizeof(bh));
         block = (sector_t)vmf->pgoff << (PAGE_SHIFT - blkbits);
+        bh.b_bdev = inode->i_sb->s_bdev;
         bh.b_size = PAGE_SIZE;
 
  repeat:
@@ -1078,6 +1080,7 @@ int dax_zero_page_range(struct inode *inode, loff_t from, unsigned length,
         BUG_ON((offset + length) > PAGE_CACHE_SIZE);
 
         memset(&bh, 0, sizeof(bh));
+        bh.b_bdev = inode->i_sb->s_bdev;
         bh.b_size = PAGE_CACHE_SIZE;
         err = get_block(inode, index, &bh, 0);
         if (err < 0)