[PATCH v2 1/1] dax: Fix stack overflow when mounting fsdax pmem device

[Adrian Huang <adrianhuang0701@gmail.com>]

From: Adrian Huang <ahuang12@lenovo.com>

When mounting fsdax pmem device, commit 6180bb446ab6 ("dax: fix
detection of dax support for non-persistent memory block devices")
introduces the stack overflow [1][2]. Here is the call path for
mounting ext4 file system:
  ext4_fill_super
    bdev_dax_supported
      __bdev_dax_supported
        dax_supported
          generic_fsdax_supported
            __generic_fsdax_supported
              bdev_dax_supported

The call path leads to the infinite calling loop, so we cannot
call bdev_dax_supported() in __generic_fsdax_supported(). The sanity
checking of the variable 'dax_dev' is moved prior to the two
bdev_dax_pgoff() checks [3][4].

[1] https://lists.01.org/hyperkitty/list/linux-nvdimm@lists.01.org/thread/BULZHRILK7N2WS2JVISNF2QZNRQK6JU4/
[2] https://lists.01.org/hyperkitty/list/linux-nvdimm@lists.01.org/thread/OOZGFY3RNQGTGJJCH52YXCSYIDXMOPXO/
[3] https://lists.01.org/hyperkitty/list/linux-nvdimm@lists.01.org/message/SMQW2LY3QHPXOAW76RKNSCGG3QJFO7HT/
[4] https://lists.01.org/hyperkitty/list/linux-nvdimm@lists.01.org/message/7E2X6UGX5RQ2ISGYNAF66VLY5BKBFI4M/

Fixes: 6180bb446ab6 ("dax: fix detection of dax support for non-persistent memory block devices")
Cc: Coly Li <colyli@suse.de>
Cc: Jan Kara <jack@suse.cz>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: John Pittman <jpittman@redhat.com>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Cc: Alasdair Kergon <agk@redhat.com>
Cc: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Adrian Huang <ahuang12@lenovo.com>
---
v1->v2
* Remove the checking for the returned value '-EOPNOTSUPP' of
  dax_direct_access(). Jan has prepared a patch to address the
  issue in dm.
---
 drivers/dax/super.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index e5767c83ea23..11d0541e6f8f 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -85,6 +85,12 @@ bool __generic_fsdax_supported(struct dax_device *dax_dev,
 		return false;
 	}
 
+	if (!dax_dev) {
+		pr_debug("%s: error: dax unsupported by block device\n",
+				bdevname(bdev, buf));
+		return false;
+	}
+
 	err = bdev_dax_pgoff(bdev, start, PAGE_SIZE, &pgoff);
 	if (err) {
 		pr_info("%s: error: unaligned partition for dax\n",
@@ -100,12 +106,6 @@ bool __generic_fsdax_supported(struct dax_device *dax_dev,
 		return false;
 	}
 
-	if (!dax_dev || !bdev_dax_supported(bdev, blocksize)) {
-		pr_debug("%s: error: dax unsupported by block device\n",
-				bdevname(bdev, buf));
-		return false;
-	}
-
 	id = dax_read_lock();
 	len = dax_direct_access(dax_dev, pgoff, 1, &kaddr, &pfn);
 	len2 = dax_direct_access(dax_dev, pgoff_end, 1, &end_kaddr, &end_pfn);
-- 
2.17.1
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