[PATCH v2 1/8] dax: replace exported dax_dev_get() with non-allocating dax_dev_find()

[Miklos Szeredi <mszeredi@redhat.com>]

From: John Groves <John@Groves.net>

This fix is in response to a Sashiko review, and some subsequent
analysis.

dax_dev_get() uses iget5_locked() which creates a new inode if no
matching one exists. This is correct for the internal caller
(alloc_dax), but dangerous for external callers that look up devices
from user-supplied or metadata-supplied dev_t values:

1. A new inode is created with DAXDEV_ALIVE set but no backing driver,
   no ops, and no IDA-allocated minor number.

2. On teardown, dax_destroy_inode() warns because kill_dax() was never
   called, and dax_free_inode() calls ida_free() for a minor that was
   never ida_alloc'd -- potentially freeing the minor of a real device.

Add dax_dev_find() which uses ilookup5() for lookup-only semantics:
it returns an existing dax_device with an elevated inode reference, or
NULL if no device with the given dev_t exists. It never creates inodes.
A dax_alive() check under dax_read_lock() guards against returning a
device that is concurrently being torn down by kill_dax().

Make dax_dev_get() static again (internal to super.c for alloc_dax),
export dax_dev_find() instead.  Also add the missing CONFIG_DAX=n stub.

About the 'fixes' tag: this removes the export of dax_dev_get(),
which was flawed, and replaces is with dax_dev_find(). It feels like
the fixes tag makes sense for correcting an ABI error.

Fixes: 2ae624d5a555d ("dax: export dax_dev_get()")
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: John Groves <john@groves.net>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---
 drivers/dax/super.c | 38 ++++++++++++++++++++++++++++++++++++--
 include/linux/dax.h |  6 +++++-
 2 files changed, 41 insertions(+), 3 deletions(-)

diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index 45f84b0eb909..824e1f6df378 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -565,7 +565,7 @@ static int dax_set(struct inode *inode, void *data)
 	return 0;
 }
 
-struct dax_device *dax_dev_get(dev_t devt)
+static struct dax_device *dax_dev_get(dev_t devt)
 {
 	struct dax_device *dax_dev;
 	struct inode *inode;
@@ -588,7 +588,41 @@ struct dax_device *dax_dev_get(dev_t devt)
 
 	return dax_dev;
 }
-EXPORT_SYMBOL_GPL(dax_dev_get);
+
+/**
+ * dax_dev_find - look up an existing dax_device by dev_t
+ * @devt: the device number to find
+ *
+ * Returns a dax_device with an elevated inode reference, or NULL if no
+ * device with the given dev_t exists. Unlike dax_dev_get(), this never
+ * allocates a new inode -- it is safe for external callers that are looking
+ * up devices from user-supplied or metadata-supplied dev_t values.
+ *
+ * Caller must put_dax() the returned device when done.
+ */
+struct dax_device *dax_dev_find(dev_t devt)
+{
+	struct dax_device *dax_dev;
+	struct inode *inode;
+	int id;
+
+	inode = ilookup5(dax_superblock, hash_32(devt + DAXFS_MAGIC, 31),
+			 dax_test, &devt);
+	if (!inode)
+		return NULL;
+
+	dax_dev = to_dax_dev(inode);
+	id = dax_read_lock();
+	if (!dax_alive(dax_dev)) {
+		dax_read_unlock(id);
+		iput(inode);
+		return NULL;
+	}
+	dax_read_unlock(id);
+
+	return dax_dev;
+}
+EXPORT_SYMBOL_GPL(dax_dev_find);
 
 struct dax_device *alloc_dax(void *private, const struct dax_operations *ops)
 {
diff --git a/include/linux/dax.h b/include/linux/dax.h
index fe6c3ded1b50..29113eb95e72 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -54,7 +54,7 @@ struct dax_device *alloc_dax(void *private, const struct dax_operations *ops);
 void *dax_holder(struct dax_device *dax_dev);
 void put_dax(struct dax_device *dax_dev);
 void kill_dax(struct dax_device *dax_dev);
-struct dax_device *dax_dev_get(dev_t devt);
+struct dax_device *dax_dev_find(dev_t devt);
 void dax_write_cache(struct dax_device *dax_dev, bool wc);
 bool dax_write_cache_enabled(struct dax_device *dax_dev);
 bool dax_synchronous(struct dax_device *dax_dev);
@@ -92,6 +92,10 @@ static inline void put_dax(struct dax_device *dax_dev)
 static inline void kill_dax(struct dax_device *dax_dev)
 {
 }
+static inline struct dax_device *dax_dev_find(dev_t devt)
+{
+	return NULL;
+}
 static inline void dax_write_cache(struct dax_device *dax_dev, bool wc)
 {
 }
-- 
2.54.0