On 5/30/26 9:51 AM, John Groves wrote:
> From: John Groves <[email protected]>
>
> 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, and update the two external callers
> (famfs_inode.c, famfs.c). Also add the missing CONFIG_DAX=n stub.
>
> Fixes: 2ae624d5a555d ("dax: export dax_dev_get()")
Not sure about the Fixes tag when the caller isn't in tree?
> Signed-off-by: John Groves <[email protected]>
Reviewed-by: Dave Jiang <[email protected]>
> ---
> 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 4c56ac2faacdb..6cb271e034a70 100644
> --- a/drivers/dax/super.c
> +++ b/drivers/dax/super.c
> @@ -550,7 +550,7 @@ 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;
> @@ -573,7 +573,41 @@ 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 fe6c3ded1b50f..29113eb95e72d 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)
> {
> }
