fuse: create fuse_dev on /dev/fuse open instead of mount

Allocate struct fuse_dev when opening the device.  This means that unlike
before, ->private_data is always set to a valid pointer.

The use of USE_DEV_SYNC_INIT magic pointer for the private_data is now
replaced with a simple bool sync_init member.

If sync INIT is not set, I/O on the device returns error before mount.
Keep this behavior by checking for the ->fc member.  If fud->fc is set, the
mount has succeeded.  Testing this used READ_ONCE(file->private_data) and
smp_mb() to try and provide the necessary semantics.  Switch this to
smp_store_release() and smp_load_acquire().

Setting fud->fc is protected by fuse_mutex, this is unchanged.

Will need this later so the /dev/fuse open file reference is not held
during FSCONFIG_CMD_CREATE.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

fs/fuse/dev.c

@@ -1548,32 +1548,24 @@ static ssize_t fuse_dev_do_read(struct fuse_dev *fud, struct file *file,
 
 static int fuse_dev_open(struct inode *inode, struct file *file)
 {
-	/*
-	 * The fuse device's file's private_data is used to hold
-	 * the fuse_conn(ection) when it is mounted, and is used to
-	 * keep track of whether the file has been mounted already.
-	 */
-	file->private_data = NULL;
+	struct fuse_dev *fud = fuse_dev_alloc();
+
+	if (!fud)
+		return -ENOMEM;
+
+	file->private_data = fud;
 	return 0;
 }
 
 struct fuse_dev *fuse_get_dev(struct file *file)
 {
-	struct fuse_dev *fud = __fuse_get_dev(file);
+	struct fuse_dev *fud = fuse_file_to_fud(file);
 	int err;
 
-	if (likely(fud))
-		return fud;
-
-	err = wait_event_interruptible(fuse_dev_waitq,
-				       READ_ONCE(file->private_data) != FUSE_DEV_SYNC_INIT);
+	err = wait_event_interruptible(fuse_dev_waitq, fuse_dev_fc_get(fud) != NULL);
 	if (err)
 		return ERR_PTR(err);
 
-	fud = __fuse_get_dev(file);
-	if (!fud)
-		return ERR_PTR(-EPERM);
-
 	return fud;
 }
 
@@ -2547,10 +2539,10 @@ void fuse_wait_aborted(struct fuse_conn *fc)
 
 int fuse_dev_release(struct inode *inode, struct file *file)
 {
-	struct fuse_dev *fud = __fuse_get_dev(file);
+	struct fuse_dev *fud = fuse_file_to_fud(file);
+	struct fuse_conn *fc = fuse_dev_fc_get(fud);
 
-	if (fud) {
-		struct fuse_conn *fc = fud->fc;
+	if (fc) {
 		struct fuse_pqueue *fpq = &fud->pq;
 		LIST_HEAD(to_end);
 		unsigned int i;
@@ -2568,8 +2560,8 @@ int fuse_dev_release(struct inode *inode, struct file *file)
 			WARN_ON(fc->iq.fasync != NULL);
 			fuse_abort_conn(fc);
 		}
-		fuse_dev_free(fud);
 	}
+	fuse_dev_free(fud);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(fuse_dev_release);
@@ -2587,16 +2579,12 @@ static int fuse_dev_fasync(int fd, struct file *file, int on)
 
 static int fuse_device_clone(struct fuse_conn *fc, struct file *new)
 {
-	struct fuse_dev *fud;
+	struct fuse_dev *new_fud = fuse_file_to_fud(new);
 
-	if (__fuse_get_dev(new))
+	if (fuse_dev_fc_get(new_fud))
 		return -EINVAL;
 
-	fud = fuse_dev_alloc_install(fc);
-	if (!fud)
-		return -ENOMEM;
-
-	new->private_data = fud;
+	fuse_dev_install(new_fud, fc);
 	atomic_inc(&fc->dev_count);
 
 	return 0;
@@ -2667,10 +2655,11 @@ static long fuse_dev_ioctl_backing_close(struct file *file, __u32 __user *argp)
 static long fuse_dev_ioctl_sync_init(struct file *file)
 {
 	int err = -EINVAL;
+	struct fuse_dev *fud = fuse_file_to_fud(file);
 
 	mutex_lock(&fuse_mutex);
-	if (!__fuse_get_dev(file)) {
-		WRITE_ONCE(file->private_data, FUSE_DEV_SYNC_INIT);
+	if (!fuse_dev_fc_get(fud)) {
+		fud->sync_init = true;
 		err = 0;
 	}
 	mutex_unlock(&fuse_mutex);

fs/fuse/fuse_dev_i.h

@@ -39,18 +39,35 @@ struct fuse_copy_state {
 	} ring;
 };
 
-#define FUSE_DEV_SYNC_INIT ((struct fuse_dev *) 1)
-#define FUSE_DEV_PTR_MASK (~1UL)
+/*
+ * Lockless access is OK, because fud->fc is set once during mount and is valid
+ * until the file is released.
+ */
+static inline struct fuse_conn *fuse_dev_fc_get(struct fuse_dev *fud)
+{
+	/* Pairs with smp_store_release() in fuse_dev_fc_set() */
+	return smp_load_acquire(&fud->fc);
+}
+
+static inline void fuse_dev_fc_set(struct fuse_dev *fud, struct fuse_conn *fc)
+{
+	/* Pairs with smp_load_acquire() in fuse_dev_fc_get() */
+	smp_store_release(&fud->fc, fc);
+}
+
+static inline struct fuse_dev *fuse_file_to_fud(struct file *file)
+{
+	return file->private_data;
+}
 
 static inline struct fuse_dev *__fuse_get_dev(struct file *file)
 {
-	/*
-	 * Lockless access is OK, because file->private data is set
-	 * once during mount and is valid until the file is released.
-	 */
-	struct fuse_dev *fud = READ_ONCE(file->private_data);
+	struct fuse_dev *fud = fuse_file_to_fud(file);
 
-	return (typeof(fud)) ((unsigned long) fud & FUSE_DEV_PTR_MASK);
+	if (!fuse_dev_fc_get(fud))
+		return NULL;
+
+	return fud;
 }
 
 struct fuse_dev *fuse_get_dev(struct file *file);

fs/fuse/fuse_i.h

@@ -577,6 +577,9 @@ struct fuse_pqueue {
  * Fuse device instance
  */
 struct fuse_dev {
+	/** Issue FUSE_INIT synchronously */
+	bool sync_init;
+
 	/** Fuse connection for this device */
 	struct fuse_conn *fc;
 
@@ -623,9 +626,6 @@ struct fuse_fs_context {
 
 	/* DAX device, may be NULL */
 	struct dax_device *dax_dev;
-
-	/* fuse_dev pointer to fill in, should contain NULL on entry */
-	void **fudptr;
 };
 
 struct fuse_sync_bucket {

fs/fuse/inode.c

@@ -1641,7 +1641,7 @@ EXPORT_SYMBOL_GPL(fuse_dev_alloc);
 
 void fuse_dev_install(struct fuse_dev *fud, struct fuse_conn *fc)
 {
-	fud->fc = fuse_conn_get(fc);
+	fuse_dev_fc_set(fud, fuse_conn_get(fc));
 	spin_lock(&fc->lock);
 	list_add_tail(&fud->entry, &fc->devices);
 	spin_unlock(&fc->lock);
@@ -1663,7 +1663,7 @@ EXPORT_SYMBOL_GPL(fuse_dev_alloc_install);
 
 void fuse_dev_free(struct fuse_dev *fud)
 {
-	struct fuse_conn *fc = fud->fc;
+	struct fuse_conn *fc = fuse_dev_fc_get(fud);
 
 	if (fc) {
 		spin_lock(&fc->lock);
@@ -1826,7 +1826,7 @@ EXPORT_SYMBOL_GPL(fuse_init_fs_context_submount);
 
 int fuse_fill_super_common(struct super_block *sb, struct fuse_fs_context *ctx)
 {
-	struct fuse_dev *fud = NULL;
+	struct fuse_dev *fud = ctx->file ? fuse_file_to_fud(ctx->file) : NULL;
 	struct fuse_mount *fm = get_fuse_mount_super(sb);
 	struct fuse_conn *fc = fm->fc;
 	struct inode *root;
@@ -1860,18 +1860,11 @@ int fuse_fill_super_common(struct super_block *sb, struct fuse_fs_context *ctx)
 			goto err;
 	}
 
-	if (ctx->fudptr) {
-		err = -ENOMEM;
-		fud = fuse_dev_alloc_install(fc);
-		if (!fud)
-			goto err_free_dax;
-	}
-
 	fc->dev = sb->s_dev;
 	fm->sb = sb;
 	err = fuse_bdi_init(fc, sb);
 	if (err)
-		goto err_dev_free;
+		goto err_free_dax;
 
 	/* Handle umasking inside the fuse code */
 	if (sb->s_flags & SB_POSIXACL)
@@ -1893,15 +1886,15 @@ int fuse_fill_super_common(struct super_block *sb, struct fuse_fs_context *ctx)
 	set_default_d_op(sb, &fuse_dentry_operations);
 	root_dentry = d_make_root(root);
 	if (!root_dentry)
-		goto err_dev_free;
+		goto err_free_dax;
 
 	mutex_lock(&fuse_mutex);
 	err = -EINVAL;
-	if (ctx->fudptr && *ctx->fudptr) {
-		if (*ctx->fudptr == FUSE_DEV_SYNC_INIT)
-			fc->sync_init = 1;
-		else
+	if (fud) {
+		if (fuse_dev_fc_get(fud))
 			goto err_unlock;
+		if (fud->sync_init)
+			fc->sync_init = 1;
 	}
 
 	err = fuse_ctl_add_conn(fc);
@@ -1910,8 +1903,8 @@ int fuse_fill_super_common(struct super_block *sb, struct fuse_fs_context *ctx)
 
 	list_add_tail(&fc->entry, &fuse_conn_list);
 	sb->s_root = root_dentry;
-	if (ctx->fudptr) {
-		*ctx->fudptr = fud;
+	if (fud) {
+		fuse_dev_install(fud, fc);
 		wake_up_all(&fuse_dev_waitq);
 	}
 	mutex_unlock(&fuse_mutex);
@@ -1920,9 +1913,6 @@ int fuse_fill_super_common(struct super_block *sb, struct fuse_fs_context *ctx)
  err_unlock:
 	mutex_unlock(&fuse_mutex);
 	dput(root_dentry);
- err_dev_free:
-	if (fud)
-		fuse_dev_free(fud);
  err_free_dax:
 	if (IS_ENABLED(CONFIG_FUSE_DAX))
 		fuse_dax_conn_free(fc);
@@ -1948,13 +1938,10 @@ static int fuse_fill_super(struct super_block *sb, struct fs_context *fsc)
 	if ((ctx->file->f_op != &fuse_dev_operations) ||
 	    (ctx->file->f_cred->user_ns != sb->s_user_ns))
 		return -EINVAL;
-	ctx->fudptr = &ctx->file->private_data;
 
 	err = fuse_fill_super_common(sb, ctx);
 	if (err)
 		return err;
-	/* file->private_data shall be visible on all CPUs after this */
-	smp_mb();
 
 	fm = get_fuse_mount_super(sb);
 

fs/fuse/virtio_fs.c

@@ -1590,8 +1590,6 @@ static int virtio_fs_fill_super(struct super_block *sb, struct fs_context *fsc)
 			goto err_free_fuse_devs;
 	}
 
-	/* virtiofs allocates and installs its own fuse devices */
-	ctx->fudptr = NULL;
 	if (ctx->dax_mode != FUSE_DAX_NEVER) {
 		if (ctx->dax_mode == FUSE_DAX_ALWAYS && !fs->dax_dev) {
 			err = -EINVAL;