fuse: return -ECONNABORTED on /dev/fuse read after abort

Currently the userspace has no way of knowing whether the fuse
connection ended because of umount or abort via sysfs. It makes it hard
for filesystems to free the mountpoint after abort without worrying
about removing some new mount.

The patch fixes it by returning different errors when userspace reads
from /dev/fuse (-ENODEV for umount and -ECONNABORTED for abort).

Add a new capability flag FUSE_ABORT_ERROR. If set and the connection is
gone because of sysfs abort, reading from the device will return
-ECONNABORTED.

Signed-off-by: Szymon Lukasz <noh4hss@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Author: noh4hss

fs/fuse/control.c

@@ -35,7 +35,7 @@ static ssize_t fuse_conn_abort_write(struct file *file, const char __user *buf,
 {
 	struct fuse_conn *fc = fuse_ctl_file_conn_get(file);
 	if (fc) {
-		fuse_abort_conn(fc);
+		fuse_abort_conn(fc, true);
 		fuse_conn_put(fc);
 	}
 	return count;

fs/fuse/cuse.c

@@ -406,7 +406,7 @@ static void cuse_process_init_reply(struct fuse_conn *fc, struct fuse_req *req)
 err_region:
 	unregister_chrdev_region(devt, 1);
 err:
-	fuse_abort_conn(fc);
+	fuse_abort_conn(fc, false);
 	goto out;
 }
 
@@ -581,7 +581,7 @@ static ssize_t cuse_class_abort_store(struct device *dev,
 {
 	struct cuse_conn *cc = dev_get_drvdata(dev);
 
-	fuse_abort_conn(&cc->fc);
+	fuse_abort_conn(&cc->fc, false);
 	return count;
 }
 static DEVICE_ATTR(abort, 0200, NULL, cuse_class_abort_store);

fs/fuse/dev.c

@@ -1234,9 +1234,10 @@ static ssize_t fuse_dev_do_read(struct fuse_dev *fud, struct file *file,
 	if (err)
 		goto err_unlock;
 
-	err = -ENODEV;
-	if (!fiq->connected)
+	if (!fiq->connected) {
+		err = (fc->aborted && fc->abort_err) ? -ECONNABORTED : -ENODEV;
 		goto err_unlock;
+	}
 
 	if (!list_empty(&fiq->interrupts)) {
 		req = list_entry(fiq->interrupts.next, struct fuse_req,
@@ -1287,7 +1288,7 @@ static ssize_t fuse_dev_do_read(struct fuse_dev *fud, struct file *file,
 	spin_lock(&fpq->lock);
 	clear_bit(FR_LOCKED, &req->flags);
 	if (!fpq->connected) {
-		err = -ENODEV;
+		err = (fc->aborted && fc->abort_err) ? -ECONNABORTED : -ENODEV;
 		goto out_end;
 	}
 	if (err) {
@@ -2076,7 +2077,7 @@ static void end_polls(struct fuse_conn *fc)
  * is OK, the request will in that case be removed from the list before we touch
  * it.
  */
-void fuse_abort_conn(struct fuse_conn *fc)
+void fuse_abort_conn(struct fuse_conn *fc, bool is_abort)
 {
 	struct fuse_iqueue *fiq = &fc->iq;
 
@@ -2089,6 +2090,7 @@ void fuse_abort_conn(struct fuse_conn *fc)
 
 		fc->connected = 0;
 		fc->blocked = 0;
+		fc->aborted = is_abort;
 		fuse_set_initialized(fc);
 		list_for_each_entry(fud, &fc->devices, entry) {
 			struct fuse_pqueue *fpq = &fud->pq;
@@ -2151,7 +2153,7 @@ int fuse_dev_release(struct inode *inode, struct file *file)
 		/* Are we the last open device? */
 		if (atomic_dec_and_test(&fc->dev_count)) {
 			WARN_ON(fc->iq.fasync != NULL);
-			fuse_abort_conn(fc);
+			fuse_abort_conn(fc, false);
 		}
 		fuse_dev_free(fud);
 	}

fs/fuse/fuse_i.h

@@ -515,6 +515,9 @@ struct fuse_conn {
 	    abort and device release */
 	unsigned connected;
 
+	/** Connection aborted via sysfs */
+	bool aborted;
+
 	/** Connection failed (version mismatch).  Cannot race with
 	    setting other bitfields since it is only set once in INIT
 	    reply, before any other request, and never cleared */
@@ -526,6 +529,9 @@ struct fuse_conn {
 	/** Do readpages asynchronously?  Only set in INIT */
 	unsigned async_read:1;
 
+	/** Return an unique read error after abort.  Only set in INIT */
+	unsigned abort_err:1;
+
 	/** Do not send separate SETATTR request before open(O_TRUNC)  */
 	unsigned atomic_o_trunc:1;
 
@@ -851,7 +857,7 @@ void fuse_request_send_background_locked(struct fuse_conn *fc,
 					 struct fuse_req *req);
 
 /* Abort all requests */
-void fuse_abort_conn(struct fuse_conn *fc);
+void fuse_abort_conn(struct fuse_conn *fc, bool is_abort);
 
 /**
  * Invalidate inode attributes

fs/fuse/inode.c

@@ -371,7 +371,7 @@ void fuse_unlock_inode(struct inode *inode)
 
 static void fuse_umount_begin(struct super_block *sb)
 {
-	fuse_abort_conn(get_fuse_conn_super(sb));
+	fuse_abort_conn(get_fuse_conn_super(sb), false);
 }
 
 static void fuse_send_destroy(struct fuse_conn *fc)
@@ -393,7 +393,7 @@ static void fuse_put_super(struct super_block *sb)
 
 	fuse_send_destroy(fc);
 
-	fuse_abort_conn(fc);
+	fuse_abort_conn(fc, false);
 	mutex_lock(&fuse_mutex);
 	list_del(&fc->entry);
 	fuse_ctl_remove_conn(fc);
@@ -918,6 +918,8 @@ static void process_init_reply(struct fuse_conn *fc, struct fuse_req *req)
 				fc->posix_acl = 1;
 				fc->sb->s_xattr = fuse_acl_xattr_handlers;
 			}
+			if (arg->flags & FUSE_ABORT_ERROR)
+				fc->abort_err = 1;
 		} else {
 			ra_pages = fc->max_read / PAGE_SIZE;
 			fc->no_lock = 1;
@@ -948,7 +950,8 @@ static void fuse_send_init(struct fuse_conn *fc, struct fuse_req *req)
 		FUSE_FLOCK_LOCKS | FUSE_HAS_IOCTL_DIR | FUSE_AUTO_INVAL_DATA |
 		FUSE_DO_READDIRPLUS | FUSE_READDIRPLUS_AUTO | FUSE_ASYNC_DIO |
 		FUSE_WRITEBACK_CACHE | FUSE_NO_OPEN_SUPPORT |
-		FUSE_PARALLEL_DIROPS | FUSE_HANDLE_KILLPRIV | FUSE_POSIX_ACL;
+		FUSE_PARALLEL_DIROPS | FUSE_HANDLE_KILLPRIV | FUSE_POSIX_ACL |
+		FUSE_ABORT_ERROR;
 	req->in.h.opcode = FUSE_INIT;
 	req->in.numargs = 1;
 	req->in.args[0].size = sizeof(*arg);

include/uapi/linux/fuse.h

@@ -113,6 +113,9 @@
  *  7.26
  *  - add FUSE_HANDLE_KILLPRIV
  *  - add FUSE_POSIX_ACL
+ *
+ *  7.27
+ *  - add FUSE_ABORT_ERROR
  */
 
 #ifndef _LINUX_FUSE_H
@@ -148,7 +151,7 @@
 #define FUSE_KERNEL_VERSION 7
 
 /** Minor version number of this interface */
-#define FUSE_KERNEL_MINOR_VERSION 26
+#define FUSE_KERNEL_MINOR_VERSION 27
 
 /** The node ID of the root inode */
 #define FUSE_ROOT_ID 1
@@ -245,6 +248,7 @@ struct fuse_file_lock {
  * FUSE_PARALLEL_DIROPS: allow parallel lookups and readdir
  * FUSE_HANDLE_KILLPRIV: fs handles killing suid/sgid/cap on write/chown/trunc
  * FUSE_POSIX_ACL: filesystem supports posix acls
+ * FUSE_ABORT_ERROR: reading the device after abort returns ECONNABORTED
  */
 #define FUSE_ASYNC_READ		(1 << 0)
 #define FUSE_POSIX_LOCKS	(1 << 1)
@@ -267,6 +271,7 @@ struct fuse_file_lock {
 #define FUSE_PARALLEL_DIROPS    (1 << 18)
 #define FUSE_HANDLE_KILLPRIV	(1 << 19)
 #define FUSE_POSIX_ACL		(1 << 20)
+#define FUSE_ABORT_ERROR	(1 << 21)
 
 /**
  * CUSE INIT request/reply flags