fusefs: Implement support for the auto_unmount option kernel-side

libfuse clients may pass the "-o auto_unmount" flag to ensure that the mountpoint
will get unmounted even if the server terminate abnormally. Without this flag
sending KILL to a FUSE daemon leaves its mountpoint mounted.

Approved by:	asomers
Differential Revision:	https://reviews.freebsd.org/D53086

sbin/mount_fusefs/mount_fusefs.8

@@ -32,7 +32,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd October 9, 2021
+.Dd January 24, 2026
 .Dt MOUNT_FUSEFS 8
 .Os
 .Sh NAME
@@ -171,6 +171,9 @@ to the file system name as reported by
 .Xr statfs 2 .
 This option can be used to identify the file system implemented by
 .Ar fuse_daemon .
+.It Cm auto_unmount
+Instructs the kernel to unmount the mount point when the FUSE device gets closed.
+This ensures a proper cleanup if the FUSE daemon exits abnormally.
 .El
 .El
 .Pp

sbin/mount_fusefs/mount_fusefs.c

@@ -84,6 +84,8 @@ static struct mntopt mopts[] = {
 	{ "automounted",	0, ALTF_AUTOMOUNTED, 1 },
 	#define ALTF_INTR 0x200
 	{ "intr",		0, ALTF_INTR, 1 },
+	#define ALTF_AUTOUNMOUNT 0x400
+	{ "auto_unmount",		0, ALTF_AUTOUNMOUNT, 1 },
 	/* Linux specific options, we silently ignore them */
 	{ "fd=",                 0, 0x00, 1 },
 	{ "rootmode=",           0, 0x00, 1 },

sys/fs/fuse/fuse_device.c

@@ -65,7 +65,6 @@
 #include <sys/module.h>
 #include <sys/systm.h>
 #include <sys/errno.h>
-#include <sys/param.h>
 #include <sys/kernel.h>
 #include <sys/conf.h>
 #include <sys/uio.h>
@@ -177,6 +176,11 @@ fdata_dtor(void *arg)
 	fuse_lck_mtx_unlock(fdata->ms_mtx);
 	FUSE_UNLOCK();
 
+	if (fdata->mp && fdata->dataflags & FSESS_AUTO_UNMOUNT) {
+		vfs_ref(fdata->mp);
+		dounmount(fdata->mp, MNT_FORCE, curthread);
+	}
+
 	fdata_trydestroy(fdata);
 }
 

sys/fs/fuse/fuse_ipc.h

@@ -242,8 +242,9 @@ struct fuse_data {
 #define FSESS_WARN_INODE_MISMATCH 0x4000000 /* ino != nodeid */
 #define FSESS_MNTOPTS_MASK	( \
 	FSESS_DAEMON_CAN_SPY | FSESS_PUSH_SYMLINKS_IN | \
-	FSESS_DEFAULT_PERMISSIONS | FSESS_INTR)
+	FSESS_DEFAULT_PERMISSIONS | FSESS_INTR | FSESS_AUTO_UNMOUNT)
 #define	FSESS_SETXATTR_EXT	  0x8000000 /* extended fuse_setxattr_in */
+#define FSESS_AUTO_UNMOUNT	  0x10000000 /* perform unmount when server dies */
 
 extern int fuse_data_cache_mode;
 

sys/fs/fuse/fuse_vfsops.c

@@ -337,6 +337,7 @@ fuse_vfsop_mount(struct mount *mp)
 	FUSE_FLAGOPT(push_symlinks_in, FSESS_PUSH_SYMLINKS_IN);
 	FUSE_FLAGOPT(default_permissions, FSESS_DEFAULT_PERMISSIONS);
 	FUSE_FLAGOPT(intr, FSESS_INTR);
+	FUSE_FLAGOPT(auto_unmount, FSESS_AUTO_UNMOUNT);
 
 	(void)vfs_scanopt(opts, "max_read=", "%u", &max_read);
 	(void)vfs_scanopt(opts, "linux_errnos", "%d", &linux_errnos);