fdescfs: Fix a file ref leak

In fdesc_lookup(), vn_vget_ino_gen() may fail without invoking the
callback, in which case the ref on fp is leaked.  This happens if the
fdescfs mount is being concurrently unmounted.  Moreover, we cannot
safely drop the ref while the dvp is locked.

So:
- Use a flag variable to indicate whether the ref is dropped.
- Reorganize things to handle the leak.

Reported by:	C Turt <ecturt@gmail.com>
Reviewed by:	mjg, kib
Tested by:	pho
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D39189

(cherry picked from commit 0f5b6f9a041e9cca3b376f6ec909374938887a3b)

1 files changed, 25 insertions, 17 deletions

diff --git a/sys/fs/fdescfs/fdesc_vnops.c b/sys/fs/fdescfs/fdesc_vnops.c
index 17320b2c8354..81b6690efc80 100644
--- a/sys/fs/fdescfs/fdesc_vnops.c
+++ b/sys/fs/fdescfs/fdesc_vnops.c
@@ -258,6 +258,7 @@ struct fdesc_get_ino_args {
 	int ix;
 	struct file *fp;
 	struct thread *td;
+	bool fdropped;
 };
 
 static int
@@ -280,6 +281,7 @@ fdesc_get_ino_alloc(struct mount *mp, void *arg, int lkflags,
 		error = fdesc_allocvp(a->ftype, a->fd_fd, a->ix, mp, rvp);
 	}
 	fdrop(a->fp, a->td);
+	a->fdropped = true;
 	return (error);
 }
 
@@ -300,6 +302,7 @@ fdesc_lookup(struct vop_lookup_args *ap)
 	int nlen = cnp->cn_namelen;
 	u_int fd, fd1;
 	int error;
+	bool fdropped;
 	struct vnode *fvp;
 
 	if ((cnp->cn_flags & ISLASTCN) &&
@@ -343,24 +346,10 @@ fdesc_lookup(struct vop_lookup_args *ap)
 	 */
 	if ((error = fget(td, fd, &cap_no_rights, &fp)) != 0)
 		goto bad;
+	fdropped = false;
 
-	/* Check if we're looking up ourselves. */
-	if (VTOFDESC(dvp)->fd_ix == FD_DESC + fd) {
-		/*
-		 * In case we're holding the last reference to the file, the dvp
-		 * will be re-acquired.
-		 */
-		vhold(dvp);
-		VOP_UNLOCK(dvp);
-		fdrop(fp, td);
-
-		/* Re-aquire the lock afterwards. */
-		vn_lock(dvp, LK_RETRY | LK_EXCLUSIVE);
-		vdrop(dvp);
-		fvp = dvp;
-		if (VN_IS_DOOMED(dvp))
-			error = ENOENT;
-	} else {
+	/* Make sure we're not looking up the dvp itself. */
+	if (VTOFDESC(dvp)->fd_ix != FD_DESC + fd) {
 		/*
 		 * Unlock our root node (dvp) when doing this, since we might
 		 * deadlock since the vnode might be locked by another thread
@@ -374,8 +363,27 @@ fdesc_lookup(struct vop_lookup_args *ap)
 		arg.ix = FD_DESC + fd;
 		arg.fp = fp;
 		arg.td = td;
+		arg.fdropped = fdropped;
 		error = vn_vget_ino_gen(dvp, fdesc_get_ino_alloc, &arg,
 		    LK_EXCLUSIVE, &fvp);
+		fdropped = arg.fdropped;
+	}
+
+	if (!fdropped) {
+		/*
+		 * In case we're holding the last reference to the file, the dvp
+		 * will be re-acquired.
+		 */
+		vhold(dvp);
+		VOP_UNLOCK(dvp);
+		fdrop(fp, td);
+		fdropped = true;
+
+		vn_lock(dvp, LK_RETRY | LK_EXCLUSIVE);
+		vdrop(dvp);
+		fvp = dvp;
+		if (error == 0 && VN_IS_DOOMED(dvp))
+			error = ENOENT;
 	}
 
 	if (error)