Fix autofs triggering problem. Assume you have an NFS server,

192.168.1.1, with share "share". This commit fixes a problem
where "mkdir /net/192.168.1.1/share/meh" would return spurious
error instead of creating the directory if the target filesystem
wasn't mounted yet; subsequent attempts would work correctly.

The failure scenario is kind of complicated to explain, but it all
boils down to calling VOP_MKDIR() for the target filesystem (NFS)
with wrong dvp - the autofs vnode instead of the filesystem root
mounted over it.

Reviewed by:	kib@
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D5442

sys/fs/autofs/autofs_vnops.c

@@ -214,7 +214,7 @@ autofs_lookup(struct vop_lookup_args *ap)
 	struct autofs_mount *amp;
 	struct autofs_node *anp, *child;
 	struct componentname *cnp;
-	int error, lock_flags;
+	int error;
 
 	dvp = ap->a_dvp;
 	vpp = ap->a_vpp;
@@ -257,23 +257,13 @@ autofs_lookup(struct vop_lookup_args *ap)
 			return (error);
 
 		if (newvp != NULL) {
-			error = VOP_LOOKUP(newvp, ap->a_vpp, ap->a_cnp);
-
 			/*
-			 * Instead of figuring out whether our vnode should
-			 * be locked or not given the error and cnp flags,
-			 * just "copy" the lock status from vnode returned
-			 * by mounted filesystem's VOP_LOOKUP().  Get rid
-			 * of that new vnode afterwards.
+			 * The target filesystem got automounted.
+			 * Let the lookup(9) go around with the same
+			 * path component.
 			 */
-			lock_flags = VOP_ISLOCKED(newvp);
-			if (lock_flags == 0) {
-				VOP_UNLOCK(dvp, 0);
-				vrele(newvp);
-			} else {
-				vput(newvp);
-			}
-			return (error);
+			vput(newvp);
+			return (ERELOOKUP);
 		}
 	}
 

sys/kern/vfs_lookup.c

@@ -495,6 +495,7 @@ lookup(struct nameidata *ndp)
 	int rdonly;			/* lookup read-only flag bit */
 	int error = 0;
 	int dpunlocked = 0;		/* dp has already been unlocked */
+	int relookup = 0;		/* do not consume the path component */
 	struct componentname *cnp = &ndp->ni_cnd;
 	int lkflags_save;
 	int ni_dvp_unlocked;
@@ -745,6 +746,14 @@ lookup(struct nameidata *ndp)
 			goto unionlookup;
 		}
 
+		if (error == ERELOOKUP) {
+			vref(dp);
+			ndp->ni_vp = dp;
+			error = 0;
+			relookup = 1;
+			goto good;
+		}
+
 		if (error != EJUSTRETURN)
 			goto bad;
 		/*
@@ -777,6 +786,8 @@ lookup(struct nameidata *ndp)
 		goto success;
 	} else
 		cnp->cn_lkflags = lkflags_save;
+
+good:
 #ifdef NAMEI_DIAGNOSTIC
 	printf("found\n");
 #endif
@@ -856,6 +867,14 @@ lookup(struct nameidata *ndp)
 	 */
 	KASSERT((cnp->cn_flags & ISLASTCN) || *ndp->ni_next == '/',
 	    ("lookup: invalid path state."));
+	if (relookup) {
+		relookup = 0;
+		if (ndp->ni_dvp != dp)
+			vput(ndp->ni_dvp);
+		else
+			vrele(ndp->ni_dvp);
+		goto dirloop;
+	}
 	if (*ndp->ni_next == '/') {
 		cnp->cn_nameptr = ndp->ni_next;
 		while (*cnp->cn_nameptr == '/') {

sys/sys/errno.h

@@ -190,6 +190,7 @@ __END_DECLS
 #define	EJUSTRETURN	(-2)		/* don't modify regs, just return */
 #define	ENOIOCTL	(-3)		/* ioctl not handled by this layer */
 #define	EDIRIOCTL	(-4)		/* do direct ioctl in GEOM */
+#define	ERELOOKUP	(-5)		/* retry the directory lookup */
 #endif
 
 #endif