renameat2(2): implement AT_RENAME_NOREPLACE flag

For msdosfs, tmpfs, and ufs.

Reviewed by:	markj
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D55539

sys/fs/msdosfs/msdosfs_vnops.c

@@ -49,12 +49,12 @@
  * October 1992
  */
 
-#include <sys/param.h>
 #include <sys/systm.h>
 #include <sys/bio.h>
 #include <sys/buf.h>
 #include <sys/clock.h>
 #include <sys/dirent.h>
+#include <sys/fcntl.h>
 #include <sys/lock.h>
 #include <sys/lockf.h>
 #include <sys/malloc.h>
@@ -970,7 +970,7 @@ msdosfs_rename(struct vop_rename_args *ap)
 		goto abortit;
 	}
 
-	if (ap->a_flags != 0) {
+	if ((ap->a_flags & ~(AT_RENAME_NOREPLACE)) != 0) {
 		error = EOPNOTSUPP;
 		goto abortit;
 	}
@@ -1041,6 +1041,11 @@ relock:
 		vrele(tvp);
 		tvp = NULL;
 	}
+	if (error == 0 && tvp != NULL &&
+	    (ap->a_flags & AT_RENAME_NOREPLACE) != 0) {
+		error = EEXIST;
+		goto unlock;
+	}
 	if (error == 0) {
 		nip = NULL;
 		error = deget(pmp, scn, blkoff, LK_EXCLUSIVE | LK_NOWAIT,

sys/fs/tmpfs/tmpfs_vnops.c

@@ -993,7 +993,7 @@ tmpfs_rename(struct vop_rename_args *v)
 		goto out;
 	}
 
-	if (v->a_flags != 0) {
+	if ((v->a_flags & ~(AT_RENAME_NOREPLACE)) != 0) {
 		error = EOPNOTSUPP;
 		goto out;
 	}
@@ -1018,9 +1018,14 @@ tmpfs_rename(struct vop_rename_args *v)
 			    "tmpfs_rename: fdvp not locked");
 			ASSERT_VOP_ELOCKED(tdvp,
 			    "tmpfs_rename: tdvp not locked");
-			if (tvp != NULL)
+			if (tvp != NULL) {
 				ASSERT_VOP_ELOCKED(tvp,
 				    "tmpfs_rename: tvp not locked");
+				if ((v->a_flags & AT_RENAME_NOREPLACE) != 0) {
+					error = EEXIST;
+					goto out_locked;
+				}
+			}
 			if (fvp == tvp) {
 				error = 0;
 				goto out_locked;

sys/kern/vfs_syscalls.c

@@ -3783,7 +3783,7 @@ kern_renameat(struct thread *td, int oldfd, const char *old, int newfd,
 	int error;
 	short irflag;
 
-	if (flags != 0)
+	if ((flags & ~(AT_RENAME_NOREPLACE)) != 0)
 		return (EINVAL);
 again:
 	tmp = mp = NULL;
@@ -3820,6 +3820,19 @@ again:
 	}
 	tdvp = tond.ni_dvp;
 	tvp = tond.ni_vp;
+	if (tvp != NULL && (flags & AT_RENAME_NOREPLACE) != 0) {
+		/*
+		 * Often filesystems need to relock the vnodes in
+		 * VOP_RENAME(), which opens a window for invalidation
+		 * of this check.  Then, not all filesystems might
+		 * implement AT_RENAME_NOREPLACE.  This leads to
+		 * situation where sometimes EOPNOTSUPP might be
+		 * returned from the VOP due to race, while most of
+		 * the time this check works.
+		 */
+		error = EEXIST;
+		goto out;
+	}
 	error = vn_start_write(fvp, &mp, V_NOWAIT);
 	if (error != 0) {
 again1:
@@ -3912,9 +3925,12 @@ out:
 		vrele(fromnd.ni_dvp);
 		vrele(fvp);
 	}
-	lockmgr(&tmp->mnt_renamelock, LK_RELEASE, 0);
-	vfs_rel(tmp);
-	vn_finished_write(mp);
+	if (tmp != NULL) {
+		lockmgr(&tmp->mnt_renamelock, LK_RELEASE, 0);
+		vfs_rel(tmp);
+	}
+	if (mp != NULL)
+		vn_finished_write(mp);
 out1:
 	if (error == ERESTART)
 		return (0);

sys/sys/fcntl.h

@@ -245,6 +245,9 @@ typedef	__pid_t		pid_t;
 #define	AT_RESOLVE_BENEATH	0x2000	/* Do not allow name resolution
 					   to walk out of dirfd */
 #define	AT_EMPTY_PATH		0x4000	/* Operate on dirfd if path is empty */
+
+#define	AT_RENAME_NOREPLACE	0x0001	/* Fail rename if target exists */
+#define	RENAME_NOREPLACE	AT_RENAME_NOREPLACE
 #endif	/* __BSD_VISIBLE */
 
 /*

sys/ufs/ufs/ufs_vnops.c

@@ -1294,7 +1294,7 @@ ufs_rename(
 		goto releout;
 	}
 
-	if (ap->a_flags != 0) {
+	if ((ap->a_flags & ~(AT_RENAME_NOREPLACE)) != 0) {
 		error = EOPNOTSUPP;
 		mp = NULL;
 		goto releout;
@@ -1394,6 +1394,11 @@ relock:
 		}
 	}
 
+	if (tvp != NULL && (ap->a_flags & AT_RENAME_NOREPLACE) != 0) {
+		error = EEXIST;
+		goto unlockout;
+	}
+
 	if (DOINGSUJ(fdvp) &&
 	    (seqc_in_modify(fdvp_s) || !vn_seqc_consistent(fdvp, fdvp_s) ||
 	     seqc_in_modify(fvp_s) || !vn_seqc_consistent(fvp, fvp_s) ||