Fix renameat(2) for CAPABILITIES kernels.

When renameat(2) is used with:
- absolute path for to;
- tofd not set to AT_FDCWD;
- the target exists
kern_renameat() requires CAP_UNLINK capability on tofd, but
corresponding namei ni_filecap is not initialized at all because the
lookup is absolute.  As result, the check was done against empty filecap
and syscall fails erronously.

Fix it by creating a return flags namei member and reporting if the
lookup was absolute, then do not touch to.ni_filecaps at all.

PR:	222258
Reviewed by:	jilles, ngie
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
X-MFC-note:	KBI breakage
Differential revision:	https://reviews.freebsd.org/D19096

sys/kern/vfs_lookup.c

@@ -410,6 +410,7 @@ namei(struct nameidata *ndp)
 	dp = NULL;
 	cnp->cn_nameptr = cnp->cn_pnbuf;
 	if (cnp->cn_pnbuf[0] == '/') {
+		ndp->ni_resflags |= NIRES_ABS;
 		error = namei_handle_root(ndp, &dp);
 	} else {
 		if (ndp->ni_startdir != NULL) {
@@ -1302,6 +1303,7 @@ NDINIT_ALL(struct nameidata *ndp, u_long op, u_long flags, enum uio_seg segflg,
 	ndp->ni_dirp = namep;
 	ndp->ni_dirfd = dirfd;
 	ndp->ni_startdir = startdir;
+	ndp->ni_resflags = 0;
 	filecaps_init(&ndp->ni_filecaps);
 	ndp->ni_cnd.cn_thread = td;
 	if (rightsp != NULL)

sys/kern/vfs_syscalls.c

@@ -3544,10 +3544,10 @@ again:
 			goto out;
 		}
 #ifdef CAPABILITIES
-		if (newfd != AT_FDCWD) {
+		if (newfd != AT_FDCWD && (tond.ni_resflags & NIRES_ABS) == 0) {
 			/*
 			 * If the target already exists we require CAP_UNLINKAT
-			 * from 'newfd'.
+			 * from 'newfd', when newfd was used for the lookup.
 			 */
 			error = cap_check(&tond.ni_filecaps.fc_rights,
 			    &cap_unlinkat_rights);

sys/sys/namei.h

@@ -87,6 +87,10 @@ struct nameidata {
 	 */
 	struct	vnode *ni_vp;		/* vnode of result */
 	struct	vnode *ni_dvp;		/* vnode of intermediate directory */
+	/*
+	 * Results: flags returned from namei
+	 */
+	u_int	ni_resflags;
 	/*
 	 * Shared between namei and lookup/commit routines.
 	 */
@@ -159,6 +163,11 @@ struct nameidata {
 #define	NOCAPCHECK	0x20000000 /* do not perform capability checks */
 #define	PARAMASK	0x3ffffe00 /* mask of parameter descriptors */
 
+/*
+ * Namei results flags
+ */
+#define	NIRES_ABS	0x00000001 /* Path was absolute */
+
 /*
  * Flags in ni_lcf, valid for the duration of the namei call.
  */