The following seems to work, for the simplest cases at least.
I would appreciate if someone who knows this code could have a look to
see if I've missed any steps, or botched any locks, and to answer the
embedded questions.
--- union_vnops.c.~1.74.~ 2021-03-07 17:23:02.000000000 -0800
+++ union_vnops.c 2021-12-04 00:36:56.318634542 -0800
@@ -622,6 +622,12 @@
un->un_uppervp->v_writecount++;
mutex_exit(un->un_uppervp->v_interlock);
}
+#ifdef UNION_DIAGNOSTIC
+ else {
+ printf("union_open: failed to open upper file for write %s: %d\n",
+ un->un_path, error);
+ }
+#endif
return (error);
}
@@ -636,6 +642,12 @@
error = VOP_OPEN(tvp, mode, cred);
VOP_UNLOCK(tvp);
+#ifdef UNION_DIAGNOSTIC
+ if (error != 0) {
+ printf("union_open: failed to open lower file %s: %d\n",
+ un->un_path, error);
+ }
+#endif
return (error);
}
/*
@@ -651,6 +663,12 @@
tvp->v_writecount++;
mutex_exit(tvp->v_interlock);
}
+#ifdef UNION_DIAGNOSTIC
+ if (error != 0) {
+ printf("union_open: failed to open upper file %s: %d\n",
+ un->un_path, error);
+ }
+#endif
return (error);
}
@@ -718,17 +736,63 @@
struct union_mount *um = MOUNTTOUNIONMOUNT(vp->v_mount);
/*
- * Disallow write attempts on read-only file systems;
- * unless the file is a socket, fifo, or a block or
- * character device resident on the file system.
+ * Disallow write attempts on read-only file systems; unless the file is
+ * readable by the user, or is a socket, fifo, or a block or character
+ * device resident on the file system.
*/
- if (ap->a_accmode & VWRITE) {
+ if (ap->a_accmode & VWRITE && un->un_uppervp == NULLVP) {
switch (vp->v_type) {
case VDIR:
case VLNK:
case VREG:
- if (vp->v_mount->mnt_flag & MNT_RDONLY)
+ /*
+ * XXX ToDo: First check for read (and write?) access
+ * on the underlying file system object (un->un_lowervp)
+ *
+ * If un->un_lowervp is readable then copy it up and
+ * finally check the upper node (i.e. continue to the
+ * next if).
+ *
+ * Does this work for VLNK and VDIR too?
+ */
+ if (un->un_lowervp != NULLVP) {
+ struct vop_access_args ra;
+ int mode = ap->a_accmode;
+
+ ra = *ap;
+ ra.a_vp = un->un_lowervp;
+ ra.a_accmode &= !VWRITE;
+ ra.a_accmode |= VREAD;
+ vn_lock(un->un_lowervp, LK_EXCLUSIVE | LK_RETRY); /* xxx ? */
+ error = VCALL(un->un_lowervp, VOFFSET(vop_access), &ra);
+ VOP_UNLOCK(un->un_lowervp); /* xxx ? */
+ if (error != 0) {
+#ifdef UNION_DIAGNOSTIC
+ printf("union_access: read access on lvp denied for %s: %d\n",
+ un->un_path, error);
+#endif
+ return (error);
+ }
+ /*
+ * XXX in theory we only need to make the vnode,
+ * not copy the file, but there's no logic
+ * elsewhere to complete the copy later if/when
+ * an open is attempted.
+ */
+ error = union_copyup(un, ((mode & O_TRUNC) == 0), ap->a_cred, curlwp);
+ if (error != 0) {
+#ifdef UNION_DIAGNOSTIC
+ printf("union_access: failed copyup for %s: %d\n",
+ un->un_path, error);
+#endif
+ return (error);
+ }
+ } else if (vp->v_mount->mnt_flag & MNT_RDONLY) {
+#ifdef UNION_DIAGNOSTIC
+ printf("union_access: EROFS for %s\n", un->un_path);
+#endif
return (EROFS);
+ }
break;
case VBAD:
case VBLK:
@@ -744,7 +808,14 @@
if ((vp = un->un_uppervp) != NULLVP) {
ap->a_vp = vp;
- return (VCALL(vp, VOFFSET(vop_access), ap));
+ error = VCALL(vp, VOFFSET(vop_access), ap);
+#ifdef UNION_DIAGNOSTIC
+ if (error) {
+ printf("union_access: failed upper access check for %s: %d\n",
+ un->un_path, error);
+ }
+#endif
+ return (error);
}
if ((vp = un->un_lowervp) != NULLVP) {
@@ -758,8 +829,13 @@
}
}
VOP_UNLOCK(vp);
- if (error)
+ if (error) {
+#ifdef UNION_DIAGNOSTIC
+ printf("union_access: failed lower access check for %s: %d\n",
+ un->un_path, error);
+#endif
return (error);
+ }
}
return (error);
@@ -1231,7 +1307,7 @@
*/
vp = NULLVP;
if (dun->un_uppervp == NULLVP)
- panic("union: null upperdvp?");
+ panic("union: null uppervp?");
error = relookup(ap->a_dvp, &vp, ap->a_cnp, 0);
if (error) {
VOP_UNLOCK(ap->a_vp);
@@ -1242,6 +1318,10 @@
* The name we want to create has
* mysteriously appeared (a race?)
*/
+#ifdef UNION_DIAGNOSTIC
+ printf("union_link: %s: already exists?\n",
+ un->un_path);
+#endif
error = EEXIST;
VOP_UNLOCK(ap->a_vp);
vput(vp);
[ 2.3112534] root file system type: cd9660
[ 2.3262533] kern.module.path=/stand/amd64/9.99.81/modules
[ 2.3362553] warning: no /dev/console
[ 2.3362553] warning: no /dev/constty
Created tmpfs /dev (2064384 byte, 4000 inodes)
[ 4.2142574] union_mount(mp = 0xffff888007b3e000)
[ 4.2142574] union_mount: from <above>:/tmp/uvar, on /var
[ 4.2142574] union_statvfs(mp = 0xffff888007b3e000, lvp = 0xffff888008cfcd40, uvp = 0xffff888008cfcac0)
[ 4.3342534] union_newsize: wtmpx: lower size now 0
[ 4.3352568] union: copied up wtmpx
[ 4.3352568] union_newsize: wtmpx: upper size now 520
[ 4.3352568] union_newsize: wtmp: lower size now 0
[ 4.3352568] union: copied up wtmp
[ 4.3402531] union_newsize: wtmp: upper size now 0
[ 4.3452562] union_newsize: wtmp: upper size now 40
[ 4.3502564] union_newsize: utmpx: lower size now 0
[ 4.3502564] union: copied up utmpx
[ 4.3502564] union_newsize: utmpx: upper size now 0
[ 4.3602577] union_newsize: utmpx: upper size now 520
[ 4.3602577] union_newsize: utmpx: upper size now 1040
[ 4.3602577] union_newsize: utmpx: upper size now 1560
[ 4.3742532] union_newsize: utmpx: upper size now 2080
init: kernel security level changed from 0 to 1
[ 4.4112513] union_newsize: utmpx: upper size now 2600
--
Greg A. Woods <gwoods%acm.org@localhost>
Kelowna, BC +1 250 762-7675 RoboHack <woods%robohack.ca@localhost>
Planix, Inc. <woods%planix.com@localhost> Avoncote Farms <woods%avoncote.ca@localhost>
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