libbe(3): move altroot augmentation bits around a little bit

We could perhaps have a method that does this given a dataset, but it's yet
clear that we'll always want to bypass the altroot when we grab the
mountpoint. For now, we'll refactor things a bit so we grab the altroot
length when libbe is initialized and have a common method that does the
necessary augmentation (replace with / if it's the root, return a pointer to
later in the string if not).

This will be used in some upcoming work to make be_mount work properly for
deep BEs.

MFC after:	1 week

lib/libbe/be.c

@@ -90,6 +90,7 @@ be_locate_rootfs(libbe_handle_t *lbh)
 libbe_handle_t *
 libbe_init(const char *root)
 {
+	char altroot[MAXPATHLEN];
 	libbe_handle_t *lbh;
 	char *poolname, *pos;
 	int pnamelen;
@@ -140,6 +141,11 @@ libbe_init(const char *root)
 	    sizeof(lbh->bootfs), NULL, true) != 0)
 		goto err;
 
+	if (zpool_get_prop(lbh->active_phandle, ZPOOL_PROP_ALTROOT,
+	    altroot, sizeof(altroot), NULL, true) == 0 &&
+	    strcmp(altroot, "-") != 0)
+		lbh->altroot_len = strlen(altroot);
+
 	return (lbh);
 err:
 	if (lbh != NULL) {
@@ -314,7 +320,6 @@ be_create(libbe_handle_t *lbh, const char *name)
 	return (set_error(lbh, err));
 }
 
-
 static int
 be_deep_clone_prop(int prop, void *cb)
 {
@@ -345,12 +350,9 @@ be_deep_clone_prop(int prop, void *cb)
 
 	/* Augment mountpoint with altroot, if needed */
 	val = pval;
-	if (prop == ZFS_PROP_MOUNTPOINT && *dccb->altroot != '\0') {
-		if (pval[strlen(dccb->altroot)] == '\0')
-			strlcpy(pval, "/", sizeof(pval));
-		else
-			val = pval + strlen(dccb->altroot);
-	}
+	if (prop == ZFS_PROP_MOUNTPOINT)
+		val = be_mountpoint_augmented(dccb->lbh, val);
+
 	nvlist_add_string(dccb->props, zfs_prop_to_name(prop), val);
 
 	return (ZPROP_CONT);
@@ -392,12 +394,9 @@ be_deep_clone(zfs_handle_t *ds, void *data)
 	nvlist_alloc(&props, NV_UNIQUE_NAME, KM_SLEEP);
 	nvlist_add_string(props, "canmount", "noauto");
 
+	dccb.lbh = isdc->lbh;
 	dccb.zhp = ds;
 	dccb.props = props;
-	if (zpool_get_prop(isdc->lbh->active_phandle, ZPOOL_PROP_ALTROOT,
-	    dccb.altroot, sizeof(dccb.altroot), NULL, true) != 0 ||
-	    strcmp(dccb.altroot, "-") == 0)
-		*dccb.altroot = '\0';
 	if (zprop_iter(be_deep_clone_prop, &dccb, B_FALSE, B_FALSE,
 	    ZFS_TYPE_FILESYSTEM) == ZPROP_INVAL)
 		return (-1);

lib/libbe/be_access.c

@@ -195,3 +195,25 @@ be_unmount(libbe_handle_t *lbh, char *bootenv, int flags)
 
 	return (BE_ERR_SUCCESS);
 }
+
+/*
+ * This function will blow away the input buffer as needed if we're discovered
+ * to be looking at a root-mount.  If the mountpoint is naturally beyond the
+ * root, however, the buffer may be left intact and a pointer to the section
+ * past altroot will be returned instead for the caller's perusal.
+ */
+char *
+be_mountpoint_augmented(libbe_handle_t *lbh, char *mountpoint)
+{
+
+	if (lbh->altroot_len == 0)
+		return (mountpoint);
+	if (mountpoint == NULL || *mountpoint == '\0')
+		return (mountpoint);
+
+	if (mountpoint[lbh->altroot_len] == '\0') {
+		*(mountpoint + 1) = '\0';
+		return (mountpoint);
+	} else
+		return (mountpoint + lbh->altroot_len);
+}

lib/libbe/be_impl.h

@@ -36,11 +36,12 @@
 #include "be.h"
 
 struct libbe_handle {
-	libzfs_handle_t *lzh;
-	zpool_handle_t *active_phandle;
 	char root[BE_MAXPATHLEN];
 	char rootfs[BE_MAXPATHLEN];
 	char bootfs[BE_MAXPATHLEN];
+	size_t altroot_len;
+	zpool_handle_t *active_phandle;
+	libzfs_handle_t *lzh;
 	be_error_t error;
 	bool print_on_err;
 };
@@ -53,9 +54,9 @@ struct libbe_deep_clone {
 };
 
 struct libbe_dccb {
+	libbe_handle_t *lbh;
 	zfs_handle_t *zhp;
 	nvlist_t *props;
-	char altroot[MAXPATHLEN];
 };
 
 typedef struct prop_data {
@@ -67,6 +68,8 @@ typedef struct prop_data {
 int prop_list_builder_cb(zfs_handle_t *, void *);
 int be_proplist_update(prop_data_t *);
 
+char *be_mountpoint_augmented(libbe_handle_t *lbh, char *mountpoint);
+
 /* Clobbers any previous errors */
 int set_error(libbe_handle_t *, be_error_t);