mount-copybind: add MOUNT_COPYBIND_AVOID_OVERLAYFS env var to skip OverlayFS

In some cases we don't want to even attempt to set up OverlayFS, for
example because SELinux in enforcing mode would kill the process
attempting to use the mount. See:

https://lore.kernel.org/all/CA+FmFJBDwt52Z-dVGfuUcnRMiMtGPhK4cCQJ=J_fg0r3x-b6ng@mail.gmail.com/T/#mef98aa406324096d1889d3d467251f30456f403c

If MOUNT_COPYBIND_AVOID_OVERLAYFS=1 is set, skip directly to copy and
bind mount.

Signed-off-by: Luca Boccassi <luca.boccassi@microsoft.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

diff --git a/meta/recipes-core/volatile-binds/files/mount-copybind b/meta/recipes-core/volatile-binds/files/mount-copybind
index 8bbb406b3a0b8916eed8a4f985bda274c4439840..ddc43576159b3da65bacaa1714a7c08955b24157 100755 (executable)
--- a/meta/recipes-core/volatile-binds/files/mount-copybind
+++ b/meta/recipes-core/volatile-binds/files/mount-copybind
@@ -2,6 +2,9 @@
 #
 # Perform a bind mount, copying existing files as we do so to ensure the
 # overlaid path has the necessary content.
+# If the target is a directory and overlayfs is available (and the environment
+# variable MOUNT_COPYBIND_AVOID_OVERLAYFS=1 is not set), then an overlay mount
+# will be attempted first.
 
 if [ $# -lt 2 ]; then
     echo >&2 "Usage: $0 spec mountpoint [OPTIONS]"
@@ -51,7 +54,7 @@ if [ -d "$mountpoint" ]; then
             mountcontext=",rootcontext=$(matchpathcon -n "$mountpoint")"
         fi
     fi
-    if ! mount -t overlay overlay -olowerdir="$mountpoint",upperdir="$spec",workdir="$overlay_workdir""$mountcontext" "$mountpoint" > /dev/null 2>&1; then
+    if [ "$MOUNT_COPYBIND_AVOID_OVERLAYFS" = 1 ] || ! mount -t overlay overlay -olowerdir="$mountpoint",upperdir="$spec",workdir="$overlay_workdir""$mountcontext" "$mountpoint" > /dev/null 2>&1; then
 
         if [ "$specdir_existed" != "yes" ]; then
             cp -aPR "$mountpoint"/. "$spec/"