Comment 5 for bug 1427264

The issue is around mount propagation. The /home/ directory is being recursively bound into the schroot tree. When the schroot tree is recursively unmounted and SCHROOT_DIR/home/foo is unmounted, that unmount event is being propagated to the original /home/foo mount point. To prevent that from happening, the recursively bound SCHROOT_DIR/home mount point needs to be recursively private. Using the example in comment #4, with eCryptfs instead of ext4, that would look like this:

foo@sec-vivid-amd64:~$ grep home/foo /proc/mounts
/home/foo/.Private /home/foo ecryptfs rw,nosuid,nodev,relatime,ecryptfs_fnek_sig=176339c8321afc83,ecryptfs_sig=35e81f7d08221a22,ecryptfs_cipher=aes,ecryptfs_key_bytes=16,ecryptfs_unlink_sigs 0 0
foo@sec-vivid-amd64:~$ sudo mount -R /home /tmp/home
foo@sec-vivid-amd64:~$ sudo mount --make-rslave /tmp/home
foo@sec-vivid-amd64:~$ grep home/foo /proc/mounts
/home/foo/.Private /home/foo ecryptfs rw,nosuid,nodev,relatime,ecryptfs_fnek_sig=176339c8321afc83,ecryptfs_sig=35e81f7d08221a22,ecryptfs_cipher=aes,ecryptfs_key_bytes=16,ecryptfs_unlink_sigs 0 0
/home/foo/.Private /tmp/home/foo ecryptfs rw,nosuid,nodev,relatime,ecryptfs_fnek_sig=176339c8321afc83,ecryptfs_sig=35e81f7d08221a22,ecryptfs_cipher=aes,ecryptfs_key_bytes=16,ecryptfs_unlink_sigs 0 0
foo@sec-vivid-amd64:~$ sudo umount -R /tmp/home
foo@sec-vivid-amd64:~$ grep home/foo /proc/mounts
/home/foo/.Private /home/foo ecryptfs rw,nosuid,nodev,relatime,ecryptfs_fnek_sig=176339c8321afc83,ecryptfs_sig=35e81f7d08221a22,ecryptfs_cipher=aes,ecryptfs_key_bytes=16,ecryptfs_unlink_sigs 0 0

Note that two separate mount commands are required. The -R and --make-rslave options cannot be specified in the same mount command. The mount(8) program will turn this into two mount(2) syscalls and strip the MS_REC flag off of the first mount(2) syscall, resulting in a non-recursive bind mount:

mount("/home", "/tmp/home", 0x1880f30, MS_MGC_VAL|MS_BIND, NULL) = 0
mount("none", "/tmp/home", NULL, MS_REC|MS_SLAVE, NULL) = 0

This means that we can add a new line to the click/fstab file, specifying that the schroot's /home/ recursive bind mount point should be made recursively private. While I haven't had a chance to do extensive testing, the following change seems to fix this bug:

$ diff -Nurp /etc/schroot/click/fstab.orig /etc/schroot/click/fstab
--- /etc/schroot/click/fstab.orig 2015-03-12 15:23:37.631296891 -0500
+++ /etc/schroot/click/fstab 2015-03-12 15:24:09.183296891 -0500
@@ -8,5 +8,6 @@
 /dev /dev none rw,bind 0 0
 /dev/pts /dev/pts none rw,bind 0 0
 /home /home none rw,rbind 0 0
+none /home none rslave 0 0
 /tmp /tmp none rw,bind 0 0
 /run/shm /run/shm none rw,bind 0 0