C is slave of B is slave of A. If a process can see (i.e. has under its root) A and C but not B then for C it will show master:B,propagate_from:A. This piece of information is shown because it can't see the immediate master (B) and so cannot determine the chain of propagation between the mounts it can see.
Concrete example: # mount --bind / /mnt # mount --bind /proc /mnt/proc # mount --make-private /mnt # mount --make-shared /mnt # mkdir /tmp/etc # mount --bind /mnt/etc /tmp/etc # mount --make-slave /tmp/etc # mount --make-shared /tmp/etc # mount --bind /tmp/etc /mnt/tmp/etc # mount --make-slave /mnt/tmp/etc # cat /proc/self/mountinfo | grep /tmp/etc 164 40 253:1 /etc /tmp/etc rw,relatime shared:100 master:97 - ... # chroot /mnt # cat /proc/self/mountinfo 129 62 253:1 / / rw,relatime shared:97 - ... 168 129 253:1 /etc /tmp/etc rw,relatime master:100 propagate_from:97 - ... Thanks, Miklos
