Detemine "which device a file is in" in chroot environment?

Question

According to this: Determine what device a directory is located on

I can find which device a file is in by df command in a normal environment:

fpemud-workstation / # df /bin 
Filesystem     1K-blocks     Used Available Use% Mounted on 
/dev/dm-0      476458008 48307576 403924640  11% /

But I can't use that method in chroot:

fpemud-workstation / # mount -t proc n /test/proc 
fpemud-workstation / # mount --rbind /sys /test/sys 
fpemud-workstation / # mount --make-rslave /test/sys 
fpemud-workstation / # mount --rbind /dev /test/dev 
fpemud-workstation / # mount --make-rslave /test/dev 
fpemud-workstation / # chroot /test df /bin 
Filesystem     1K-blocks     Used Available Use% Mounted on 
-              476458008 48307604 403924612  11% /

EDIT:

Yes I have /etc/mtab in my chroot, this is a detailed result:

fpemud-workstation / # mount -t proc n /test/proc 
fpemud-workstation / # mount --rbind /sys /test/sys 
fpemud-workstation / # mount --make-rslave /test/sys 
fpemud-workstation / # mount --rbind /dev /test/dev 
fpemud-workstation / # mount --make-rslave /test/dev 

fpemud-workstation / # chroot /test ls /etc/mtab -l 
lrwxrwxrwx 1 root root 19 May 26 22:37 /etc/mtab -> ../proc/self/mounts 

fpemud-workstation / # chroot /test df /bin 
Filesystem     1K-blocks     Used Available Use% Mounted on 
-              476458008 51387464 400844752  12% / 

fpemud-workstation / # chroot /test cat /proc/mounts 
n /proc proc rw,relatime 0 0 
sysfs /sys sysfs rw,nosuid,nodev,noexec,relatime 0 0 
tmpfs /sys/fs/cgroup tmpfs ro,nosuid,nodev,noexec,mode=755 0 0 
cgroup /sys/fs/cgroup/systemd cgroup rw,nosuid,nodev,noexec,relatime,xattr,release_agent=/usr/lib/systemd/systemd-cgroups-agent,name=systemd 0 0
cgroup /sys/fs/cgroup/cpu,cpuacct cgroup rw,nosuid,nodev,noexec,relatime,cpu,cpuacct 0 0 
cgroup /sys/fs/cgroup/freezer cgroup rw,nosuid,nodev,noexec,relatime,freezer 0 0 
cgroup /sys/fs/cgroup/pids cgroup rw,nosuid,nodev,noexec,relatime,pids 0 0 
cgroup /sys/fs/cgroup/devices cgroup rw,nosuid,nodev,noexec,relatime,devices 0 0 
cgroup /sys/fs/cgroup/cpuset cgroup rw,nosuid,nodev,noexec,relatime,cpuset 0 0 
cgroup /sys/fs/cgroup/net_cls,net_prio cgroup rw,nosuid,nodev,noexec,relatime,net_cls,net_prio 0 0 
cgroup /sys/fs/cgroup/perf_event cgroup rw,nosuid,nodev,noexec,relatime,perf_event 0 0 
cgroup /sys/fs/cgroup/blkio cgroup rw,nosuid,nodev,noexec,relatime,blkio 0 0 
cgroup /sys/fs/cgroup/memory cgroup rw,nosuid,nodev,noexec,relatime,memory 0 0 
pstore /sys/fs/pstore pstore rw,nosuid,nodev,noexec,relatime 0 0 
configfs /sys/kernel/config configfs rw,relatime 0 0 
debugfs /sys/kernel/debug debugfs rw,relatime 0 0 
fusectl /sys/fs/fuse/connections fusectl rw,relatime 0 0 
devtmpfs /dev devtmpfs rw,nosuid,size=1997472k,nr_inodes=499368,mode=755 0 0 
tmpfs /dev/shm tmpfs rw,nosuid,nodev 0 0 
devpts /dev/pts devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0 
hugetlbfs /dev/hugepages hugetlbfs rw,relatime 0 0 
mqueue /dev/mqueue mqueue rw,relatime 0 0

score 2 · Answer 1 · answered May 26 '16 at 23:21

df reads /etc/mtab to find what device is mounted on a particular mount point. This can either be a file which is updated by the mount command, or a symbolic link to /proc/mounts which contains current information provided by the kernel.

/etc/mtab has the advantage that it can provide more precise information: it's what was passed to mount, not some information massaged by the kernel. But it also has limitations: it can get out of synch if mount can't write to it for some reason (e.g. while / is read-only, or if mount is run in a chroot), and it doesn't work in a chroot.

In a system intended to be used chrooted, make /etc/mtab a symbolic link to /proc/mounts.

Detemine "which device a file is in" in chroot environment?

1 Answers1