Using trace open

The trace open gadget streams events related to files opened inside pods.

On Kubernetes

Here we deploy a small demo pod "mypod":

$ kubectl run --restart=Never -ti --image=busybox mypod -- sh -c 'while /bin/true ; do whoami ; sleep 3 ; done'

Using the trace open gadget, we can see which processes open what files. We can simply filter for the pod "mypod" and omit specifying the node, thus tracing on all nodes for a pod called "mypod":

$ kubectl gadget trace open --podname mypod
K8S.NODE         K8S.NAMESPACE    K8S.PODNAME      K8S.CONTAINER   PID    COMM               FD ERR PATH
ip-10-0-30-247   default          mypod            mypod           18455  whoami              3   0 /etc/passwd
ip-10-0-30-247   default          mypod            mypod           18521  whoami              3   0 /etc/passwd
ip-10-0-30-247   default          mypod            mypod           18525  whoami              3   0 /etc/passwd
ip-10-0-30-247   default          mypod            mypod           18530  whoami              3   0 /etc/passwd
^
Terminating!

Seems the whoami command opens "/etc/passwd" to map the user ID to a user name. We can leave trace open by hitting Ctrl-C.

Finally, we need to clean up our pod:

$ kubectl delete pod mypod

With ig

Let's start the gadget in a terminal:

$ sudo ig trace open -c test-trace-open
RUNTIME.CONTAINERNAME                                      PID        COMM             FD    ERR PATH

Run a container that opens some files:

$ docker run --name test-trace-open -it --rm busybox /bin/sh -c 'while /bin/true ; do whoami ; sleep 3 ; done'

The tool will show the different files opened by the container:

$ sudo ig trace open -c test-trace-open
RUNTIME.CONTAINERNAME                                      PID        COMM             FD    ERR PATH
test-trace-open                                            630417     whoami           3     0   /etc/passwd
test-trace-open                                            630954     whoami           3     0   /etc/passwd

If you want to get full paths in from the tool, you can run it with the --full-path flag. This will add the column FULLPATH that contains the absolute path of the file with symlinks resolved.

$ sudo ./ig trace open -c test-trace-open-fullpath --full-path
RUNTIME.CONTAINERNAME         PID        COMM             FD  ERR PATH                            FULLPATH
test-trace-open-fullpath      1330356    cat              3   0   /etc/passwd                     /etc/passwd
test-trace-open-fullpath      1330401    cat              3   0   ../etc/mtab                     /proc/22/mounts

You can also filter by prefix paths using --prefixes. It will then only report events where opened files matched one of the given prefixes.

RUNTIME.CONTAINERNAME                            PID        COMM             FD    ERR PATH
test-trace-open-fullpath                         64069      touch            3     0   /tmp/foo/quux.txt
test-trace-open-fullpath                         64034      bash             3     0   /tmp/bar.txt

Note that this filtering occurs in eBPF.