Using traceloop
The traceloop gadget is used to trace system calls issued by containers.
On Kubernetes
Start traceloop
Traceloop is disabled by default from version 0.4.0. It can be enabled by using:
$ kubectl gadget traceloop start
Multiplication demo
Let’s run a pod to compute an important multiplication:
$ kubectl create ns test
$ kubectl run --restart=Never -n test -ti --image=busybox mypod -- sh -c 'RANDOM=output ; echo "3*7*2" | bc > /tmp/file-$RANDOM ; cat /tmp/file-$RANDOM'
cat: can't open '/tmp/file-3240': No such file or directory
pod default/mypod terminated (Error)
$ kubectl delete pod -n test mypod
pod "mypod" deleted
Oh no! We made a mistake in the shell script: we opened the wrong file. Is the result lost forever? Let’s check with the traceloop gadget:
$ kubectl gadget traceloop list
K8S.NODE K8S.NAMESPACE K8S.POD K8S.CONTAINER CONTAINERID
minikube kube-system kube-scheduler-minikube kube-scheduler 2b63eb745ce2cf
...
minikube test mypod mypod ef6f2d3f44b555
Let’s inspect the traceloop log:
$ kubectl gadget traceloop show ef6f2d3f44b555
CPU PID COMM NAME PARAMS RET
...
1 337825 sh write fd=1, buf=34716864 3*7*2
, count=6 6
...
1 337826 sh open filename=34717192 /tmp/file-24581, flags=577, mode=438 3
...
1 337826 sh execve filename=34717344 /bin/bc, argv=34717176, envp=34717224 0
...
1 337826 bc read fd=0, buf=5355360 3*7*2
, count=4096 6
1 337826 bc write fd=1, buf=5359456 42
, count=3 3
...
0 337813 cat open filename=140736754175569 /tmp/file-3240, flags=0, mode=0 -2
0 337813 cat write fd=2, buf=140736754168912 cat: can't open '/tmp/file-3240': No such file or directory
, coun… 60
0 337813 cat exit_group error_code=1 X
Thanks to the traceloop gadget, we can recover the result of the
multiplication: 42. And we can understand the mistake in the shell script: the
result was saved in /tmp/file-24581 but we attempted to open
/tmp/file-3240.
Note that, return value of exit_group() is X as this syscall never returns.
The same applies for exit() and rt_sigreturn().
You can now remove the trace associated to this container:
$ kubectl gadget traceloop delete ef6f2d3f44b555
And if you no more need the traceloop gadget, you can stop it:
$ kubectl gadget traceloop stop
Listing files demo
With traceloop, we can strace pods in the past, even after they terminated.
Example: let’s list the programs in /bin:
$ kubectl run -n test --restart=Never -ti --image=debian mypod -- sh -c 'ls -l /bin | grep mv'
-rwxr-xr-x 1 root root 147080 Sep 24 2020 mv
$ kubectl delete pod mypod
pod "mypod" deleted
Because of the grep mv, we only see one entry. But traceloop can recover other entries:
$ kubectl gadget traceloop list
...
minikube test mypod mypod 9c691a53cd43a0
$ kubectl gadget traceloop show 9c691a53cd43a0
...
1 97851 ls lgetxattr pathname=140729093707632 /bin/more, name=139950439515987 security.selinux, value=94534240126… -61
1 97851 ls getxattr pathname=140729093707632 /bin/more, name=94534234714099 system.posix_acl_access, value=0, si… -61
1 97851 ls statx dfd=4294967196, filename=140729093707632 /bin/vdir, flags=256, mask=606, buffer=140729093707… 0
1 97851 ls lgetxattr pathname=140729093707632 /bin/vdir, name=139950439515987 security.selinux, value=94534240126… -61
1 97851 ls getxattr pathname=140729093707632 /bin/vdir, name=94534234714099 system.posix_acl_access, value=0, si… -61
1 97851 ls statx dfd=4294967196, filename=140729093707632 /bin/rmdir, flags=256, mask=606, buffer=14072909370… 0
1 97851 ls lgetxattr pathname=140729093707632 /bin/rmdir, name=139950439515987 security.selinux, value=9453424012… -61
1 97851 ls getxattr pathname=140729093707632 /bin/rmdir, name=94534234714099 system.posix_acl_access, value=0, s… -61
1 97851 ls statx dfd=4294967196, filename=140729093707632 /bin/tempfile, flags=256, mask=606, buffer=14072909… 0
1 97851 ls lgetxattr pathname=140729093707632 /bin/tempfile, name=139950439515987 security.selinux, value=9453424… -61
1 97851 ls getxattr pathname=140729093707632 /bin/tempfile, name=94534234714099 system.posix_acl_access, value=0… -61
1 97851 ls statx dfd=4294967196, filename=140729093707632 /bin/zfgrep, flags=256, mask=606, buffer=1407290937… 0
1 97851 ls lgetxattr pathname=140729093707632 /bin/zfgrep, name=139950439515987 security.selinux, value=945342401… -61
1 97851 ls getxattr pathname=140729093707632 /bin/zfgrep, name=94534234714099 system.posix_acl_access, value=0, … -61
1
...
With ig
Start a container in interactive mode:
$ docker run -it --rm --name test-traceloop busybox /bin/sh
Start traceloop in another terminal:
$ sudo ig traceloop -c test-traceloop
Run a command inside the container
$ docker run -it --rm --name test-traceloop busybox /bin/sh
/ # ls
Press Ctrl+C on the gadget terminal, it’ll print the systemcalls performed by the container:
$ sudo ig traceloop -c test-traceloop
Tracing syscalls... Hit Ctrl-C to end
^C
CPU PID COMM NAME PARAMS RET
...
6 150829 sh execve filename=18759352 /bin/ls, argv=18759280, envp=18759296 0
6 150829 ls brk brk=0 36…
6 150829 ls brk brk=36440320 36…
...
6 150829 ls write fd=1, buf=5355360 bin dev etc home pro… 158
6 150829 ls exit_group error_code=0 ...
Filter by syscalls
If you are only interested by specific syscalls, you can use the --syscall-filters flag:
$ kubectl gadget traceloop start --syscall-filters openat,write,execve
$ kubectl create ns test
$ kubectl run --restart=Never -n test --rm -ti --image=busybox mypod -- sh -c 'RANDOM=output ; echo "3*7*2" | bc > /tmp/file-$RANDOM ; cat /tmp/file-$RANDOM'
cat: can't open '/tmp/file-3240': No such file or directory
pod default/mypod terminated (Error)
$ kubectl delete pod -n test mypod
pod "mypod" deleted
$ kubectl gadget traceloop list
NODE NAMESPACE POD CONTAINER CONTAINERID
...
minikube test mypod mypod 50f702a9e3f0cd2
$ kubectl gadget traceloop show 50f702a9e3f0cd2
CPU PID COMM SYSCALL PARAMS RET
...
1 118483 sh execve filename="/bin/bc", argv=0x5576c8219668, envp=0x5576c8219698 0
...
5 118470 sh execve filename="/bin/cat", argv=0x5576c8219688, envp=0x5576c82196a0 0
...
1 10685 cat openat dfd=4294967196, filename="/tmp/file-3240", flags=0, mode=0 -1 (…
1 10685 cat write fd=2, buf="cat: can't open '/tmp/file-3240': No such file or director… 60
This options is also available with ig:
$ sudo ig traceloop -c test-traceloop --syscall-filters openat,write
RUNTIME.CONTAINERNAME CPU PID COMM SYSCALL PARAMS RET
# Run a container in a second terminal
$ docker run --rm --name test-traceloop busybox ls
bin
dev
...
# Go back to first terminal
^C
...
test-traceloop 1 135771 ls openat dfd=4294967196, filename=".", flags=591872,… 3
test-traceloop 1 135771 ls write fd=1, buf="bin\ndev\netc\nhome\nlib\nlib64\… 53
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