R&D Center Base System Development Department Copyright 2019 Sony Corporation Debug application inside Kubernetes using Linux Kernel tools Kenta Tada R&D Center Sony Corporation
Background ⚫We developed the lightweight and secure runC-based container platform for embedded system ⚫That platform needs to launch secure(restricted and rootless) containers for third party ⚫We developed the ftrace-based system call analyzer to generate secure configs too ⚫Currently, we are porting those tools to our Kubernetes environments
Use kernel tools to trace applications transparently ⚫Existing methods are very useful but additional packages and additional capabilities are needed to debug ⚫On the other hand, we just want to investigate system calls sometimes • Needed capabilities • Correct file permissions • seccomp settings for security ⚫Let’s use kernel tools to trace applications transparently
Kernel technologies our syscall analyzer used ⚫ftrace • Tracing framework for the Linux kernel • ftrace can collect various information although it is typically considered the function tracer • Easy to set up(Just write settings to tracefs) –No eBPF compiler(No LLVM) ⚫Tracepoints • Static trace points inside kernel
What is needed to integrate 1. Divide ftrace ring buffer using ftrace instances for for each containers • https://speakerdeck.com/kentatada/container-debug-using-ftrace 2. Set up ftrace inside container startup today’s topic
Set up ftrace inside container startup ⚫How to insert the ftrace setting tool before container startup ⚫How to get PID1’s process inside the container ⚫What ftrace settings are needed to trace container’s processes
How to insert the ftrace setting tool before container startup ⚫Container Lifecycle and related hook ⚫Our ftrace-based tracer should be executed at prestart because we want to trace from the process start like strace process lifetime poststart poststop prestart process start process stop Setup ftrace Collect logs
How to get PID1’s process inside the container ⚫From OCI runtime spec, the state of the container which includes container initial PID must be passed to hooks over stdin • https://github.com/opencontainers/runtime- spec/blob/master/config.md ⚫So, we get the info about PID1’s process inside the container from stdin ⚫This approach can be useful on any low level runtimes if they comply with OCI runtime spec
What ftrace settings are needed to trace container’s processes ⚫Enable system call events which you want to trace (e.g. From /sys/kernel/debug/tracing/events/syscalls) ⚫Only trace the specified PID (e.g. # echo [PID] > /sys/kernel/debug/tracing/set_event_pid) ⚫Trace processes which PID of “set_event_pid” forked (e.g. echo 1 > /sys/kernel/debug/tracing/options/event-fork)
How to set up prestart hook in Kubernetes ⚫Kubernetes Pod Lifecycle and related hook ⚫Kubernetes did not provide prestart hook • https://github.com/kubernetes/kubernetes/issues/140 ⚫Next, we investigate prestart hook in the layer of high level runtime process lifetime process start process stop prestop poststart
How to set up prestart hook in containerd ⚫ In the first place, CRI does not currently provide a way to specify the hook into the container’s config.json ⚫ High level runtime has their own implementation ⚫ Below is the containerd’s ongoing project • https://github.com/containerd/cri/pull/1248 • https://github.com/containerd/cri/issues/405
How to set up prestart hook in CRI-O ⚫ CRI-O has already provided their own solution "oci-hooks“ • podman has the same feature ⚫ oci-hooks provides a way for users to configure the intended hooks for Open Container Initiative containers so they will only be executed for containers that need their functionality, and then only for the stages where they're needed https://github.com/containers/libpod/blob/master/pkg/hooks/docs/oci-hooks.5.md
oci-seccomp-bpf-hook ⚫oci-seccomp-bpf-hook generates seccomp profiles by tracing the syscalls made by the container using eBPF ⚫The perf is used to log syscalls ⚫This tool has a few limitations • Needs CAP_SYS_ADMIN to run • Compiles C code on the fly using LLVM • Cannot use podman run --rm along with this ability
When oci-ftrace-syscall-analyzer is used ⚫Your production system doesn’t want to provide privileges with users ⚫Your production kernel didn’t prepare for eBPF configurations ⚫Your production system doesn’t want to use LLVM • GCC will support the BPF backend? –Compiling to BPF with GCC : https://lwn.net/Articles/800606/
What is the problem? ⚫Process core dump will be recorded at the path of /proc/sys/kernel/core_pattern ⚫But containers have their own Linux namespace and kernel does not support Linux namespace ⚫Kubernetes users cannot get their process core dump. ⚫Kubernetes issues •https://github.com/kubernetes/kubernetes/issues/48787
Wrap up ⚫oci-ftrace-syscall-analyzer is seamlessly integrated with Kubernetes using CRI-O • https://github.com/KentaTada/oci-ftrace-syscall-analyzer ⚫We should consider how to get process coredump on Kubernetes
Challenging of oci-ftrace-syscall-analyzer ⚫Integrate our tool with containerd ⚫Implement the user space logging facility originated from our internal container tools ⚫Use kprobes to hook system call to investigate syscall args ⚫Implement seccomp generator ⚫Get rid of unnecessary syscall logs recorded from prestart to actual runC’s exec • oci-seccomp-bpf-hook used prctl(2) as starting point. Is it actually standard??
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