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Hello world gadget

This is a short getting started guide to write your first gadget. This guide will get you familiar with the key concepts by implementing a simplified version of the "trace open" (opensnoop) tool.

Starting from a template​

If you want to create a new repository for your gadget, you can use the gadget-template repository. This is a GitHub tempate repository.

You can also look for examples in gadgets published on Artifact Hub:

Artifact Hub: Gadgets

Starting from scratch​

If you already have a git repository for your project and want to add a gadget to it, you can start from scratch. The rest of this guide assumes you will start from scratch.

The first step is to create an empty folder where the source code of the gadget will be stored:

$ mkdir mygadget

Implementing the eBPF program​

The eBPF code contains the source code for the programs that are injected in the kernel to collect information. Let's create a file called program.bpf.c and put the following contents in there.

The first thing we need is to include some header files.

// Kernel types definitions
// Check https://blog.aquasec.com/vmlinux.h-ebpf-programs for more details
#include <vmlinux.h>

// eBPF helpers signatures
// Check https://man7.org/linux/man-pages/man7/bpf-helpers.7.html to learn
// more about different available helpers
#include <bpf/bpf_helpers.h>

// Inspektor Gadget buffer
#include <gadget/buffer.h>

// Inspektor Gadget macros
#include <gadget/macros.h>

Then, we have to specify a structure with all the information our gadget will provide. Let's only put the pid for the time being.

struct event {
	__u32 pid;
};

Then, create a buffer eBPF map to send events to user space:

// events is the name of the buffer map and 1024 * 256 is its size.
GADGET_TRACER_MAP(events, 1024 * 256);

This macro will automatically create a ring buffer if the kernel supports it. Otherwise, a perf array will be created.

And define a tracer by using the GADGET_TRACER macro with the following parameters:

  • Tracer's Name: open
  • Buffer Map Name: events
  • Event Structure Name: event
// Define a tracer
GADGET_TRACER(open, events, event);

After that, we need to define a program that is attached to a hook that provides the information we need, in this case we'll attach to a tracepoint that is called each time the openat() syscall is executed.

This program collects the information to fill the event (only pid for now), and then calls gadget_submit_buf() helper to send the event to user space.

SEC("tracepoint/syscalls/sys_enter_openat")
int enter_openat(struct syscall_trace_enter *ctx)
{
	struct event *event;

	event = gadget_reserve_buf(&events, sizeof(*event));
	if (!event)
		return 0;

	event->pid = bpf_get_current_pid_tgid() >> 32;

	gadget_submit_buf(ctx, &events, event, sizeof(*event));

	return 0;
}

Finally, it's needed to define the license of the eBPF code.

char LICENSE[] SEC("license") = "GPL";

The full file should look like:

// program.bpf.c

// Kernel types definitions
// Check https://blog.aquasec.com/vmlinux.h-ebpf-programs for more details
#include <vmlinux.h>

// eBPF helpers signatures
// Check https://man7.org/linux/man-pages/man7/bpf-helpers.7.html to learn
// more about different available helpers
#include <bpf/bpf_helpers.h>

// Inspektor Gadget buffer
#include <gadget/buffer.h>

// Inspektor Gadget macros
#include <gadget/macros.h>

struct event {
	__u32 pid;
};

// events is the name of the buffer map and 1024 * 256 is its size.
GADGET_TRACER_MAP(events, 1024 * 256);

// Define a tracer
GADGET_TRACER(open, events, event);

SEC("tracepoint/syscalls/sys_enter_openat")
int enter_openat(struct syscall_trace_enter *ctx)
{
	struct event *event;

	event = gadget_reserve_buf(&events, sizeof(*event));
	if (!event)
		return 0;

	event->pid = bpf_get_current_pid_tgid() >> 32;

	gadget_submit_buf(ctx, &events, event, sizeof(*event));

	return 0;
}

char LICENSE[] SEC("license") = "GPL";

Check Gadget eBPF API to learn all functions, macros and types that Inspektor Gadget exposes to the eBPF programs.

Building the gadget for the first time​

We can now compile our gadget. You don't need to have any build tools installed on the machine, the image build by default uses docker to run a container with all dependencies to compile the code.

$ cd mygadget
$ sudo ig image build -t mygadget:latest .
Successfully built ghcr.io/inspektor-gadget/gadget/mygadget:latest@sha256:dd3f5c357983bb863ef86942e36f4c851933eec4b32ba65ee375acb1c514f628

Take into account that it is possible to customize the build process by defining a build.yaml file. Check the Building a Gadget section for more details.

(Optional) Pushing the gadget image to a container registry​

You could push the gadget to a remote container registry. If you're using the same machine for building and running the gadget, this step can be skipped.

$ sudo ig image tag mygadget:latest ghcr.io/my-org/mygadget:latest
Successfully tagged with ghcr.io/my-org/mygadget:latest@sha256:dd3f5c357983bb863ef86942e36f4c851933eec4b32ba65ee375acb1c514f628

$ sudo ig image push ghcr.io/my-org/mygadget:latest
Pushing ghcr.io/my-org/mygadget:latest...
Successfully pushed ghcr.io/my-org/mygadget:latest@sha256:dd3f5c357983bb863ef86942e36f4c851933eec4b32ba65ee375acb1c514f628

For the sake of simplicity this guide doesn't cover signing the gadget image, however, we strongly encourage you to sign them. Please check signing to get more details.

Running the gadget​

We're now all set to run our gadget for the first time.

$ sudo ig run mygadget:latest --verify-image=false
PID
1113
1113
1113
1113
1113
1113
1113
1113
1113
1113
1113
1113
1113
1113
1113
1219
220121

Great, our program already shows the PID! Can we improve it further?

Adding more information to the gadget​

Let's add some more information to our event, like command and file name.

Add the fields in the event structure.

#define NAME_MAX 255

struct event {
	__u32 pid;
	char comm[TASK_COMM_LEN];
	char filename[NAME_MAX];
};

Now create the logic to fill those fields in the enter_openat program. Insert them after you have reserved space for your event structure and before you submit the buffer.

	bpf_get_current_comm(event->comm, sizeof(event->comm));
	bpf_probe_read_user_str(event->filename, sizeof(event->filename), (const char *)ctx->args[1]);

Build and run the gadget again. Now it provides more information.

$ sudo ig image build -t mygadget:latest .
....
$ sudo ig run mygadget:latest --verify-image=false
PID                      COMM                     FILENAME
11305                    Chrome_ChildIOT          /dev/shm/.org.chromium.…
11305                    ThreadPoolForeg          /home/mvb/.config/Slack…
11305                    Chrome_ChildIOT          /dev/shm/.org.chromium.…
11305                    ThreadPoolForeg          /home/mvb/.config/Slack…
11305                    Chrome_ChildIOT          /dev/shm/.org.chromium.…
1349                     containerd               /var/lib/containerd/io.…
1349                     containerd               /var/lib/containerd/io.…
1349                     containerd               /var/lib/containerd/io.…

Creating a metadata file​

The above formatting is not totally great, the pid column is taking a lot of space while the filename is being trimmed. The metadata file contains extra information about the gadget, among other things, it can be used to specify the format to be used.

An initial version of the metadata file can be created by passing --update-metadata to the build command:

$ sudo ig image build . -t mygadget --update-metadata

It'll create a gadget.yaml file:

name: 'TODO: Fill the gadget name'
description: 'TODO: Fill the gadget description'
homepageURL: 'TODO: Fill the gadget homepage URL'
documentationURL: 'TODO: Fill the gadget documentation URL'
sourceURL: 'TODO: Fill the gadget source code URL'
datasources:
  open:
    fields:
      comm:
        annotations:
          description: 'TODO: Fill field description'
      filename:
        annotations:
          description: 'TODO: Fill field description'
      pid:
        annotations:
          description: 'TODO: Fill field description'

Let's edit the file to customize the output. We define some templates for well-known fields like pid, comm, etc.

name: mygadget
description: Example gadget
homepageURL: http://mygadget.com
documentationURL: https://mygadget.com/docs
sourceURL: https://github.com/my-org/mygadget/
datasources:
  open:
    fields:
      comm:
        annotations:
          description: Name of the process opening a file
          template: comm
      filename:
        annotations:
           description: Path of the file being opened
           columns.width: 64
      pid:
        annotations:
          description: PID of the process opening a file
          template: pid

Now we can build and run the gadget again

$ sudo ig image build . -t mygadget
...

$ sudo ig run mygadget:latest --verify-image=false
              PID COMM              FILENAME
             1094 systemd-oomd      /sys/fs/cgroup/user.slice/user-1001.slice/[email protected]/memor…
             1094 systemd-oomd      /sys/fs/cgroup/user.slice/user-1001.slice/[email protected]/memor…
             1094 systemd-oomd      /sys/fs/cgroup/user.slice/user-1001.slice/[email protected]/memor…
             1094 systemd-oomd      /sys/fs/cgroup/user.slice/user-1001.slice/[email protected]/memor…
             1094 systemd-oomd      /sys/fs/cgroup/user.slice/user-1001.slice/[email protected]/memor…
             1094 systemd-oomd      /sys/fs/cgroup/user.slice/user-1001.slice/[email protected]/memor…
             1094 systemd-oomd      /proc/meminfo
            20100 tmux: server      /proc/118135/cmdline
             1094 systemd-oomd      /proc/meminfo
             1094 systemd-oomd      /proc/meminfo
             5803 FSBroker114558    /proc/114558/statm
             5803 FSBroker114558    /proc/114558/statm
             5803 FSBroker114558    /proc/114558/smaps
            20100 tmux: server      /proc/118135/cmdline
             1094 systemd-oomd      /proc/meminfo
           118137 ig                /proc

Now the output is much better.

Filtering and container enrichement​

The gadget we created provides information about all events happening on the host, however it (a) doesn't provide any information about the container generating the event nor (b) allows it to filter events by a given container.

Inspektor Gadget provides the logic to filter and enrich events with container information. The first step is to include these two addional header files:

// Inspektor Gadget filtering
#include <gadget/mntns_filter.h>

// Inspektor Gadget types
#include <gadget/types.h>
  • gadget/mntns_filter.h: Defines an eBPF map and some helper functions used to filter events by containers
  • gadget/types.h: Inspektor Gadget specific types, like gadget_mntns_id.

Add the following field to the event structure.

struct event {
	...
	gadget_mntns_id mntns_id;
	...
}

And then, on the program, set this field. gadget_get_mntns_id is a helper function provided by Inspektor Gadget to get the current mount namespace.

	struct event *event;
	u64 mntns_id;

	mntns_id = gadget_get_mntns_id();

Finally, we need to discard the events we're not interested in:

	if (gadget_should_discard_mntns_id(mntns_id))
		return 0;

	event = gadget_reserve_buf(&events, sizeof(*event));
	if (!event)
		return 0;

	event->mntns_id = mntns_id;
	...

The gadget_should_discard_mntns_id function is provided to determine if a given event should be traced or not. This function should be called as early as possible in the program to avoid unnecessary work.

After adding the gadget_mntns_id field to the event structure, compiling and running again, Inspektor Gadget will automatically add the container name column to the output:

$ sudo ig run mygadget:latest --verify-image=false
RUNTIME.CONTAINERNAME        PID             COMM            FILENAME                        MNTNS_ID

However, the output is empty. It's because now the gadget is filtering only events generated by containers. Create a container and run some commands there:

$ docker run --rm -ti --name=mycontainer busybox cat /dev/null

Only events generated in containers are now printed, and they include the name of the container generating them.

$ sudo ig run mygadget:latest --verify-image=false
RUNTIME.CONTAINERNAME MNTNS_ID            PID COMM        FILENAME
mycontainer           4026536181       119341 runc:[2:IN… /proc/self/fd
mycontainer           4026536181       119341 sh          /etc/ld.so.cache
mycontainer           4026536181       119341 sh          /lib/x86_64-linux-gnu/glibc-hwcaps/x86-64-v…
mycontainer           4026536181       119341 sh          /lib/x86_64-linux-gnu/glibc-hwcaps/x86-64-v…
mycontainer           4026536181       119341 sh          /lib/x86_64-linux-gnu/tls/x86_64/x86_64/lib…
mycontainer           4026536181       119341 sh          /lib/x86_64-linux-gnu/tls/x86_64/libm.so.6
mycontainer           4026536181       119341 sh          /lib/x86_64-linux-gnu/tls/x86_64/libm.so.6
mycontainer           4026536181       119341 sh          /lib/x86_64-linux-gnu/tls/libm.so.6
mycontainer           4026536181       119341 sh          /lib/x86_64-linux-gnu/x86_64/x86_64/libm.so…
mycontainer           4026536181       119341 sh          /lib/x86_64-linux-gnu/x86_64/libm.so.6
mycontainer           4026536181       119341 sh          /lib/x86_64-linux-gnu/x86_64/libm.so.6
mycontainer           4026536181       119341 sh          /lib/x86_64-linux-gnu/libm.so.6
mycontainer           4026536181       119341 sh          /usr/lib/x86_64-linux-gnu/glibc-hwcaps/x86-…
mycontainer           4026536181       119341 sh          /usr/lib/x86_64-linux-gnu/glibc-hwcaps/x86-
...

Additionally, after adding the gadget_mntns_id field to the event structure, Inspektor Gadget will automatically add the flag --containername/-c to the gadget. This flag allows filtering events by container name.

The following command doesn't show any event as there is no container with the specified name:

$ sudo ig run mygadget:latest -c non_existing_container --verify-image=false
RUNTIME.CONTAINERNAME MNTNS_ID            PID COMM        FILENAME

Updating the gadget​

Some times we have to update our gadgets, like adding more fields to the generated event for instance. Let's add the following fields to the event struct:

struct event {
	...
	__u32 uid;
	__u32 gid;
	...
}

and add the logic in the eBPF program to fill them:

	__u64 uid_gid = bpf_get_current_uid_gid();
	event->uid = (__u32)uid_gid;
	event->gid = (__u32)(uid_gid >> 32);

Let's build the gadget with the --update-metadata file, so our new fields are automatically added to the metadata file. Notice the -v option is used to get debugging messages.

$ sudo ig image build . -t mygadget --update-metadata -v
...
DEBU[0000] Metadata file found, updating it
DEBU[0000] Adding field "mntns_id"
DEBU[0000] Field "pid" already exists, skipping
DEBU[0000] Field "comm" already exists, skipping
DEBU[0000] Field "filename" already exists, skipping
DEBU[0000] Adding field "uid"
DEBU[0000] Adding field "gid"
...

The uid, gid and mntns_id (added in the previous step fields were added to the metadata file:

      gid:
        annotations:
          description: 'TODO: Fill field description'
      mntns_id:
        annotations:
          description: 'TODO: Fill field description'
      pid:
        annotations:
          description: PID of the process opening a file
          template: pid
      uid:
        annotations:
          description: 'TODO: Fill field description'

Edit them, build and run the gadget again:

      comm:
        annotations:
          description: Name of the process opening a file
          template: comm
      filename:
        annotations:
          description: Path of the file being opened
          columns.width: 64
      gid:
        annotations:
          description: Group ID opening the file
          template: uid
      mntns_id:
        annotations:
          description: Mount namespace inode id
          template: ns
      pid:
        annotations:
          description: PID of the process opening a file
          template: pid
      uid:
        annotations:
          description: User ID opening the file
          template: uid
$ sudo ig image build . -t mygadget --update-metadata -v
...

$ sudo ig run mygadget:latest --verify-image=false
RUNTIME.CONTAINERN…        PID COMM       FILENAME                                       UID       GID

Now, the UID and GID columns have the expected format. Notice also that the MNTNS_ID column is not showed because the template ns hides it by default.

Closing​

Congratulations! You've implemented your first gadget. Check out our documentation to get more information.