examples/c: Add task_iter

This is a simple example of using BPF iterators, which iterates
over all tasks on the host.

Signed-off-by: Jordan Rome <[email protected]>

README.md

@@ -303,6 +303,21 @@ interface:lo    protocol: UDP   127.0.0.1:51845(src) -> 127.0.0.1:53(dst)
 interface:lo    protocol: UDP   127.0.0.1:41552(src) -> 127.0.0.1:53(dst)
 ```
 
+## task_iter
+
+`task_iter` is an example of using [BPF Iterators](https://docs.kernel.org/bpf/bpf_iterators.html). 
+This example iterates over all tasks on the host and gets their pid, process name, 
+kernel stack, and their state. Note: you can use BlazeSym to symbolize the kernel stacktraces 
+(like in `profile`) but that code is omitted for simplicity.
+
+```shell
+$ sudo ./task_iter
+Task Info. Pid: 3647645. Process Name: TTLSFWorker59. Kernel Stack Len: 3. State: INTERRUPTIBLE
+Task Info. Pid: 1600495. Process Name: tmux: client. Kernel Stack Len: 6. State: INTERRUPTIBLE
+Task Info. Pid: 1600497. Process Name: tmux: server. Kernel Stack Len: 0. State: RUNNING
+Task Info. Pid: 1600498. Process Name: bash. Kernel Stack Len: 5. State: INTERRUPTIBLE
+```
+
 # Building
 
 libbpf-bootstrap supports multiple build systems that do the same thing.

examples/c/.gitignore

@@ -10,5 +10,6 @@
 /sockfilter
 /tc
 /ksyscall
+/task_iter
 /cmake-build-debug/
-/cmake-build-release/
+/cmake-build-release/

examples/c/Makefile

@@ -24,7 +24,7 @@ INCLUDES := -I$(OUTPUT) -I../../libbpf/include/uapi -I$(dir $(VMLINUX)) -I$(LIBB
 CFLAGS := -g -Wall
 ALL_LDFLAGS := $(LDFLAGS) $(EXTRA_LDFLAGS)
 
-APPS = minimal minimal_legacy bootstrap uprobe kprobe fentry usdt sockfilter tc ksyscall
+APPS = minimal minimal_legacy bootstrap uprobe kprobe fentry usdt sockfilter tc ksyscall task_iter
 
 CARGO ?= $(shell which cargo)
 ifeq ($(strip $(CARGO)),)

examples/c/task_iter.bpf.c

@@ -0,0 +1,67 @@
+// SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause
+/* Copyright (c) 2023 Meta */
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include <bpf/bpf_core_read.h>
+#include "task_iter.h"
+
+char LICENSE[] SEC("license") = "Dual BSD/GPL";
+
+struct {
+	__uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, __u32);
+	__type(value, struct task_info);
+} task_info_buf SEC(".maps");
+
+struct task_struct___post514 {
+	unsigned int __state;
+} __attribute__((preserve_access_index));
+
+struct task_struct___pre514 {
+	long state;
+} __attribute__((preserve_access_index));
+
+static __u32 get_task_state(void *arg)
+{
+	if (bpf_core_field_exists(struct task_struct___pre514, state)) {
+		struct task_struct___pre514 *task = arg;
+		return task->state;
+	} else {
+		struct task_struct___post514 *task = arg;
+		return task->__state;
+	}
+}
+
+static __u32 zero = 0;
+
+SEC("iter/task")
+int get_tasks(struct bpf_iter__task *ctx)
+{
+	struct seq_file *seq = ctx->meta->seq;
+	struct task_struct *task = ctx->task;
+	struct task_info *t;
+	long res;
+
+	if (!task) {
+		return 0;
+	}
+
+	t = bpf_map_lookup_elem(&task_info_buf, &zero);
+	if (!t) {
+		return 0;
+	}
+
+	t->pid = task->tgid;
+	t->tid = task->pid;
+	t->state = get_task_state(task);
+
+	bpf_probe_read_kernel_str(t->comm, TASK_COMM_LEN, task->comm);
+
+	res = bpf_get_task_stack(task, t->kstack, sizeof(__u64) * MAX_STACK_LEN, 0);
+	t->kstack_len = res <= 0 ? res : res / sizeof(t->kstack[0]);
+
+	bpf_seq_write(seq, t, sizeof(struct task_info));
+	return 0;
+}

examples/c/task_iter.c

@@ -0,0 +1,119 @@
+// SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
+/* Copyright (c) 2023 Meta */
+#include <argp.h>
+#include <signal.h>
+#include <stdio.h>
+#include <sys/resource.h>
+#include <bpf/bpf.h>
+#include <bpf/libbpf.h>
+#include <unistd.h>
+#include "task_iter.h"
+#include "task_iter.skel.h"
+
+static struct env {
+	bool verbose;
+} env;
+
+static int libbpf_print_fn(enum libbpf_print_level level, const char *format, va_list args)
+{
+	if (level == LIBBPF_DEBUG && !env.verbose)
+		return 0;
+	return vfprintf(stderr, format, args);
+}
+
+static volatile bool exiting = false;
+
+static void sig_handler(int sig)
+{
+	exiting = true;
+}
+
+static const char *get_task_state(__u32 state)
+{
+	// taken from
+	// https://elixir.bootlin.com/linux/latest/source/include/linux/sched.h#L85
+	// there are a lot more states not covered here but these are common ones
+	switch (state) {
+	case 0x0000:
+		return "RUNNING";
+	case 0x0001:
+		return "INTERRUPTIBLE";
+	case 0x0002:
+		return "UNINTERRUPTIBLE";
+	case 0x0200:
+		return "WAKING";
+	case 0x0400:
+		return "NOLOAD";
+	case 0x0402:
+		return "IDLE";
+	case 0x0800:
+		return "NEW";
+	default:
+		return "<unknown>";
+	}
+}
+
+int main(int argc, char **argv)
+{
+	struct task_iter_bpf *skel;
+	struct task_info buf;
+	int get_tasks_fd;
+	ssize_t ret;
+	int err;
+
+	/* Set up libbpf errors and debug info callback */
+	libbpf_set_print(libbpf_print_fn);
+
+	/* Cleaner handling of Ctrl-C */
+	signal(SIGINT, sig_handler);
+	signal(SIGTERM, sig_handler);
+
+	/* Open, load, and verify BPF application */
+	skel = task_iter_bpf__open_and_load();
+	if (!skel) {
+		fprintf(stderr, "Failed to open and load BPF skeleton\n");
+		goto cleanup;
+	}
+
+	/* Attach tracepoints */
+	err = task_iter_bpf__attach(skel);
+	if (err) {
+		fprintf(stderr, "Failed to attach BPF skeleton\n");
+		goto cleanup;
+	}
+
+	get_tasks_fd = bpf_iter_create(bpf_link__fd(skel->links.get_tasks));
+	if (get_tasks_fd < 0) {
+		err = -1;
+		fprintf(stderr, "Failed to create iter\n");
+		goto cleanup;
+	}
+
+	while (true) {
+		ret = read(get_tasks_fd, &buf, sizeof(struct task_info));
+		if (ret < 0) {
+			if (errno == EAGAIN) {
+				continue;
+			}
+			err = -errno;
+			break;
+		}
+		if (ret == 0) {
+			break;
+		}
+		if (buf.kstack_len <= 0) {
+			printf("Error getting kernel stack for task. Task Info. Pid: %d. Process Name: %s. Kernel Stack Error: %d. State: %s \n",
+			       buf.pid, buf.comm, buf.kstack_len, get_task_state(buf.state));
+		} else {
+			printf("Task Info. Pid: %d. Process Name: %s. Kernel Stack Len: %d. State: %s \n",
+			       buf.pid, buf.comm, buf.kstack_len, get_task_state(buf.state));
+		}
+	}
+
+cleanup:
+	/* Clean up */
+	close(get_tasks_fd);
+	task_iter_bpf__destroy(skel);
+
+	return err < 0 ? -err : 0;
+}

examples/c/task_iter.h

@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
+/* Copyright (c) 2023 Meta */
+
+#define TASK_COMM_LEN 16
+#define MAX_STACK_LEN 127
+
+struct task_info {
+	pid_t pid;
+	pid_t tid;
+	__u32 state;
+	char comm[TASK_COMM_LEN];
+
+	int kstack_len;
+
+	__u64 kstack[MAX_STACK_LEN];
+};