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eBPF Tutorial by Example 6: Capturing Signal Sending and Store State with Hash Maps

eBPF (Extended Berkeley Packet Filter) is a powerful network and performance analysis tool on the Linux kernel that allows developers to dynamically load, update, and run user-defined code at runtime.

This article is the sixth part of the eBPF Tutorial by Example. It mainly introduces how to implement an eBPF tool that captures a collection of system calls that send signals to processes and uses a hash map to store state.

sigsnoop

The example code is as follows:

#include <vmlinux.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_tracing.h>

#define MAX_ENTRIES 10240
#define TASK_COMM_LEN 16

struct event {
 unsigned int pid;
 unsigned int tpid;
 int sig;
 int ret;
 char comm[TASK_COMM_LEN];
};

struct {
 __uint(type, BPF_MAP_TYPE_HASH);
 __uint(max_entries, MAX_ENTRIES);
 __type(key, __u32);
 __type(value, struct event);
} values SEC(".maps");


static int probe_entry(pid_t tpid, int sig)
{
 struct event event = {};
 __u64 pid_tgid;
 __u32 tid;

 pid_tgid = bpf_get_current_pid_tgid();
 tid = (__u32)pid_tgid;
 event.pid = pid_tgid >> 32;
 event.tpid = tpid;
 event.sig = sig;
 bpf_get_current_comm(event.comm, sizeof(event.comm));
 bpf_map_update_elem(&values, &tid, &event, BPF_ANY);
 return 0;
}

static int probe_exit(void *ctx, int ret)
{
 __u64 pid_tgid = bpf_get_current_pid_tgid();
 __u32 tid = (__u32)pid_tgid;
 struct event *eventp;

 eventp = bpf_map_lookup_elem(&values, &tid);
 if (!eventp)
  return 0;

 eventp->ret = ret;
 bpf_printk("PID %d (%s) sent signal %d ",
           eventp->pid, eventp->comm, eventp->sig);
 bpf_printk("to PID %d, ret = %d",
           eventp->tpid, ret);

cleanup:
 bpf_map_delete_elem(&values, &tid);
 return 0;
}

SEC("tracepoint/syscalls/sys_enter_kill")
int kill_entry(struct trace_event_raw_sys_enter *ctx)
{
 pid_t tpid = (pid_t)ctx->args[0];
 int sig = (int)ctx->args[1];

 return probe_entry(tpid, sig);
}

SEC("tracepoint/syscalls/sys_exit_kill")
int kill_exit(struct trace_event_raw_sys_exit *ctx)
{
 return probe_exit(ctx, ctx->ret);
}

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

The above code defines an eBPF program for capturing system calls that send signals to processes, including kill, tkill, and tgkill. It captures the enter and exit events of system calls by using tracepoints, and executes specified probe functions such as probe_entry and probe_exit when these events occur.

In the probe function, we use the bpf_map to store the captured event information, including the process ID of the sending signal, the process ID of the receiving signal, the signal value, and the name of the executable for the current task. When the system call exits, we retrieve the event information stored in the bpf_map and use bpf_printk to print the process ID, process name, sent signal, and return value of the system call.

Finally, we also need to use the SEC macro to define the probe and specify the name of the system call to be captured and the probe function to be executed.

eunomia-bpf is an open-source eBPF dynamic loading runtime and development toolchain that combines with Wasm. Its purpose is to simplify the development, building, distribution, and running of eBPF programs. You can refer to https://github.com/eunomia-bpf/eunomia-bpf for downloading and installing the ecc compilation toolchain and ecli runtime. We use eunomia-bpf to compile and run this example.

Compile and run the above code:

docker run -it -v `pwd`/:/src/ ghcr.io/eunomia-bpf/ecc-`uname -m`:latest

or

$ ecc sigsnoop.bpf.c
Compiling bpf object...
Generating export types...
Packing ebpf object and config into package.json...
$ sudo ecli run package.json
Running eBPF program...

After running this program, you can view the output of the eBPF program by checking the /sys/kernel/debug/tracing/trace_pipe file:

$ sudo cat /sys/kernel/debug/tracing/trace_pipe
    systemd-journal-363     [000] d...1   672.563868: bpf_trace_printk: PID 363 (systemd-journal) sent signal 0
     systemd-journal-363     [000] d...1   672.563869: bpf_trace_printk: to PID 1400, ret = 0
     systemd-journal-363     [000] d...1   672.563870: bpf_trace_printk: PID 363 (systemd-journal) sent signal 0
     systemd-journal-363     [000] d...1   672.563870: bpf_trace_printk: to PID 1527, ret = -3

Summary

This article mainly introduces how to implement an eBPF tool to capture the collection of system calls sent by processes using signals and save the state using a hash map. Using a hash map requires defining a struct:

struct {
 __uint(type, BPF_MAP_TYPE_HASH);
 __uint(max_entries, MAX_ENTRIES);
 __type(key, __u32);
 __type(value, struct event);
} values SEC(".maps");

And using corresponding APIs for access, such as bpf_map_lookup_elem, bpf_map_update_elem, bpf_map_delete_elem, etc.

If you want to learn more about eBPF knowledge and practice, you can visit our tutorial code repository https://github.com/eunomia-bpf/bpf-developer-tutorial or website https://eunomia.dev/tutorials/ to get more examples and complete tutorials.