examples/c: add ksyscall example

Add an example to trace `kill` and `tgkill` syscalls using libbpf's
`SEC(ksyscall)` program type.

Signed-off-by: syogaraj <[email protected]>

Author: syogaraj

examples/c/.gitignore

@@ -9,3 +9,4 @@
 /usdt
 /sockfilter
 /tc
+/ksyscall

examples/c/Makefile

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

examples/c/ksyscall.bpf.c

@@ -0,0 +1,52 @@
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include <bpf/bpf_core_read.h>
+
+#define TASK_COMM_LEN 16
+
+SEC("ksyscall/tgkill")
+int BPF_KSYSCALL(tgkill_entry, pid_t tgid, pid_t tid, int sig) 
+{
+	char comm[TASK_COMM_LEN];
+	__u32 caller_pid = bpf_get_current_pid_tgid() >> 32;
+
+	if (sig == 0) {
+		/*
+			If sig is 0, then no signal is sent, but existence and permission
+			checks are still performed; this can be used to check for the
+			existence of a process ID or process group ID that the caller is
+			permitted to signal.
+		*/
+		return 0;
+	}
+
+	bpf_get_current_comm(&comm, sizeof(comm));
+	bpf_printk("tgkill syscall called by PID %d (%s) for thread id %d with pid %d and signal %d.",
+		   caller_pid, comm, tid, tgid, sig);
+	return 0;
+}
+
+SEC("ksyscall/kill")
+int BPF_KSYSCALL(entry_probe, pid_t pid, int sig) 
+{
+	char comm[TASK_COMM_LEN];
+	__u32 caller_pid = bpf_get_current_pid_tgid() >> 32;
+
+	if (sig == 0) {
+		/*
+			If sig is 0, then no signal is sent, but existence and permission
+			checks are still performed; this can be used to check for the
+			existence of a process ID or process group ID that the caller is
+			permitted to signal.
+		*/
+		return 0;
+	}
+
+	bpf_get_current_comm(&comm, sizeof(comm));
+	bpf_printk("KILL syscall called by PID %d (%s) for PID %d with signal %d.",
+		   caller_pid, comm, pid, sig);
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";

examples/c/ksyscall.c

@@ -0,0 +1,60 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <signal.h>
+#include <string.h>
+#include <errno.h>
+#include <sys/resource.h>
+#include <bpf/libbpf.h>
+#include "ksyscall.skel.h"
+
+static int libbpf_print_fn(enum libbpf_print_level level, const char *format, va_list args)
+{
+	return vfprintf(stderr, format, args);
+}
+
+static volatile sig_atomic_t stop;
+
+static void sig_int(int signo)
+{
+	stop = 1;
+}
+
+int main(int argc, char **argv)
+{
+	struct ksyscall_bpf *skel;
+	int err;
+
+	/* Set up libbpf errors and debug info callback */
+	libbpf_set_print(libbpf_print_fn);
+
+	/* Open load and verify BPF application */
+	skel = ksyscall_bpf__open_and_load();
+	if (!skel) {
+		fprintf(stderr, "Failed to open BPF skeleton\n");
+		return 1;
+	}
+
+	/* Attach tracepoint handler */
+	err = ksyscall_bpf__attach(skel);
+	if (err) {
+		fprintf(stderr, "Failed to attach BPF skeleton\n");
+		goto cleanup;
+	}
+
+	if (signal(SIGINT, sig_int) == SIG_ERR) {
+		fprintf(stderr, "can't set signal handler: %s\n", strerror(errno));
+		goto cleanup;
+	}
+
+	printf("Successfully started! Please run `sudo cat /sys/kernel/debug/tracing/trace_pipe` "
+	       "to see output of the BPF programs.\n");
+
+	while (!stop) {
+		fprintf(stderr, ".");
+		sleep(1);
+	}
+
+cleanup:
+	ksyscall_bpf__destroy(skel);
+	return -err;
+}