tetragon: Add support to detect available exit probes

Adding support to use disassociate_ctty exit probe in case
acct_process is missing in kernel.

Detecting both probes during base sensor setup and picking
up available function and adding new log line:

time="2023-12-11T14:44:28Z" level=info msg="Exit probe on acct_process" failed=false

The failed value will be true when the kallsyms detection
fails.

Signed-off-by: Jiri Olsa <[email protected]>

bpf/process/bpf_exit.c

@@ -43,10 +43,28 @@ char _license[] __attribute__((section("license"), used)) = "Dual BSD/GPL";
  * we are the last one of the thread group.
  */
 __attribute__((section("kprobe/acct_process"), used)) int
-event_exit(struct pt_regs *ctx)
+event_exit_acct_process(struct pt_regs *ctx)
 {
 	__u64 pid_tgid = get_current_pid_tgid();
 
 	event_exit_send(ctx, pid_tgid >> 32);
 	return 0;
 }
+
+/*
+ * Hooking on acct_process kernel function, which is called on the task's
+ * exit path once the task is the last one in the group. It's stable since
+ * v4.19, so it's safe to hook for us.
+ *
+ * It's called with on_exit argument != 0 when called from do_exit
+ * function with same conditions like for acct_process described above.
+ */
+__attribute__((section("kprobe/disassociate_ctty"), used)) int
+event_exit_disassociate_ctty(struct pt_regs *ctx)
+{
+	int on_exit = (int)PT_REGS_PARM1_CORE(ctx);
+
+	if (on_exit)
+		event_exit_send(ctx, get_current_pid_tgid() >> 32);
+	return 0;
+}

pkg/ksyms/ksyms.go

@@ -193,3 +193,12 @@ func (k *Ksyms) getFnOffset(addr uint64) (*FnOffset, error) {
 		Offset:  addr - sym.addr,
 	}, nil
 }
+
+func (k *Ksyms) IsAvailable(name string) bool {
+	for _, sym := range k.table {
+		if sym.name == name {
+			return true
+		}
+	}
+	return false
+}

pkg/sensors/base/base.go

@@ -4,7 +4,12 @@
 package base
 
 import (
+	"log"
+	"sync"
+
 	"github.com/cilium/tetragon/pkg/kernels"
+	"github.com/cilium/tetragon/pkg/ksyms"
+	"github.com/cilium/tetragon/pkg/logger"
 	"github.com/cilium/tetragon/pkg/sensors"
 	"github.com/cilium/tetragon/pkg/sensors/program"
 )
@@ -64,6 +69,26 @@ var (
 	sensorInit sync.Once
 )
 
+func setupExitProgram() {
+	ks, err := ksyms.KernelSymbols()
+	if err == nil {
+		has_acct_process := ks.IsAvailable("acct_process")
+		has_disassociate_ctty := ks.IsAvailable("disassociate_ctty")
+
+		/* Preffer acct_process over disassociate_ctty */
+		if has_acct_process {
+			Exit.Attach = "acct_process"
+			Exit.Label = "kprobe/acct_process"
+		} else if has_disassociate_ctty {
+			Exit.Attach = "disassociate_ctty"
+			Exit.Label = "kprobe/disassociate_ctty"
+		} else {
+			log.Fatal("Failed to detect exit probe symbol.")
+		}
+	}
+	logger.GetLogger().Infof("Exit probe on %s", Exit.Attach)
+}
+
 func GetExecveMap() *program.Map {
 	return ExecveMap
 }
@@ -104,6 +129,7 @@ func GetDefaultMaps() []*program.Map {
 // GetInitialSensor returns the base sensor
 func GetInitialSensor() *sensors.Sensor {
 	sensorInit.Do(func() {
+		setupExitProgram()
 		sensor.Progs = GetDefaultPrograms()
 		sensor.Maps = GetDefaultMaps()
 	})

pkg/testutils/sensors/load.go

@@ -131,7 +131,7 @@ func mergeSensorMaps(t *testing.T, maps1, maps2 []SensorMap, progs1, progs2 []Se
 func mergeInBaseSensorMaps(t *testing.T, sensorMaps []SensorMap, sensorProgs []SensorProg) ([]SensorMap, []SensorProg) {
 	var baseProgs = []SensorProg{
 		0: SensorProg{Name: "event_execve", Type: ebpf.TracePoint},
-		1: SensorProg{Name: "event_exit", Type: ebpf.Kprobe},
+		1: SensorProg{Name: "event_exit", Type: ebpf.Kprobe, Match: ProgMatchPartial},
 		2: SensorProg{Name: "event_wake_up_new_task", Type: ebpf.Kprobe},
 		3: SensorProg{Name: "execve_send", Type: ebpf.TracePoint},
 		4: SensorProg{Name: "tg_kp_bprm_committing_creds", Type: ebpf.Kprobe},