cgroupv2: ebpf: debug info when detaching programs in fallback mode

It seems that we are triggering the mutli-attach fallback in the fedora
CI, but we don't have enough debugging information to really know what's
going on, so add some. Unfortunately the amount of information we have
available with eBPF programs in general is fairly limited (we can't get
their bytecode for instance).

We also demote the "more than one filter" warning to an info message
because it happens very often under the systemd cgroup driver (likely
when systemd configures the cgroup it isn't deleting our old program, so
when our apply code runs after the systemd one there are two running
programs).

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

libcontainer/cgroups/ebpf/ebpf_linux.go

@@ -193,13 +193,37 @@ func LoadAttachCgroupDeviceFilter(insts asm.Instructions, license string, dirFd
 		return nil
 	}
 	if !useReplaceProg {
+		logLevel := logrus.DebugLevel
 		// If there was more than one old program, give a warning (since this
 		// really shouldn't happen with runc-managed cgroups) and then detach
 		// all the old programs.
 		if len(oldProgs) > 1 {
-			logrus.Warnf("found more than one filter (%d) attached to a cgroup -- removing extra filters!", len(oldProgs))
+			// NOTE: Ideally this should be a warning but it turns out that
+			//       systemd-managed cgroups trigger this warning (apparently
+			//       systemd doesn't delete old non-systemd programs when
+			//       setting properties).
+			logrus.Infof("found more than one filter (%d) attached to a cgroup -- removing extra filters!", len(oldProgs))
+			logLevel = logrus.InfoLevel
 		}
-		for _, oldProg := range oldProgs {
+		for idx, oldProg := range oldProgs {
+			// Output some extra debug info.
+			if info, err := oldProg.Info(); err == nil {
+				fields := logrus.Fields{
+					"type": info.Type.String(),
+					"tag":  info.Tag,
+					"name": info.Name,
+				}
+				if id, ok := info.ID(); ok {
+					fields["id"] = id
+				}
+				if runCount, ok := info.RunCount(); ok {
+					fields["run_count"] = runCount
+				}
+				if runtime, ok := info.Runtime(); ok {
+					fields["runtime"] = runtime.String()
+				}
+				logrus.WithFields(fields).Logf(logLevel, "removing old filter %d from cgroup", idx)
+			}
 			err = link.RawDetachProgram(link.RawDetachProgramOptions{
 				Target:  dirFd,
 				Program: oldProg,