libct/seccomp: enable binary tree optimization

This makes libseccomp produce a BPF which uses a binary tree for
syscalls (instead of linear set of if statements).

It does not make sense to enable binary tree for small set of rules,
so don't do that if we have less than 8 syscalls (the number is chosen
arbitrarily).

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/seccomp/seccomp_linux.go

@@ -111,6 +111,20 @@ func InitSeccomp(config *configs.Seccomp) (int, error) {
 		}
 	}
 
+	// Enable libseccomp binary tree optimization for longer rulesets.
+	//
+	// The number below chosen semi-arbitrarily, considering the following:
+	// 1. libseccomp <= 2.5.4 misbehaves when binary tree optimization
+	// is enabled and there are 0 rules.
+	// 2. All known libseccomp versions (2.5.0 to 2.5.4) generate a binary
+	// tree with 4 syscalls per node.
+	if len(config.Syscalls) > 32 {
+		if err := filter.SetOptimize(2); err != nil {
+			// The error is not fatal and is probably means we have older libseccomp.
+			logrus.Debugf("seccomp binary tree optimization not available: %v", err)
+		}
+	}
+
 	// Unset no new privs bit
 	if err := filter.SetNoNewPrivsBit(false); err != nil {
 		return -1, fmt.Errorf("error setting no new privileges: %w", err)