libct/seccomp/patchbpf: support SPEC_ALLOW

Commit 58ea21d added support for seccomp flags such as
SPEC_ALLOW, but it does not work as expected, because since commit
7a8d716 we do not use libseccomp-golang's Load(), but
handle flags separately in patchbfp.

This fixes setting SPEC_ALLOW flag.

Add a comment to not forget to amend filterFlags when adding new flags.

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

libcontainer/seccomp/patchbpf/enosys_linux.go

@@ -43,6 +43,11 @@ const uintptr_t C_SET_MODE_FILTER = SECCOMP_SET_MODE_FILTER;
 #endif
 const uintptr_t C_FILTER_FLAG_LOG = SECCOMP_FILTER_FLAG_LOG;
 
+#ifndef SECCOMP_FILTER_FLAG_SPEC_ALLOW
+#	define SECCOMP_FILTER_FLAG_SPEC_ALLOW (1UL << 2)
+#endif
+const uintptr_t C_FILTER_FLAG_SPEC_ALLOW = SECCOMP_FILTER_FLAG_SPEC_ALLOW;
+
 #ifndef SECCOMP_FILTER_FLAG_NEW_LISTENER
 #	define SECCOMP_FILTER_FLAG_NEW_LISTENER (1UL << 3)
 #endif
@@ -641,8 +646,13 @@ func filterFlags(config *configs.Seccomp, filter *libseccomp.ScmpFilter) (flags
 			flags |= uint(C.C_FILTER_FLAG_LOG)
 		}
 	}
-
-	// TODO: Support seccomp flags not yet added to libseccomp-golang...
+	if apiLevel >= 4 {
+		if ssb, err := filter.GetSSB(); err != nil {
+			return 0, false, fmt.Errorf("unable to fetch SECCOMP_FILTER_FLAG_SPEC_ALLOW bit: %w", err)
+		} else if ssb {
+			flags |= uint(C.C_FILTER_FLAG_SPEC_ALLOW)
+		}
+	}
 
 	for _, call := range config.Syscalls {
 		if call.Action == configs.Notify {

libcontainer/seccomp/seccomp_linux.go

@@ -105,6 +105,7 @@ func InitSeccomp(config *configs.Seccomp) (int, error) {
 			if err := filter.SetSSB(true); err != nil {
 				return -1, fmt.Errorf("error adding SSB flag to seccomp filter: %w", err)
 			}
+		// NOTE when adding more flags, make sure to also modify filterFlags in patchbpf.
 		default:
 			return -1, fmt.Errorf("seccomp flags %q not yet supported by runc", flag)
 		}