diff --git a/README.adoc b/README.adoc
index c65d095..ae8b3e8 100644
--- a/README.adoc
+++ b/README.adoc
@@ -12678,6 +12678,15 @@ gdb-multiarch -batch -ex 'arch arm' -ex "file v7/nop.out" -ex "disassemble/rs as
 
 Bibliography: https://stackoverflow.com/questions/1875491/nop-for-iphone-binaries
 
+==== ARM udf instruction
+
+Guaranteed undefined! Therefore raise illegal instruction signal. Used by GCC `__builtin_trap` apparently: https://stackoverflow.com/questions/16081618/programmatically-cause-undefined-instruction-exception
+
+* link:userland/arch/arm/udf.S[]
+* link:userland/arch/aarch64/udf.S[]
+
+TODO: why GNU GAS 2.29 does not have a mnemonic for it?
+
 === ARM SIMD
 
 ==== ARM VFP
diff --git a/path_properties.py b/path_properties.py
index 767aacb..703a825 100644
--- a/path_properties.py
+++ b/path_properties.py
@@ -313,6 +313,10 @@ path_properties_tuples = (
                                     },
                                 ),
                                 'freestanding': freestanding_properties,
+                                'udf.S': {
+                                    'exit_status': 132,
+                                    'receives_signal': True
+                                },
                             }
                         ),
                         'aarch64': (
@@ -327,6 +331,10 @@ path_properties_tuples = (
                                     },
                                 ),
                                 'freestanding': freestanding_properties,
+                                'udf.S': {
+                                    'exit_status': 132,
+                                    'receives_signal': True
+                                },
                             }
                         ),
                         'fail.S': {'exit_status': 1},
diff --git a/userland/arch/aarch64/udf.S b/userland/arch/aarch64/udf.S
new file mode 100644
index 0000000..b249424
--- /dev/null
+++ b/userland/arch/aarch64/udf.S
@@ -0,0 +1,10 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#arm-udf-instruction */
+
+#include "common.h"
+
+ENTRY
+    .long 0
+#if 0
+    udf 0
+#endif
+EXIT
diff --git a/userland/arch/arm/udf.S b/userland/arch/arm/udf.S
new file mode 100644
index 0000000..a8f5cca
--- /dev/null
+++ b/userland/arch/arm/udf.S
@@ -0,0 +1,7 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#arm-udf-instruction */
+
+#include "common.h"
+
+ENTRY
+    udf 0
+EXIT