● Covered by 1 source · 1 reportLow impact1 neutral
The Origin of the x86 ud2 Undefined Instruction
Key points
- ud2 is an architecturally undefined x86 instruction.
- It guarantees an invalid opcode exception.
- Compilers use ud2 for unreachable code sections.
- Earlier sequences (0F FF, 0F B9) were informally used for this purpose.
Purpose of ud2
The x86 ud2 instruction is an architecturally undefined instruction designed to reliably raise an "invalid opcode" exception. Compilers utilize ud2 to mark code paths that should never be executed, such as after a function declared with `[[noreturn]]` returns. If execution reaches a ud2 instruction, the program crashes, preventing unintended behavior from random instructions.
Historical Precursors
Before ud2, developers seeking to force an invalid opcode exception would use specific byte sequences that reliably triggered this behavior. Two such sequences were 0F FF and 0F B9. Both sequences were found to cause an invalid opcode exception, though they were internally decoded as if they took parameters, which were not actually used due to the exception being raised immediately.
The Need for ud2
The reason for the introduction of ud2, and its '2' suffix, relates to changes in processor behavior. Intel's subsequent processors sometimes altered how 0F FF or 0F B9 were handled. These sequences might have stopped raising an invalid opcode exception, performed random operations, or even been assigned to new instructions. This change in behavior broke software that had come to rely on these sequences for crashing, demonstrating Hyrum's Law: "With a sufficient number of users, all observable behaviors will be depended upon by somebody."
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Reporting from
The x86 ud2 instruction, which guarantees an invalid opcode exception, is used by compilers to mark unreachable code. Its name, ud2, stems from historical issues where earlier undefined instruction sequences (0F FF and 0F B9) were relied upon by software, only to change behavior in later processors, illustrating Hyrum's Law.