Illegal Instruction (SIGILL, signal 4) faults and UD2 amd64 (x86-64) instruction
Maybe I'm just slow, but I only recently learned of the UD family of x86-64 (amd64) instructions (UD0, UD1 & UD2, which are opcodes 0x0fff, 0x0fb9 & 0x0f0b respectively). These are reserved undefined instructions that generate an invalid opcode exception when executed, resulting in a SIGILL (illegal instruction, signal 4) trap on Unix-like operating systems.
These can be generated by Clang/LLVM (and gcc) when compiling with flags like -fsanitize=undefined -fsanitize-trap=all.
For example, a range check is introduced when doing casts that might result in undefined behavior:
#include <cstdint>
int64_t fubar(double d) {
return static_cast<int64_t>(d);
}
Results in the following sequence:
.LCPI0_0:
.quad -4332462841530417151 # double -9.2233720368547778E+18
.LCPI0_1:
.quad 4890909195324358656 # double 9.2233720368547758E+18
.text
.globl _Z5fubard
.p2align 4, 0x90
.type _Z5fubard,@function
_Z5fubard: # @_Z5fubard
.cfi_startproc
.long 846595819 # 0x327606eb
.long .L__unnamed_1-_Z5fubard
# %bb.0:
ucomisd .LCPI0_0(%rip), %xmm0
jbe .LBB0_2
# %bb.1:
movsd .LCPI0_1(%rip), %xmm1 # xmm1 = mem[0],zero
ucomisd %xmm0, %xmm1
jbe .LBB0_2
# %bb.3:
cvttsd2si %xmm0, %rax
retq
.LBB0_2:
ud2
The UD2 instruction is invoked if the double is outside of the bounds of a signed int64.
Likewise, standard library routines may also do bounds checking:
#include <vector>
#include <cstdint>
uint32_t foobar(const std::vector<uint32_t>& v, int i) {
return v[i];
}
Generates a bunch of bounds checks, all resulting in UD2:
foobar(std::vector<unsigned int, std::allocator<unsigned int> > const&, int):
test rdi, rdi
je .L7
test dil, 7
jne .L7
mov rdx, QWORD PTR [rdi]
movsx rsi, esi
sal rsi, 2
lea rax, [rdx+rsi]
js .L4
cmp rax, rdx
jb .L7
.L5:
test rax, rax
je .L7
test al, 3
jne .L7
mov eax, DWORD PTR [rax]
ret
.L4:
cmp rdx, rax
jnb .L5
.L7:
ud2
Other architectures
And it's not just amd64 (x86-64):
| Arch | Instruction |
|---|---|
| arm64 | brk #1000 |
| risc-v | ebreak |
| m68k | trap #7 |
| mips | break |
