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</details>
## **Introduction to ARM64**
ARM64, also known as ARMv8-A, is a 64-bit processor architecture used in various types of devices including smartphones, tablets, servers, and even some high-end personal computers (macOS). It's a product of ARM Holdings, a company known for its energy-efficient processor designs.
### **Registers**
ARM64 has **31 general-purpose registers**, labeled `x0` through `x30`. Each can store a **64-bit** (8-byte) value. For operations that require only 32-bit values, the same registers can be accessed in a 32-bit mode using the names w0 through w30.
1.**`x0`** to **`x7`** - These are typically used as scratch registers and for passing parameters to subroutines.
5.**`x18`** - Platform register. On some platforms, this register is reserved for platform-specific uses.
6.**`x19`** to **`x28`** - These are callee-saved registers. A function must preserve these registers' values for its caller.
7.**`x29`** - **Frame pointer**.
8.**`x30`** - Link register. It holds the return address when a `BL` (Branch with Link) or `BLR` (Branch with Link to Register) instruction is executed.
9.**`sp`** - **Stack pointer**, used to keep track of the top of the stack.
10.**`pc`** - **Program counter**, which points to the next instruction to be executed.
### **Calling Convention**
The ARM64 calling convention specifies that the **first eight parameters** to a function are passed in registers **`x0` through `x7`**. **Additional** parameters are passed on the **stack**. The **return** value is passed back in register **`x0`**, or in **`x1`** as well **if it's 128 bits**. The **`x19`** to **`x30`** and **`sp`** registers must be **preserved** across function calls.
When reading a function in assembly, look for the **function prologue and epilogue**. The **prologue** usually involves **saving the frame pointer (`x29`)**, **setting** up a **new frame pointer**, and a**llocating stack space**. The **epilogue** usually involves **restoring the saved frame pointer** and **returning** from the function.
### **Common Instructions**
ARM64 instructions generally have the **format `opcode dst, src1, src2`**, where **`opcode`** is the **operation** to be performed (such as `add`, `sub`, `mov`, etc.), **`dst`** is the **destination** register where the result will be stored, and **`src1`** and **`src2`** are the **source** registers. Immediate values can also be used in place of source registers.
* **`mov`**: **Move** a value from one **register** to another.
* Example: `mov x0, x1` — This moves the value from `x1` to `x0`.
* **`ldr`**: **Load** a value from **memory** into a **register**.
* Example: `ldr x0, [x1]` — This loads a value from the memory location pointed to by `x1` into `x0`.
* **`str`**: **Store** a value from a **register** into **memory**.
* Example: `str x0, [x1]` — This stores the value in `x0` into the memory location pointed to by `x1`.
* **`ldp`**: **Load Pair of Registers**. This instruction **loads two registers** from **consecutive memory** locations. The memory address is typically formed by adding an offset to the value in another register.
* Example: `ldp x0, x1, [x2]` — This loads `x0` and `x1` from the memory locations at `x2` and `x2 + 8`, respectively.
* **`stp`**: **Store Pair of Registers**. This instruction **stores two registers** to **consecutive memory** locations. The memory address is typically formed by adding an offset to the value in another register.
* Example: `stp x0, x1, [x2]` — This stores `x0` and `x1` to the memory locations at `x2` and `x2 + 8`, respectively.
* **`add`**: **Add** the values of two registers and store the result in a register.
* Example: `add x0, x1, x2` — This adds the values in `x1` and `x2` together and stores the result in `x0`.
* **`sub`**: **Subtract** the values of two registers and store the result in a register.
* Example: `sub x0, x1, x2` — This subtracts the value in `x2` from `x1` and stores the result in `x0`.
* **`mul`**: **Multiply** the values of **two registers** and store the result in a register.
* Example: `mul x0, x1, x2` — This multiplies the values in `x1` and `x2` and stores the result in `x0`.
* **`div`**: **Divide** the value of one register by another and store the result in a register.
* Example: `div x0, x1, x2` — This divides the value in `x1` by `x2` and stores the result in `x0`.
* **`bl`**: **Branch** with link, used to **call** a **subroutine**. Stores the **return address in `x30`**.
* Example: `bl myFunction` — This calls the function `myFunction` and stores the return address in `x30`.
* **`blr`**: **Branch** with Link to Register, used to **call** a **subroutine** where the target is **specified** in a **register**. Stores the return address in `x30`.
* Example: `blr x1` — This calls the function whose address is contained in `x1` and stores the return address in `x30`.
* **`ret`**: **Return** from **subroutine**, typically using the address in **`x30`**.
* Example: `ret` — This returns from the current subroutine using the return address in `x30`.
* **`cmp`**: **Compare** two registers and set condition flags.
* Example: `cmp x0, x1` — This compares the values in `x0` and `x1` and sets the condition flags accordingly.
* **`b.eq`**: **Branch if equal**, based on the previous `cmp` instruction.
* Example: `b.eq label` — If the previous `cmp` instruction found two equal values, this jumps to `label`.
* **`b.ne`**: **Branch if Not Equal**. This instruction checks the condition flags (which were set by a previous comparison instruction), and if the compared values were not equal, it branches to a label or address.
* Example: After a `cmp x0, x1` instruction, `b.ne label` — If the values in `x0` and `x1` were not equal, this jumps to `label`.
* **`cbz`**: **Compare and Branch on Zero**. This instruction compares a register with zero, and if they are equal, it branches to a label or address.
* Example: `cbz x0, label` — If the value in `x0` is zero, this jumps to `label`.
* **`cbnz`**: **Compare and Branch on Non-Zero**. This instruction compares a register with zero, and if they are not equal, it branches to a label or address.
* Example: `cbnz x0, label` — If the value in `x0` is non-zero, this jumps to `label`.
* **`adrp`**: Compute the **page address of a symbol** and store it in a register.
* Example: `adrp x0, symbol` — This computes the page address of `symbol` and stores it in `x0`.
* **`ldrsw`**: **Load** a signed **32-bit** value from memory and **sign-extend it to 64** bits.
* Example: `ldrsw x0, [x1]` — This loads a signed 32-bit value from the memory location pointed to by `x1`, sign-extends it to 64 bits, and stores it in `x0`.
* **`stur`**: **Store a register value to a memory location**, using an offset from another register.
* Example: `stur x0, [x1, #4]` — This stores the value in `x0` into the memory ddress that is 4 bytes greater than the address currently in `x1`.
* **`svc`** : Make a **system call**. It stands for "Supervisor Call". When the processor executes this instruction, it **switches from user mode to kernel mode** and jumps to a specific location in memory where the **kernel's system call handling** code is located.
* Example: 
```armasm
mov x8, 93 ; Load the system call number for exit (93) into register x8.
mov x0, 0 ; Load the exit status code (0) into register x0.
svc 0 ; Make the system call.
```
## macOS
### syscalls
Check out [**syscalls.master**](https://opensource.apple.com/source/xnu/xnu-1504.3.12/bsd/kern/syscalls.master).
.section __TEXT,__text ; This directive tells the assembler to place the following code in the __text section of the __TEXT segment.
.global _main ; This makes the _main label globally visible, so that the linker can find it as the entry point of the program.
.align 2 ; This directive tells the assembler to align the start of the _main function to the next 4-byte boundary (2^2 = 4).
_main:
adr x0, sh_path ; This is the address of "/bin/sh".
mov x1, xzr ; Clear x1, because we need to pass NULL as the second argument to execve.
mov x2, xzr ; Clear x2, because we need to pass NULL as the third argument to execve.
mov x16, #59 ; Move the execve syscall number (59) into x16.
svc #0x1337 ; Make the syscall. The number 0x1337 doesn't actually matter, because the svc instruction always triggers a supervisor call, and the exact action is determined by the value in x16.
.section __TEXT,__text ; This directive tells the assembler to place the following code in the __text section of the __TEXT segment.
.global _main ; This makes the _main label globally visible, so that the linker can find it as the entry point of the program.
.align 2 ; This directive tells the assembler to align the start of the _main function to the next 4-byte boundary (2^2 = 4).
_main:
; We are going to build the string "/bin/sh" and place it on the stack.
mov x1, #0x622F ; Move the lower half of "/bi" into x1. 0x62 = 'b', 0x2F = '/'.
movk x1, #0x6E69, lsl #16 ; Move the next half of "/bin" into x1, shifted left by 16. 0x6E = 'n', 0x69 = 'i'.
movk x1, #0x732F, lsl #32 ; Move the first half of "/sh" into x1, shifted left by 32. 0x73 = 's', 0x2F = '/'.
movk x1, #0x68, lsl #48 ; Move the last part of "/sh" into x1, shifted left by 48. 0x68 = 'h'.
str x1, [sp, #-8] ; Store the value of x1 (the "/bin/sh" string) at the location `sp - 8`.
; Prepare arguments for the execve syscall.
mov x1, #8 ; Set x1 to 8.
sub x0, sp, x1 ; Subtract x1 (8) from the stack pointer (sp) and store the result in x0. This is the address of "/bin/sh" string on the stack.
mov x1, xzr ; Clear x1, because we need to pass NULL as the second argument to execve.
mov x2, xzr ; Clear x2, because we need to pass NULL as the third argument to execve.
; Make the syscall.
mov x16, #59 ; Move the execve syscall number (59) into x16.
svc #0x1337 ; Make the syscall. The number 0x1337 doesn't actually matter, because the svc instruction always triggers a supervisor call, and the exact action is determined by the value in x16.
The goal is to execute `execve("/bin/cat", ["/bin/cat", "/etc/passwd"], NULL)`, so the second argument (x1) is an array of params (which in memory these means a stack of the addresses).
* Do you work in a **cybersecurity company**? Do you want to see your **company advertised in HackTricks**? or do you want to have access to the **latest version of the PEASS or download HackTricks in PDF**? Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
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* **Share your hacking tricks by submitting PRs to the** [**hacktricks repo**](https://github.com/carlospolop/hacktricks) **and** [**hacktricks-cloud repo**](https://github.com/carlospolop/hacktricks-cloud).
