How to implement system call in ARM64?

 

System calls

Sometimes it is necessary for software to request a function from a more privileged entity. This might happen when, for example, an application requests that the OS opens a file.

In A64, there are special instructions for making such system calls. These instructions cause an exception, which allows controlled entry into a more privileged Exception level.

• SVC - Supervisor call
  Causes an exception targeting EL1.
  Used by an application to call the OS.
• HVC - Hypervisor call
  Causes an exception targeting EL2.
  Used by an OS to call the hypervisor, not available at EL0.
• SMC - Secure monitor call
  Causes an exception targeting EL3.
  Used by an OS or hypervisor to call the EL3 firmware, not available at EL0.

If an exception is executed from an Exception level higher than the target exception level, then the exception is taken to the current Exception level. This means that an SVC at EL2 would cause exception entry to EL2. Similarly, an HVC at EL3 causes exception entry to EL3. This is consistent with the rule that an exception can never cause the processor to lose privilege.

Intro

Hello world in ARM64 assembly for Linux and Macos. First example is how to compile hello world for raspberry pi 4, as its supports ARMv8 instruction set. Second example is how to run assembly on Apple M1 chip that also supports ARMv8 instruction set

The two assembly examples are equivalent to C code

int main() {
    char *s="Hello ARM64";
    write(1,s,strlen(s));
    exit(0);
}

Raspberry Pi 4

Running 64bit linux. To detect with architecture and what bitness of os run command

uname

Architecture shown as aarch64 enoughs to indicate that os ir 64bit

Linux raspberrypi 5.4.42-v8+ #1319 SMP PREEMPT Wed May 20 14:18:56 BST 2020 aarch64 GNU/Linux

.data

/* Data segment: define our message string and calculate its length. */
helloworld:
    .ascii        "Hello, ARM64!\n"
helloworld_len = . - helloworld

.text

/* Our application's entry point. */
.globl _start
_start:
    /* syscall write(int fd, const void *buf, size_t count) */
    mov     x0, #1              /* fd := STDOUT_FILENO */
    ldr     x1, =helloworld     /* buf := msg */
    ldr     x2, =helloworld_len /* count := len */
    mov     w8, #64             /* write is syscall #64 */
    svc     #0                  /* invoke syscall */

    /* syscall exit(int status) */
    mov     x0, #0               /* status := 0 */
    mov     w8, #93              /* exit is syscall #1 */
    svc     #0                   /* invoke syscall */

Compile

Too compile check if you have installed gnu gcc, other compilers such as clang also should work perfectly fine.

    as hello.s -o hello.o
    gcc hello.o -o hello

Apple M1

.global _start            // Provide program starting address to linker
.align 2                  // Make sure everything is aligned properly

/* syscall write(int fd, const void *buf, size_t count) */
_start: 
    mov    X0, #1         // 1 = StdOut
    adr    X1, helloworld     // string to print
    mov    X2, helloworld_len // length of our string
    mov    X16, #4            // Unix write system call
    svc    #0x80              // Call kernel to output the string

/* syscall exit(int status) */
    mov     X0, #0            // Use 0 return code
    mov     X16, #1           // System call number 1 terminates this program
    svc     #0x80             // Call kernel to terminate the program

helloworld:      .ascii  "Hello, ARM64!\n"
helloworld_len = . - helloworld

Compile

Install xcode tools before compilation

    as -o hello.o hello.s
    ld -macosx_version_min 11.0.0 -o hello hello.o -lSystem -syslibroot `xcrun -sdk macosx --show-sdk-path` -e _start -arch arm64

svc指令

[root@centos7 aarch64-bare-metal-qemu]# cat ans.s 
 .global _start
 _start:
 mov x0, #42
 mov x8, #93
 svc #0

[root@centos7 aarch64-bare-metal-qemu]# as ans.s -o ans.o
[root@centos7 aarch64-bare-metal-qemu]# ld  ans.o  -o ans
[root@centos7 aarch64-bare-metal-qemu]# ./ans
[root@centos7 aarch64-bare-metal-qemu]# echo $?
42
[root@centos7 aarch64-bare-metal-qemu]# 

[root@centos7 aarch64-bare-metal-qemu]# cat sys.s
.data

/* Data segment: define our message string and calculate its length. */
helloworld:
    .ascii        "Hello, ARM64!\n"
helloworld_len = . - helloworld

.text

/* Our application's entry point. */
.globl _start
_start:
    /* syscall write(int fd, const void *buf, size_t count) */
    mov     x0, #1              /* fd := STDOUT_FILENO */
    ldr     x1, =helloworld     /* buf := msg */
    ldr     x2, =helloworld_len /* count := len */
    mov     w8, #64             /* write is syscall #64 */
    svc     #0                  /* invoke syscall */

    /* syscall exit(int status) */
    mov     x0, #0               /* status := 0 */
    mov     w8, #93              /* exit is syscall #1 */
    svc     #0                   /* invoke syscall */

[root@centos7 aarch64-bare-metal-qemu]# as sys.s -o sys.o
[root@centos7 aarch64-bare-metal-qemu]# ld sys.o -o sys
[root@centos7 aarch64-bare-metal-qemu]# ./sys 
Hello, ARM64!