Identify syscall in arm - linux

Question

When we looking on assembler at x86 CPU , syscall look like:
0F 05 syscall ; LINUX - sys_nanosleep
48 3D 01 F0 FF FF cmp rax, 0FFFFFFFFFFFFF001h
When are we talking about ARM CPU what is the convention how syscall looks like in assembler?

Answer 1

The source code for the musl libc library may help: all supported architectures have a small header file implementing the 'syscalls'.
x86_64:
static __inline long __syscall0(long n)
{
unsigned long ret;
__asm__ __volatile__ ("syscall" : "=a"(ret) : "a"(n) : "rcx", "r11", "memory");
return ret;
}
Arm:
#ifdef __thumb__
/* Avoid use of r7 in asm constraints when producing thumb code,
* since it's reserved as frame pointer and might not be supported. */
#define __ASM____R7__
#define __asm_syscall(...) do { \
__asm__ __volatile__ ( "mov %1,r7 ; mov r7,%2 ; svc 0 ; mov r7,%1" \
: "=r"(r0), "=&r"((int){0}) : __VA_ARGS__ : "memory"); \
return r0; \
} while (0)
#else
#define __ASM____R7__ __asm__("r7")
#define __asm_syscall(...) do { \
__asm__ __volatile__ ( "svc 0" \
: "=r"(r0) : __VA_ARGS__ : "memory"); \
return r0; \
} while (0)
#endif
Aarch64:
#define __asm_syscall(...) do { \
__asm__ __volatile__ ( "svc 0" \
: "=r"(x0) : __VA_ARGS__ : "memory", "cc"); \
return x0; \
} while (0)
Example for generated code:
/* syscall.c */
#define __asm_syscall(...) do { \
__asm__ __volatile__ ( "svc 0" \
: "=r"(x0) : __VA_ARGS__ : "memory", "cc"); \
return x0; \
} while (0)
static inline long __syscall0(long n)
{
register long x8 __asm__("x8") = n;
register long x0 __asm__("x0");
__asm_syscall("r"(x8));
}
void test(void) {
__syscall0(1);
}
/opt/arm/9/gcc-arm-9.2-2019.12-x86_64-aarch64-none-linux-gnu/bin/aarch64-none-linux-gnu-objdump -D syscall.o
/opt/arm/9/gcc-arm-9.2-2019.12-x86_64-aarch64-none-linux-gnu/bin/aarch64-none-linux-gnu-gcc -c -o syscall.o syscall.c
Disassembly of section .text:
0000000000000000 <__syscall0>:
0: d10043ff sub sp, sp, #0x10
4: f90007e0 str x0, [sp, #8]
8: f94007e8 ldr x8, [sp, #8]
c: d4000001 svc #0x0
10: 910043ff add sp, sp, #0x10
14: d65f03c0 ret
0000000000000018 <test>:
18: a9bf7bfd stp x29, x30, [sp, #-16]!
1c: 910003fd mov x29, sp
20: d2800020 mov x0, #0x1 // #1
24: 97fffff7 bl 0 <__syscall0>
28: d503201f nop
2c: a8c17bfd ldp x29, x30, [sp], #16
30: d65f03c0 ret
This being said, the arm documentation cannot really be called garbage, even though you found difficult to find the exact information you were looking for: The Exploration Tools section of their web site is excellent IMHO.
You can find the pseudo-code for the SVC instructions and their exact encodings here and here, and you even could simulate the instructions: Alastair Reid wrote a couple of fascinating articles here regarding ISA formal specifications.