from kernel to user syscall `open()` syscall in linux kernel 5.14.13 on arm64 and riscv.

syscall-open-in-linux.c

/* https://elixir.bootlin.com/linux/v5.14.13/source/fs/open.c#L1224 */

SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, umode_t, mode)
{
	if (force_o_largefile())
		flags |= O_LARGEFILE;
	return do_sys_open(AT_FDCWD, filename, flags, mode);
}

#define SYSCALL_DEFINE3(name, ...) SYSCALL_DEFINEx(3, _##name, __VA_ARGS__)

/* /include/linux/syscalls.h */
#ifdef CONFIG_FTRACE_SYSCALLS
#define SYSCALL_DEFINEx(x, sname, ...)              \
    static const char *types_##sname[] = {          \
        __SC_STR_TDECL##x(__VA_ARGS__)          \
    };                          \
    static const char *args_##sname[] = {           \
        __SC_STR_ADECL##x(__VA_ARGS__)          \
    };                          \
    SYSCALL_METADATA(sname, x);             \
    __SYSCALL_DEFINEx(x, sname, __VA_ARGS__)
#else
#define SYSCALL_DEFINEx(x, sname, ...)              \
    __SYSCALL_DEFINEx(x, sname, __VA_ARGS__)
#endif

/* /include/linux/syscalls.h */
#define __SYSCALL_DEFINEx(x, name, ...)                 \
    asmlinkage long sys##name(__SC_DECL##x(__VA_ARGS__));       \
    static inline long SYSC##name(__SC_DECL##x(__VA_ARGS__));   \
    asmlinkage long SyS##name(__SC_LONG##x(__VA_ARGS__))        \
    {                               \
        __SC_TEST##x(__VA_ARGS__);              \
        return (long) SYSC##name(__SC_CAST##x(__VA_ARGS__));    \
    }                               \
    SYSCALL_ALIAS(sys##name, SyS##name);                \
    static inline long SYSC##name(__SC_DECL##x(__VA_ARGS__))

/* https://elixir.bootlin.com/linux/latest/source/tools/include/nolibc/nolibc.h#L2114 */
static __attribute__((unused))
int open(const char *path, int flags, mode_t mode)
{
	int ret = sys_open(path, flags, mode);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

/* https://elixir.bootlin.com/linux/latest/source/tools/include/nolibc/nolibc.h#L1625 */
static __attribute__((unused))
int sys_open(const char *path, int flags, mode_t mode)
{
#ifdef __NR_openat
	return my_syscall4(__NR_openat, AT_FDCWD, path, flags, mode);
#elif defined(__NR_open)
	return my_syscall3(__NR_open, path, flags, mode);
#else
#error Neither __NR_openat nor __NR_open defined, cannot implement sys_open()
#endif
}

/* arm64 */

#define my_syscall3(num, arg1, arg2, arg3)                                    \
({                                                                            \
	register long _num  asm("x8") = (num);                                \
	register long _arg1 asm("x0") = (long)(arg1);                         \
	register long _arg2 asm("x1") = (long)(arg2);                         \
	register long _arg3 asm("x2") = (long)(arg3);                         \
									      \
	asm volatile (                                                        \
		"svc #0\n"                                                    \
		: "=r"(_arg1)                                                 \
		: "r"(_arg1), "r"(_arg2), "r"(_arg3),                         \
		  "r"(_num)                                                   \
		: "memory", "cc"                                              \
	);                                                                    \
	_arg1;                                                                \
})

#define my_syscall4(num, arg1, arg2, arg3, arg4)                              \
({                                                                            \
	register long _num  asm("x8") = (num);                                \
	register long _arg1 asm("x0") = (long)(arg1);                         \
	register long _arg2 asm("x1") = (long)(arg2);                         \
	register long _arg3 asm("x2") = (long)(arg3);                         \
	register long _arg4 asm("x3") = (long)(arg4);                         \
									      \
	asm volatile (                                                        \
		"svc #0\n"                                                    \
		: "=r"(_arg1)                                                 \
		: "r"(_arg1), "r"(_arg2), "r"(_arg3), "r"(_arg4),             \
		  "r"(_num)                                                   \
		: "memory", "cc"                                              \
	);                                                                    \
	_arg1;                                                                \
})

/* Syscalls for AARCH64 :
 *   - registers are 64-bit
 *   - stack is 16-byte aligned
 *   - syscall number is passed in x8
 *   - arguments are in x0, x1, x2, x3, x4, x5
 *   - the system call is performed by calling svc 0
 *   - syscall return comes in x0.
 *   - the arguments are cast to long and assigned into the target registers
 *     which are then simply passed as registers to the asm code, so that we
 *     don't have to experience issues with register constraints.
 *
 * On aarch64, select() is not implemented so we have to use pselect6().
 */


/* riscv */
#if   __riscv_xlen == 64
#define PTRLOG "3"
#define SZREG  "8"
#elif __riscv_xlen == 32
#define PTRLOG "2"
#define SZREG  "4"
#endif

/* Syscalls for RISCV :
 *   - stack is 16-byte aligned
 *   - syscall number is passed in a7
 *   - arguments are in a0, a1, a2, a3, a4, a5
 *   - the system call is performed by calling ecall
 *   - syscall return comes in a0
 *   - the arguments are cast to long and assigned into the target
 *     registers which are then simply passed as registers to the asm code,
 *     so that we don't have to experience issues with register constraints.
 */

/* https://elixir.bootlin.com/linux/latest/source/tools/include/nolibc/nolibc.h#L1288 */
#define my_syscall3(num, arg1, arg2, arg3)                                    \
({                                                                            \
	register long _num  asm("a7") = (num);                                \
	register long _arg1 asm("a0") = (long)(arg1);                         \
	register long _arg2 asm("a1") = (long)(arg2);                         \
	register long _arg3 asm("a2") = (long)(arg3);                         \
									      \
	asm volatile (                                                        \
		"ecall\n\t"                                                   \
		: "+r"(_arg1)                                                 \
		: "r"(_arg2), "r"(_arg3),                                     \
		  "r"(_num)                                                   \
		: "memory", "cc"                                              \
	);                                                                    \
	_arg1;                                                                \
})

#define my_syscall4(num, arg1, arg2, arg3, arg4)                              \
({                                                                            \
	register long _num  asm("a7") = (num);                                \
	register long _arg1 asm("a0") = (long)(arg1);                         \
	register long _arg2 asm("a1") = (long)(arg2);                         \
	register long _arg3 asm("a2") = (long)(arg3);                         \
	register long _arg4 asm("a3") = (long)(arg4);                         \
									      \
	asm volatile (                                                        \
		"ecall\n"                                                     \
		: "+r"(_arg1)                                                 \
		: "r"(_arg2), "r"(_arg3), "r"(_arg4),                         \
		  "r"(_num)                                                   \
		: "memory", "cc"                                              \
	);                                                                    \
	_arg1;                                                                \
})