sys: Refactor GDT initialization, enhance syscall handling, and improve user mode entry

Finally we got usermode working again, It took awhile but we got it done. A few things we did in this commit:

- Simplified GDT entry initialization in gdt.c for kernel and user segments.
- Fixed TSS structure for task switching.
- Implemented cr2 handling in a page fault.
- Enhanced user stack setup in usermode.c to return the correct RSP.
- Improved syscall implementation in syscall.c, including new syscall numbers.
- Updated syscall entry to correctly handle context switching and argument passing.
- Refactored init.c to demonstrate file operations using syscalls.
- Added new syscalls for file operations in syscalls.h.
- Modified VFS to handle leading slashes in paths correctly.

Signed-off-by: kaguya <vpshinomiya@protonmail.com>

user/crt0.S

@@ -1,12 +1,7 @@
 .section .text
 .global _start
+.global main
 _start:
-    mov $1, %rax
-    mov $'H', %rdi
-    syscall
-
-    mov $1, %rax
-    mov $'i', %rdi
-    syscall
+    call main
 
 1:  jmp 1b

user/include/syscalls.h

@@ -1,5 +1,10 @@
 #pragma once
 
+#define SYS_READ   0
+#define SYS_WRITE  1
+#define SYS_OPEN   2
+#define SYS_CLOSE  3
+
 static inline long syscall(long num, long a1, long a2, long a3)
 {
     long ret;
@@ -8,7 +13,6 @@ static inline long syscall(long num, long a1, long a2, long a3)
         "syscall"
         : "=a"(ret)
         : "a"(num), "D"(a1), "S"(a2), "d"(a3)
-        :
     );
 
     return ret;

user/programs/init.c

@@ -1,5 +1,7 @@
 #include "../include/syscalls.h"
 
+#define STDOUT 1
+
 unsigned strlen(const char* str)
 {
     unsigned len = 0;
@@ -8,15 +10,37 @@ unsigned strlen(const char* str)
         ++len;
         ++str;
     }
-
     return len;
 }
 
 void main()
 {
-    syscall(1, 'A', 0, 0);
-    syscall(1, 'B', 0, 0);
-    syscall(1, 'C', 0, 0);
+    const char* path = "/qwerty.txt";
+    const char* msg  = "Suki Suki Daisuki Kekkon Shiyo, my honey!";
 
+    char buf[128];
+
+    // ── open file ─────────────────────────────
+    long fd = syscall(SYS_OPEN, (long)path, 0, 0);
+
+    // ── write message ─────────────────────────
+    syscall(SYS_WRITE, fd, (long)msg, strlen(msg));
+
+    // ── close ────────────────────────────────
+    syscall(SYS_CLOSE, fd, 0, 0);
+
+    // ── reopen ───────────────────────────────
+    fd = syscall(SYS_OPEN, (long)path, 0, 0);
+
+    // ── read into buffer ─────────────────────
+    long n = syscall(SYS_READ, fd, (long)buf, sizeof(buf));
+
+    // ── close ────────────────────────────────
+    syscall(SYS_CLOSE, fd, 0, 0);
+
+    // ── print buffer to stdout ───────────────
+    syscall(SYS_WRITE, STDOUT, (long)buf, n);
+
+    // ── done ────────────────────────────────
     while (1);
 }