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>

src/main.c

@@ -126,15 +126,15 @@ static uacpi_interrupt_ret handle_power_button(uacpi_handle ctx) {
     return UACPI_INTERRUPT_HANDLED;
 }
 
-// The following will be our kernel's entry point.
-// If renaming kmain() to something else, make sure to change the
-// linker script accordingly.
+
 void kmain(void) {
-    // Ensure the bootloader actually understands our base revision (see spec).
     if (LIMINE_BASE_REVISION_SUPPORTED(limine_base_revision) == false) {
         hcf();
     }
 
+    uint64_t rsp;
+    asm volatile("mov %%rsp, %0" : "=r"(rsp));
+
     // Ensure we got a framebuffer.
     if (framebuffer_request.response == NULL
      || framebuffer_request.response->framebuffer_count < 1) {
@@ -179,8 +179,6 @@ void kmain(void) {
             entry->type);
     }
     x86_64_DisableInterrupts();
-    
-
 
     uint32_t msr_lo, msr_hi;
     asm volatile("rdmsr" : "=a"(msr_lo), "=d"(msr_hi) : "c"(0x1B));
@@ -202,7 +200,6 @@ void kmain(void) {
     x86_64_PIT_Initialize(1000);
     asm volatile("sti");
     calibrate_tsc();
-    
 
 
     //while (1) asm volatile("hlt");
@@ -220,102 +217,8 @@ void kmain(void) {
         hcf();
     }
 
+
     ext2_read_root_dir();
-    printf("test");
-
-
-    uint8_t* file_buffer = kmalloc(4096); // adjust if needed
-    uint32_t file_size = 0;
-    
-    if (ext2_read_file_from_root("a.txt", file_buffer, &file_size)) {
-        printf("Read file (size %d):\n", file_size);
-
-        for (uint32_t i = 0; i < file_size; i++) {
-            printf("%c", file_buffer[i]); // or printf("%c", ...)
-        }
-
-        printf("\n");
-    } else {
-        printf("Failed to read file\n");
-    }
-
-    kfree(file_buffer);
-
-    uint8_t* file2_buffer = kmalloc(4096); // adjust if needed
-    uint32_t file2_size = 0;
-
-    if (ext2_read_file_from_root("dogcatman.txt", file2_buffer, &file2_size)) {
-        printf("Read file (size %d):\n", file2_size);
-
-        for (uint32_t i = 0; i < file2_size; i++) {
-            printf("%c", file2_buffer[i]); // or printf("%c", ...)
-        }
-
-        printf("\n");
-    } else {
-        printf("Failed to read file\n");
-    }
-
-    kfree(file2_buffer);
-
-    uint8_t* file3_buffer = kmalloc(4096); // adjust if needed
-    uint32_t file3_size = 0;
-
-    if (ext2_read_file_from_root("test.txt", file3_buffer, &file3_size)) {
-        printf("Read file (size %d):\n", file3_size);
-
-        for (uint32_t i = 0; i < file3_size; i++) {
-            printf("%c", file3_buffer[i]); // or printf("%c", ...)
-        }
-
-        printf("\n");
-    } else {
-        printf("Failed to read file\n");
-    }
-
-    kfree(file3_buffer);
-
-    uint8_t* file4_buffer = kmalloc(4096); // adjust if needed
-    uint32_t file4_size = 0;
-
-    if (ext2_read_file_from_root("qwerty.txt", file4_buffer, &file4_size)) {
-        printf("Read file (size %d):\n", file4_size);
-
-        for (uint32_t i = 0; i < file4_size; i++) {
-            printf("%c", file4_buffer[i]); // or printf("%c", ...)
-        }
-
-        printf("\n");
-    } else {
-        printf("Failed to read file\n");
-    }
-
-
-
-    kfree(file4_buffer);
-
-    const char* msg = "hello i wrote from kernel!";
-
-    ext2_write_file_from_root("qwerty.txt", (const uint8_t*)msg, strlen(msg), EXT2_WRITE_APPEND);
-
-    uint8_t* file5_buffer = kmalloc(4096); // adjust if needed
-    uint32_t file5_size = 0;
-
-    if (ext2_read_file_from_root("qwerty.txt", file5_buffer, &file5_size)) {
-        printf("Read file (size %d):\n", file5_size);
-
-        for (uint32_t i = 0; i < file5_size; i++) {
-            printf("%c", file5_buffer[i]); // or printf("%c", ...)
-        }
-
-        printf("\n");
-    } else {
-        printf("Failed to read file\n");
-    }
-
-
-
-    kfree(file5_buffer);
 
     uint32_t inum = ext2_resolve_path("charlie.tga");
     if (!inum) {
@@ -364,20 +267,8 @@ void kmain(void) {
     kfree(img);
 
     //clear_screen(0xFF1E1E1E);
+    
 
-    fd_t fd = VFS_Open("/a.txt");
-    if (fd < 0) {
-        printf("asudg");
-    }
-    uint8_t buf[128];
-    int n;
-
-    while ((n = VFS_Read(fd, buf, sizeof(buf))) > 0)
-    {
-        VFS_Write(VFS_FD_STDOUT, buf, n);
-    }
-
-    VFS_Close(fd);
     printf("\nKirkOS %s\n", KIRKOS_VERSION);
 
     x86_64_EnableInterrupts();
@@ -454,8 +345,12 @@ void kmain(void) {
     input_init();
     ps2_kbd_init();
 
-    //syscall_init();
-    //start_userspace();
+    gdt_load_tss((size_t)&kernel_tss);
+    kernel_tss.rsp0 = rsp;
+
+
+    syscall_init();
+    start_userspace();
 
     // We're done, just hang...
     hcf();