Fixed Usermode Segment Loading

- Change : Introduced uint16_t data_sel and uint16_t code_sel variables.
- Fix : Modified the asm volatile block to load ds , es , fs , and gs directly from the 16-bit register operand, which satisfies the assembler's requirement for segment register moves.
- Improved Safety : Updated the pushl sequence for ss and cs to explicitly cast the 16-bit selectors to 32-bit to maintain stack alignment for iret .

Author: PythonPlumber

Makefile

@@ -18,7 +18,9 @@ C_SRCS := $(wildcard $(SRC_DIR)/*.c $(SRC_DIR)/*/*.c $(SRC_DIR)/*/*/*.c)
 ASM_OBJS := $(patsubst $(SRC_DIR)/%.s,$(BUILD_DIR)/%.o,$(ASM_SRCS))
 C_OBJS := $(patsubst $(SRC_DIR)/%.c,$(BUILD_DIR)/%.o,$(C_SRCS))
 
-OBJS := $(ASM_OBJS) $(C_OBJS)
+BOOT_OBJ := $(BUILD_DIR)/boot.o
+OTHER_OBJS := $(filter-out $(BOOT_OBJ),$(ASM_OBJS) $(C_OBJS))
+OBJS := $(BOOT_OBJ) $(OTHER_OBJS)
 
 ifneq ($(MODEL_BLOB),)
 OBJS += $(MODEL_OBJ)

src/ramdisk.c

@@ -30,6 +30,7 @@ static uint32_t ramdisk_read(fs_node_t* node, uint32_t offset, uint32_t size, ui
 }
 
 fs_node_t* ramdisk_init(uint32_t module_start, uint32_t module_size) {
+    (void)module_size;
     initrd_base = (uint8_t*)module_start;
     initrd_header = (initrd_header_t*)initrd_base;
     file_headers = (initrd_file_header_t*)(initrd_base + sizeof(initrd_header_t));

src/usermode.c

@@ -4,24 +4,26 @@
 #define USER_DATA_SEL 0x23
 
 void switch_to_user_mode(uint32_t entry_point, uint32_t user_stack) {
+    uint16_t data_sel = USER_DATA_SEL;
+    uint16_t code_sel = USER_CODE_SEL;
+
     asm volatile(
         "cli\n"
-        "mov %0, %%ax\n"
-        "mov %%ax, %%ds\n"
-        "mov %%ax, %%es\n"
-        "mov %%ax, %%fs\n"
-        "mov %%ax, %%gs\n"
-        "pushl %0\n"
-        "pushl %1\n"
+        "mov %0, %%ds\n"
+        "mov %0, %%es\n"
+        "mov %0, %%fs\n"
+        "mov %0, %%gs\n"
+        "pushl %1\n" // ss
+        "pushl %2\n" // esp
         "pushfl\n"
         "popl %%eax\n"
         "or $0x200, %%eax\n"
-        "pushl %%eax\n"
-        "pushl %2\n"
-        "pushl %3\n"
+        "pushl %%eax\n" // eflags
+        "pushl %3\n" // cs
+        "pushl %4\n" // eip
         "iret\n"
         :
-        : "r"(USER_DATA_SEL), "r"(user_stack), "r"(USER_CODE_SEL), "r"(entry_point)
-        : "eax"
+        : "r"(data_sel), "r"((uint32_t)data_sel), "r"(user_stack), "r"((uint32_t)code_sel), "r"(entry_point)
+        : "eax", "memory"
     );
 }