Hello,
I'm currently trying to switch to ring 3 on my kernel. However when I try, I get a General Protection Fault.
Here is my GDT:
Code:
.align 8
gdt_start:
gdt_null:
.quad 0
gdt_kernel_code:
.word 0xffff
.word 0
.byte 0
.byte 0b10011010
.byte 0b11001111
.byte 0
gdt_kernel_data:
.word 0xffff
.word 0
.byte 0
.byte 0b10010010
.byte 0b11001111
.byte 0
gdt_user_code:
.word 0xffff
.word 0
.byte 0
.byte 0b11111010
.byte 0b11001111
.byte 0
gdt_user_data:
.word 0xffff
.word 0
.byte 0
.byte 0b11110010
.byte 0b11001111
.byte 0
gdt_tss:
.quad 0
gdt:
.word gdt - gdt_start - 1
.long gdt_start
(My TSS is initialized somewhere else, see:
https://forum.osdev.org/viewtopic.php?f=1&t=33720)
Here is my code for context switching:
Code:
.global context_switch
context_switch:
push %ebp
mov %esp, %ebp
mov $GDT_USER_DATA_OFFSET, %ax
mov %ax, %ds
mov %ax, %es
mov %ax, %fs
mov %ax, %gs
push $GDT_USER_DATA_OFFSET
push 8(%ebp)
pushf
push $GDT_USER_CODE_OFFSET
push 16(%ebp)
iret
# TODO Error handling
mov %ebp, %esp
pop %ebp
ret
I'm calling this function from C code, its prototype is:
Code:
extern void context_switch(void *esp, void *eip);
I'm passing a pointer to the last byte of an allocated page of memory in
esp (for the stack) and a function pointer for
eip.
The error happens when reaching the
iret instruction.
When catching the General Protection Fault interrupt, the error code tells me that the problem comes from segment
0x18 (gdt_user_code).
Am I doing something wrong?
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