`Star::write` returning `Err` when using segments created by this crate

[shinobu-uwu]

Hello, I hope everyone is doing well!

I've been trying to enable the syscall/sysret instructions in my kernel, however when I try to use the segments I registered in my GDT, I get this error: Error setting STAR: Sysret CS and SS are not offset by 8. I find it confusing because I generated these segments like this:

pub static ref GDT: (GlobalDescriptorTable, Selectors) = {
    let mut gdt = GlobalDescriptorTable::new();
    let kernel_code_selector = gdt.append(Descriptor::kernel_code_segment());
    let kernel_data_selector = gdt.append(Descriptor::kernel_data_segment());
    let user_code_selector = gdt.append(Descriptor::user_code_segment());
    let user_data_selector = gdt.append(Descriptor::user_data_segment());
    let tss_selector = gdt.append(Descriptor::tss_segment(&TSS));
    (
        gdt,
        Selectors {
            kernel_code_selector,
            kernel_data_selector,
            user_code_selector,
            user_data_selector,
            tss_selector,
        },
    )
};

What is even more confusing is that cs_sysret and ss_sysret are in fact offset by 8. If anyone could point me in the right direction I would be thanked. Here is the code where I try to write to Star and its output:

let kernel_cs = GDT.1.kernel_code_selector;
let kernel_ss = GDT.1.kernel_data_selector;
let user_cs = GDT.1.user_code_selector;
let user_ss = GDT.1.user_data_selector;

debug!("User CS: {:#x?}", user_cs.0);
debug!("User SS: {:#x?}", user_ss.0);
debug!("Kernel CS: {:#x?}", kernel_cs.0);
debug!("Kernel SS: {:#x?}", kernel_ss.0);

match Star::write(user_cs, user_ss, kernel_cs, kernel_ss) {
    Ok(_) => {
        debug!("STAR MSR set successfully");
    }
    Err(e) => {
        panic!("Error setting STAR: {}", e)
    }
}

DEBUG [jon::syscall:28]: User CS: 0x1b
DEBUG [jon::syscall:29]: User SS: 0x23
DEBUG [jon::syscall:30]: Kernel CS: 0x8
DEBUG [jon::syscall:31]: Kernel SS: 0x10
ERROR [jon:61]: panicked at src/syscall/mod.rs:38:13:
Error setting STAR: Sysret CS and SS are not offset by 8.

[Freax13]

You need to swap the user data and code segments in the GDT. I'm sorry that the error message isn't very helpful here.

[shinobu-uwu]

That worked, thanks!