//! aldermon-msr — minimal setcap'd helper for VID reads.
//!
//! The kernel gates /dev/cpu/N/msr twice: file mode (0600 root:root;
//! CAP_SYS_RAWIO does NOT override DAC) AND msr_open()'s capable check —
//! so access needs BOTH a 0440 adm node (udev rule) AND the capability,
//! which file permissions can never grant. This helper is the only binary
//! that ever holds the capability: it reads exactly one hard-coded MSR
//! (IA32_PERF_STATUS 0x198), read-only, and prints one line per CPU.
//! Everything else — the app, its deps, its config writes — stays unprivileged.
//!
//! usage: aldermon-msr CPU [CPU...]  -> "CPU 0x<hex>" per cpu, "CPU -" on miss

use std::io::{Read, Seek, Write};

const PERF_STATUS: u64 = 0x198;
const CPU_MAX: u32 = 4096;

fn read_perf_status(cpu: u32) -> Option<u64> {
    let path = format!("/dev/cpu/{cpu}/msr");
    let mut f = std::fs::File::open(path).ok()?;
    f.seek(std::io::SeekFrom::Start(PERF_STATUS)).ok()?;
    let mut buf = [0u8; 8];
    f.read_exact(&mut buf).ok()?;
    Some(u64::from_le_bytes(buf))
}

fn main() {
    let mut cpus = Vec::new();
    for arg in std::env::args().skip(1) {
        match arg.parse::<u32>() {
            Ok(c) if c <= CPU_MAX => cpus.push(c),
            _ => {
                eprintln!("usage: aldermon-msr CPU [CPU...]");
                std::process::exit(2);
            }
        }
    }
    if cpus.is_empty() {
        eprintln!("usage: aldermon-msr CPU [CPU...]");
        std::process::exit(2);
    }
    let out = std::io::stdout();
    let mut out = out.lock();
    for cpu in cpus {
        let line = match read_perf_status(cpu) {
            Some(v) => format!("{cpu} {v:#x}\n"),
            None => format!("{cpu} -\n"),
        };
        if out.write_all(line.as_bytes()).is_err() {
            std::process::exit(1);
        }
    }
}