package container

import (
	"bytes"
	"fmt"
	"os"
	"strconv"
	"sync"

	"hakurei.app/fhs"
)

// mustSysctlInt reads a sysctl path via proc, interprets it as a decimal
// representation of an integer, and terminates the program if the read was
// unsuccessful or the response is invalid.
func mustSysctlInt(path string) (v int) {
	p, err := os.ReadFile(fhs.ProcSys + path)
	if err != nil {
		fmt.Println(err.Error())
		os.Exit(1)
	}

	if v, err = strconv.Atoi(string(bytes.TrimSpace(p))); err != nil {
		fmt.Printf("invalid sysctl response %q\n", string(p))
		os.Exit(1)
	}
	return
}

// cached sysctl responses for populateSysctl
var (
	kernelOverflowuid int
	kernelOverflowgid int
	kernelCapLastCap  int

	sysctlOnce sync.Once
)

// populateSysctl populates cached sysctl responses guarded by sysctlOnce.
func populateSysctl() {
	sysctlOnce.Do(func() {
		kernelOverflowuid = mustSysctlInt("kernel/overflowuid")
		kernelOverflowgid = mustSysctlInt("kernel/overflowgid")
		kernelCapLastCap = mustSysctlInt("kernel/cap_last_cap")
	})
}

// OverflowUID returns the fixed uid returned by legacy 16-bit UID system calls.
// This value is also returned for unmapped UIDs in a user namespace.
func OverflowUID() int {
	populateSysctl()
	return kernelOverflowuid
}

// OverflowGID is like [OverflowUID], but for GID instead.
func OverflowGID() int {
	populateSysctl()
	return kernelOverflowgid
}

// lastcap returns the highest valid capability of the running kernel. Exports
// CAP_LAST_CAP from the kernel.
func lastcap() uintptr {
	populateSysctl()
	return uintptr(kernelCapLastCap)
}