firecracker-microvm/firecracker · error

Cannot create cstring from cmdline string

Error message

Cannot create cstring from cmdline string

What it means

Error "Cannot create cstring from cmdline string" thrown in firecracker-microvm/firecracker.

Source

Thrown at src/vmm/src/arch/x86_64/mod.rs:262

    entry_point: EntryPoint,
    initrd: &Option<InitrdConfig>,
    boot_cmdline: Cmdline,
) -> Result<(), ConfigurationError> {
    configure_vcpus_for_boot(
        kvm,
        vcpus,
        machine_config,
        cpu_template,
        vm.guest_memory(),
        entry_point,
    )?;

    // Write the kernel command line to guest memory. This is x86_64 specific, since on
    // aarch64 the command line will be specified through the FDT.
    let cmdline_size = boot_cmdline
        .as_cstring()
        .map(|cmdline_cstring| cmdline_cstring.as_bytes_with_nul().len())
        .expect("Cannot create cstring from cmdline string");

    load_cmdline(
        vm.guest_memory(),
        GuestAddress(crate::arch::x86_64::layout::CMDLINE_START),
        &boot_cmdline,
    )
    .map_err(ConfigurationError::LoadCommandline)?;

    // Note that this puts the mptable at the last 1k of Linux's 640k base RAM
    mptable::setup_mptable(
        vm.guest_memory(),
        &mut vm.resource_allocator(),
        machine_config.vcpu_count,
    )
    .map_err(ConfigurationError::MpTableSetup)?;

    match entry_point.protocol {
        BootProtocol::PvhBoot => {

View on GitHub (pinned to 81b38b9dad)

Solutions

1. Remove NUL bytes from the kernel command-line string; CString rejects interior NULs.
2. Validate the boot arguments passed via the API or config for embedded NUL characters.

When it happens

Trigger: Thrown at src/vmm/src/arch/x86_64/mod.rs:216 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of firecracker-microvm/firecracker@81b38b9dad (2026-08-19). Data as JSON: /api/errors/3427ff4b33ec7037. Report an issue: GitHub.