/proc/self/mountinfo is unavailable when target_os != linux

Error message

/proc/self/mountinfo is unavailable when target_os != linux

What it means

`docker_inspect_self_mountinfo` reads `/proc/self/mountinfo` to discover whether cross itself is running inside a container. That file only exists on Linux, so the function bails immediately when compiled/targeted for any other target_os instead of attempting a nonexistent path.

Solutions

Only use this code path on Linux (it is guarded; if you hit it, the caller should skip mountinfo detection on non-Linux).
Run cross on a Linux host or inside a Linux container where /proc/self/mountinfo exists.
On macOS/Windows, rely on Docker Desktop defaults instead of self-mountinfo detection.

Defensive patterns

Strategy: type-guard

Validate before calling

function mountinfoAvailable() { return process.platform === 'linux'; }

Type guard

const isLinux = () => process.platform === 'linux';
if (!isLinux()) {
  // skip mountinfo-based container detection
}

Prevention

Gate mountinfo-dependent logic behind a Linux check.
Run cross on Linux hosts or in Linux containers.
Use Docker Desktop defaults on macOS/Windows instead of self-inspection.

When it happens

Trigger: Running cross on a non-Linux host (macOS, Windows) along a code path that inspects self mountinfo to determine if cross is containerized.

Common situations: Developers running cross natively on macOS/Windows expecting container-in-container detection; cross built for a non-Linux target where this inspection is requested.

Understand the failure class

Background: "unsupported platform" / "not supported on this platform" errors: what they mean and how to fix them — this error's family across 47 libraries.

Related errors

AI-assisted analysis of cross-rs/cross@8c1a8aa4b6 (2026-09-13). Data as JSON: /api/errors/4be9bab5abd84c8a. Report an issue: GitHub.

Appendix: source

Thrown at src/docker/shared.rs:1331

                        .join(", ")
                )
            }),
        ));
    };

    let image_name = pick.default_image_name();
    if pick.platforms.is_empty() {
        return Err(GetImageError::SpecifiedImageNoPlatform(image_name));
    }

    let mut image: PossibleImage = image_name.into();
    image.toolchain = pick.platforms.to_vec();
    Ok(image)
}

fn docker_inspect_self_mountinfo(engine: &Engine, msg_info: &mut MessageInfo) -> Result<String> {
    if cfg!(not(target_os = "linux")) {
        eyre::bail!("/proc/self/mountinfo is unavailable when target_os != linux");
    }

    // The ID for the current Docker container might be in mountinfo,
    // somewhere in a mount root. Full IDs are 64-char hexadecimal
    // strings, so the first matching path segment in a mount root
    // containing /docker/ is likely to be what we're looking for. See:
    // https://www.kernel.org/doc/Documentation/filesystems/proc.txt
    // https://community.toradex.com/t/15240/4
    let mountinfo = file::read("/proc/self/mountinfo")?;
    let container_id = mountinfo
        .lines()
        .filter_map(|s| s.split(' ').nth(3))
        .filter(|s| s.contains("/docker/"))
        .flat_map(|s| s.split('/'))
        .find(|s| s.len() == 64 && s.as_bytes().iter().all(u8::is_ascii_hexdigit))
        .ok_or_else(|| eyre::eyre!("couldn't find container id in mountinfo"))?;

    engine

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