main() panics on os.Args[1] when invoked with no arguments

[dolph]

Summary

main() accesses os.Args[1] on line 30 of ussher.go before the length check on line 48. Invoking ussher with no arguments crashes with a Go runtime panic instead of producing the intended usage: ussher <username> message.

Reproduction

$ ./ussher
panic: runtime error: index out of range [1] with length 1

(The clean log.Fatal("usage: ussher <username>") further down at line 49 is unreachable for the no-args case.)

Why this matters specifically for ussher

This binary lives in the SSH authentication path. A panic — rather than a clean error — produces a Go stack trace on stderr, which is mildly informative for an operator running ./ussher by hand, but for any caller that processes ussher's output (sshd parsing stdout, a probe/health-check, an --version script that misorders args), the trace is unexpected and the exit code is 2 (Go runtime panic) rather than 1 (clean fatal). It's also the kind of crash a reachability scan would flag as "potentially exploitable" at a glance, even though it isn't.

The --version short-circuit ironically suffers the same issue — bare ./ussher can never print version info because the args check happens first and panics before the version branch can match.

Approach

Move the length check to the very top of main, before any os.Args[1] access:

func main() {
    if len(os.Args) < 2 {
        // Pre-init-log fatal: write to stderr (the default log destination).
        log.Fatal("usage: ussher <username>")
    }

    if os.Args[1] == "--version" {
        PrintVersion()
        return
    }

    // ... existing security checks, initLog, etc.
}

This is also a natural seam for the parseArgs extraction proposed in #13 — the new function would do the length and --version handling and return either a trigger to print version, a typed error, or a username string.

Acceptance criteria

• ./ussher (no args) prints usage: ussher <username> to stderr and exits non-zero, no Go panic.
• ./ussher --version continues to work.
• ./ussher root (or any valid user) continues to work.
• A test case in ussher_test.go (per 0% test coverage on Run, main, and the security startup gates #13) covers the no-args path.

Files

• /home/user/ussher/ussher.go