From 6c63d1dfea79bd7737486ad9476dfbcc839cd6fd Mon Sep 17 00:00:00 2001
From: ZauberNerd <zaubernerd@zaubernerd.de>
Date: Tue, 23 Jun 2026 20:59:36 +0000
Subject: [PATCH] init: tolerate closed stdio when the console is not yet ready

The kernel's handoff of stdin/stdout/stderr to PID 1 is best-effort: if
hvc0 is not registered when the kernel execs init, init inherits closed
fds 0/1/2 ("unable to open an initial console" in the kernel log). Rust
std's sanitize_standard_fds() then runs before main(), cannot backfill
/dev/null (devtmpfs is unmounted), and aborts -- rebooting before the
workload runs.

This is the flaky configure-vm-2cpu-1GiB failure (empty output): boot
timing intermittently delays hvc0 (gated on the secondary vCPU) past the
kernel's initial-console-open. KRUN_ENOMEM_WORKAROUND widens the window;
the race is latent without it.

A C init tolerates closed stdio. Restore that with an .init_array
constructor that reopens 0/1/2 (console, else /dev/null) before the
runtime check; setup_redirects() rewires them to the krun ports as before.

Assisted-by: Claude Code / claude-opus-4.8
Signed-off-by: ZauberNerd <zaubernerd@zaubernerd.de>
---
 init/src/main.rs | 44 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/init/src/main.rs b/init/src/main.rs
index 1d2de30..6e54788 100644
--- a/init/src/main.rs
+++ b/init/src/main.rs
@@ -10,6 +10,50 @@ mod fs;
 #[cfg(feature = "timesync")]
 mod timesync;
 
+/// Ensure init has open stdin/stdout/stderr.
+///
+/// The kernel hands init closed fds 0/1/2 when the console (hvc0) is not yet
+/// registered as it execs init, and Rust std's sanitize_standard_fds() aborts on
+/// that before main() runs. Open them first (the console, else /dev/null);
+/// setup_redirects() rewires them to the krun ports later.
+#[cfg(target_os = "linux")]
+extern "C" fn ensure_stdio() {
+    use nix::fcntl::{self, OFlag};
+    use nix::mount::{self, MsFlags};
+    use nix::sys::stat::Mode;
+    use nix::unistd;
+    use std::os::fd::AsRawFd;
+
+    if unsafe { libc::fcntl(libc::STDIN_FILENO, libc::F_GETFD) } >= 0 {
+        return;
+    }
+    let _ = std::fs::create_dir_all("/dev");
+    let _ = mount::mount(
+        Some("devtmpfs"),
+        "/dev",
+        Some("devtmpfs"),
+        MsFlags::empty(),
+        None::<&str>,
+    );
+
+    let Ok(fd) = fcntl::open(c"/dev/console", OFlag::O_RDWR, Mode::empty())
+        .or_else(|_| fcntl::open(c"/dev/null", OFlag::O_RDWR, Mode::empty()))
+    else {
+        return;
+    };
+    let _ = unistd::dup2_stdin(&fd);
+    let _ = unistd::dup2_stdout(&fd);
+    let _ = unistd::dup2_stderr(&fd);
+    if fd.as_raw_fd() <= libc::STDERR_FILENO {
+        // fd is a stdio slot that dup2 just wrote to — don't close it.
+        std::mem::forget(fd);
+    }
+}
+#[cfg(target_os = "linux")]
+#[used]
+#[unsafe(link_section = ".init_array")]
+static ENSURE_STDIO_CTOR: extern "C" fn() = ensure_stdio;
+
 fn main() -> anyhow::Result<()> {
     #[cfg(target_os = "freebsd")]
     freebsd::open_console();
-- 
2.51.2