feat: add support for CODEX_SECURE_MODE=1 to restrict process observability (#4220)

Because the `codex` process could contain sensitive information in
memory, such as API keys, we add logic so that when
`CODEX_SECURE_MODE=1` is specified, we avail ourselves of whatever the
operating system provides to restrict observability/tampering, which
includes:

- disabling `ptrace(2)`, so it is not possible to attach to the process
with a debugger, such as `gdb`
- disabling core dumps

Admittedly, a user with root privileges can defeat these safeguards.

For now, we only add support for this in the `codex` multitool, but we
may ultimately want to support this in some of the smaller CLIs that are
buildable out of our Cargo workspace.

codex-rs/cli/src/main.rs

@@ -21,6 +21,7 @@ use std::path::PathBuf;
 use supports_color::Stream;
 
 mod mcp_cmd;
+mod pre_main_hardening;
 
 use crate::mcp_cmd::McpCli;
 use crate::proto::ProtoCli;
@@ -194,6 +195,34 @@ fn print_exit_messages(exit_info: AppExitInfo) {
     }
 }
 
+pub(crate) const CODEX_SECURE_MODE_ENV_VAR: &str = "CODEX_SECURE_MODE";
+
+/// As early as possible in the process lifecycle, apply hardening measures
+/// if the CODEX_SECURE_MODE environment variable is set to "1".
+#[ctor::ctor]
+fn pre_main_hardening() {
+    let secure_mode = match std::env::var(CODEX_SECURE_MODE_ENV_VAR) {
+        Ok(value) => value,
+        Err(_) => return,
+    };
+
+    if secure_mode == "1" {
+        #[cfg(any(target_os = "linux", target_os = "android"))]
+        crate::pre_main_hardening::pre_main_hardening_linux();
+
+        #[cfg(target_os = "macos")]
+        crate::pre_main_hardening::pre_main_hardening_macos();
+
+        #[cfg(windows)]
+        crate::pre_main_hardening::pre_main_hardening_windows();
+    }
+
+    // Always clear this env var so child processes don't inherit it.
+    unsafe {
+        std::env::remove_var(CODEX_SECURE_MODE_ENV_VAR);
+    }
+}
+
 fn main() -> anyhow::Result<()> {
     arg0_dispatch_or_else(|codex_linux_sandbox_exe| async move {
         cli_main(codex_linux_sandbox_exe).await?;