linux-sandbox: use standalone bundled bwrap (#21255)

**Summary**
- Add `codex-bwrap`, a standalone `bwrap` binary built from the existing
vendored bubblewrap sources.
- Remove the linked vendored bwrap path from `codex-linux-sandbox`;
runtime now prefers system `bwrap` and falls back to bundled
`codex-resources/bwrap`.
- Add bundled SHA-256 verification with missing/all-zero digest as the
dev-mode skip value, then exec the verified file through
`/proc/self/fd`.
- Keep `launcher.rs` focused on choosing and dispatching the preferred
launcher. Bundled lookup, digest verification, and bundled exec now live
in `linux-sandbox/src/bundled_bwrap.rs`; Bazel runfiles lookup lives in
`linux-sandbox/src/bazel_bwrap.rs`; shared argv/fd exec helpers live in
`linux-sandbox/src/exec_util.rs`.
- Teach Bazel tests to surface the Bazel-built `//codex-rs/bwrap:bwrap`
through `CARGO_BIN_EXE_bwrap`; `codex-linux-sandbox` only honors that
fallback in debug Bazel runfiles environments so release/user runtime
lookup stays tied to `codex-resources/bwrap`.
- Allow `codex-exec-server` filesystem helpers to preserve just the
Bazel bwrap/runfiles variables they need in debug Bazel builds, since
those helpers intentionally rebuild a small environment before spawning
`codex-linux-sandbox`.
- Verify the Bazel bwrap target in Linux release CI with a build-only
check. Running `bwrap --version` is too strong for GitHub runners
because bubblewrap still attempts namespace setup there.

**Verification**
- Latest update: `cargo test -p codex-linux-sandbox`
- Latest update: `just fix -p codex-linux-sandbox`
- `cargo check --target x86_64-unknown-linux-gnu -p codex-linux-sandbox`
could not run locally because this macOS machine does not have
`x86_64-linux-gnu-gcc`; GitHub Linux Bazel CI is expected to cover the
Linux-only modules.
- Earlier in this PR: `cargo test -p codex-bwrap`
- Earlier in this PR: `cargo test -p codex-exec-server`
- Earlier in this PR: `cargo check --release -p codex-exec-server`
- Earlier in this PR: `just fix -p codex-linux-sandbox -p
codex-exec-server`
- Earlier in this PR: `bazel test --nobuild
//codex-rs/linux-sandbox:linux-sandbox-all-test
//codex-rs/core:core-all-test
//codex-rs/exec-server:exec-server-file_system-test
//codex-rs/app-server:app-server-all-test` (analysis completed; Bazel
then refuses to run tests under `--nobuild`)
- Earlier in this PR: `bazel build --nobuild //codex-rs/bwrap:bwrap`
- Prior to this update: `just bazel-lock-update`, `just
bazel-lock-check`, and YAML parse check for
`.github/workflows/bazel.yml`


---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/21255).
* #21257
* #21256
* __->__ #21255

codex-rs/bwrap/BUILD.bazel

@@ -0,0 +1,35 @@
+load("@rules_cc//cc:defs.bzl", "cc_library")
+load("//:defs.bzl", "codex_rust_crate")
+
+codex_rust_crate(
+    name = "bwrap",
+    crate_name = "codex_bwrap",
+    # Bazel wires vendored bubblewrap + libcap via :bwrap-ffi below and sets
+    # bwrap_available explicitly, so we skip Cargo's build.rs in Bazel builds.
+    build_script_enabled = False,
+    deps_extra = select({
+        "@platforms//os:linux": [":bwrap-ffi"],
+        "//conditions:default": [],
+    }),
+    rustc_flags_extra = select({
+        "@platforms//os:linux": ["--cfg=bwrap_available"],
+        "//conditions:default": [],
+    }),
+)
+
+cc_library(
+    name = "bwrap-ffi",
+    srcs = ["//codex-rs/vendor:bubblewrap_c_sources"],
+    hdrs = [
+        "config.h",
+        "//codex-rs/vendor:bubblewrap_headers",
+    ],
+    copts = [
+        "-D_GNU_SOURCE",
+        "-Dmain=bwrap_main",
+    ],
+    includes = ["."],
+    deps = ["@libcap//:libcap"],
+    target_compatible_with = ["@platforms//os:linux"],
+    visibility = ["//visibility:private"],
+)

codex-rs/bwrap/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "codex-bwrap"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[[bin]]
+name = "bwrap"
+path = "src/main.rs"
+
+[lints]
+workspace = true
+
+[target.'cfg(target_os = "linux")'.dependencies]
+libc = { workspace = true }
+
+[build-dependencies]
+cc = "1"
+pkg-config = "0.3"

codex-rs/bwrap/build.rs

@@ -0,0 +1,99 @@
+use std::env;
+use std::path::Path;
+use std::path::PathBuf;
+
+fn main() {
+    println!("cargo:rustc-check-cfg=cfg(bwrap_available)");
+    println!("cargo:rerun-if-env-changed=CODEX_BWRAP_SOURCE_DIR");
+    println!("cargo:rerun-if-env-changed=PKG_CONFIG_ALLOW_CROSS");
+    println!("cargo:rerun-if-env-changed=PKG_CONFIG_PATH");
+    println!("cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR");
+    println!("cargo:rerun-if-env-changed=CODEX_SKIP_BWRAP_BUILD");
+
+    let manifest_dir = PathBuf::from(env::var("CARGO_MANIFEST_DIR").unwrap_or_default());
+    let vendor_dir = manifest_dir.join("../vendor/bubblewrap");
+    for source in ["bubblewrap.c", "bind-mount.c", "network.c", "utils.c"] {
+        println!(
+            "cargo:rerun-if-changed={}",
+            vendor_dir.join(source).display()
+        );
+    }
+
+    let target_os = env::var("CARGO_CFG_TARGET_OS").unwrap_or_default();
+    if target_os != "linux" || env::var_os("CODEX_SKIP_BWRAP_BUILD").is_some() {
+        return;
+    }
+
+    if let Err(err) = try_build_bwrap() {
+        panic!("failed to compile bubblewrap for Linux target: {err}");
+    }
+}
+
+fn try_build_bwrap() -> Result<(), String> {
+    let manifest_dir =
+        PathBuf::from(env::var("CARGO_MANIFEST_DIR").map_err(|err| err.to_string())?);
+    let out_dir = PathBuf::from(env::var("OUT_DIR").map_err(|err| err.to_string())?);
+    let src_dir = resolve_bwrap_source_dir(&manifest_dir)?;
+    let libcap = pkg_config::Config::new()
+        .probe("libcap")
+        .map_err(|err| format!("libcap not available via pkg-config: {err}"))?;
+
+    let config_h = out_dir.join("config.h");
+    std::fs::write(
+        &config_h,
+        r#"#pragma once
+#define PACKAGE_STRING "bubblewrap built for Codex"
+"#,
+    )
+    .map_err(|err| format!("failed to write {}: {err}", config_h.display()))?;
+
+    let mut build = cc::Build::new();
+    build
+        .file(src_dir.join("bubblewrap.c"))
+        .file(src_dir.join("bind-mount.c"))
+        .file(src_dir.join("network.c"))
+        .file(src_dir.join("utils.c"))
+        .include(&out_dir)
+        .include(&src_dir)
+        .define("_GNU_SOURCE", None)
+        // Rename `main` so the Rust wrapper can expose the Cargo-built binary.
+        .define("main", Some("bwrap_main"));
+    for include_path in libcap.include_paths {
+        // Use -idirafter so target sysroot headers win (musl cross builds),
+        // while still allowing libcap headers from the host toolchain.
+        build.flag(format!("-idirafter{}", include_path.display()));
+    }
+
+    build.compile("standalone_bwrap");
+    println!("cargo:rustc-cfg=bwrap_available");
+    Ok(())
+}
+
+/// Resolve the bubblewrap source directory used for build-time compilation.
+///
+/// Priority:
+/// 1. `CODEX_BWRAP_SOURCE_DIR` points at an existing bubblewrap checkout.
+/// 2. The vendored bubblewrap tree under `codex-rs/vendor/bubblewrap`.
+fn resolve_bwrap_source_dir(manifest_dir: &Path) -> Result<PathBuf, String> {
+    if let Ok(path) = env::var("CODEX_BWRAP_SOURCE_DIR") {
+        let src_dir = PathBuf::from(path);
+        if src_dir.exists() {
+            return Ok(src_dir);
+        }
+        return Err(format!(
+            "CODEX_BWRAP_SOURCE_DIR was set but does not exist: {}",
+            src_dir.display()
+        ));
+    }
+
+    let vendor_dir = manifest_dir.join("../vendor/bubblewrap");
+    if vendor_dir.exists() {
+        return Ok(vendor_dir);
+    }
+
+    Err(format!(
+        "expected vendored bubblewrap at {}, but it was not found.\n\
+Set CODEX_BWRAP_SOURCE_DIR to an existing checkout or vendor bubblewrap under codex-rs/vendor.",
+        vendor_dir.display()
+    ))
+}

codex-rs/bwrap/config.h

@@ -0,0 +1 @@
+#define PACKAGE_STRING "bubblewrap built for Codex"

codex-rs/bwrap/src/main.rs

@@ -0,0 +1,45 @@
+#[cfg(all(target_os = "linux", bwrap_available))]
+fn main() {
+    use std::ffi::CStr;
+    use std::ffi::CString;
+    use std::os::raw::c_char;
+    use std::os::unix::ffi::OsStrExt;
+
+    unsafe extern "C" {
+        fn bwrap_main(argc: libc::c_int, argv: *const *const c_char) -> libc::c_int;
+    }
+
+    let cstrings = std::env::args_os()
+        .map(|arg| {
+            CString::new(arg.as_os_str().as_bytes())
+                .unwrap_or_else(|err| panic!("failed to convert argv to CString: {err}"))
+        })
+        .collect::<Vec<_>>();
+    let mut argv_ptrs = cstrings
+        .iter()
+        .map(CString::as_c_str)
+        .map(CStr::as_ptr)
+        .collect::<Vec<*const c_char>>();
+    argv_ptrs.push(std::ptr::null());
+
+    // SAFETY: We provide a null-terminated argv vector whose pointers remain
+    // valid for the duration of the call.
+    let exit_code = unsafe { bwrap_main(cstrings.len() as libc::c_int, argv_ptrs.as_ptr()) };
+    std::process::exit(exit_code);
+}
+
+#[cfg(all(target_os = "linux", not(bwrap_available)))]
+fn main() {
+    panic!(
+        r#"bubblewrap is not available in this build.
+Notes:
+- ensure the target OS is Linux
+- libcap headers must be available via pkg-config
+- bubblewrap sources expected at codex-rs/vendor/bubblewrap (default)"#
+    );
+}
+
+#[cfg(not(target_os = "linux"))]
+fn main() {
+    panic!("bwrap is only supported on Linux");
+}