build(linux-musl): statically link Bun's C++ runtime

scripts/build/flags.ts

@@ -1201,13 +1201,8 @@ export const linkerFlags: Flag[] = [
   },
   {
     flag: ["-static-libstdc++", "-static-libgcc"],
-    when: c => c.linux && c.abi === "gnu",
-    desc: "Static C++ runtime (don't depend on host libstdc++)",
-  },
-  {
-    flag: ["-lstdc++", "-lgcc"],
-    when: c => c.linux && c.abi === "musl",
-    desc: "Dynamic C++ runtime on musl (static unavailable)",
+    when: c => c.linux && (c.abi === "gnu" || c.abi === "musl"),
+    desc: "Static C++ runtime (don't depend on host libstdc++/libgcc_s)",
   },
   {
     flag: c => [

src/jsc/bindings/BunProcess.cpp

@@ -400,6 +400,7 @@ extern "C" void Bun__unlink(const char*, size_t);
 extern "C" void CrashHandler__setDlOpenAction(const char* action);
 extern "C" bool Bun__VM__allowAddons(void* vm);
 extern "C" int32_t Bun__addonNeedsGlibcOnMusl(const char* path, size_t len, char* soname_out, size_t soname_cap);
+extern "C" void Bun__loadMuslCxxRuntimeIfPresent();
 
 JSC_DEFINE_HOST_FUNCTION(Process_functionDlopen, (JSC::JSGlobalObject * globalObject_, JSC::CallFrame* callFrame))
 {
@@ -564,6 +565,7 @@ JSC_DEFINE_HOST_FUNCTION(Process_functionDlopen, (JSC::JSGlobalObject * globalOb
             return throwError(globalObject, scope, ErrorCode::ERR_DLOPEN_FAILED, msg.toString());
         }
     }
+    Bun__loadMuslCxxRuntimeIfPresent();
 #endif
     CrashHandler__setDlOpenAction(utf8.data());
     void* handle = dlopen(utf8.data(), RTLD_LAZY);

src/runtime/napi/libc_check.rs

@@ -1,14 +1,49 @@
-//! Pre-`dlopen` libc-mismatch detection for native addons on Linux.
+//! Native-addon loader preflight helpers for Linux.
 //!
 //! A glibc-linked `.node` loaded into a musl process (typically via Alpine's
 //! `gcompat` shim, which satisfies the `libc.so.6` soname but not the ABI)
 //! segfaults inside the dynamic loader during relocation. That crash is not
 //! catchable from JS and looks like a Bun bug. Instead, inspect the addon's
 //! ELF `PT_DYNAMIC` segment before calling `dlopen` and surface a
 //! `ERR_DLOPEN_FAILED` that names the problem. See issue #15753.
+//!
+//! On Linux-musl, this module also performs a one-time best-effort load of the
+//! optional host C++ runtime into the process-global namespace before addon
+//! `dlopen`.
 
 use core::ffi::c_char;
 
+/// A musl Bun embeds its own C++ runtime so the executable can start without
+/// Alpine's `libstdc++` and `libgcc` packages. Before the first native addon,
+/// opportunistically restore the shared runtime provider that musl addons
+/// historically inherited from Bun's startup dependencies. Keeping the
+/// handle open for the process lifetime makes its symbols available to this
+/// and later addons. Missing packages are not an error here: self-contained
+/// addons may still load, and `process.dlopen` reports the original loader
+/// error for addons which need them.
+#[unsafe(no_mangle)]
+pub(crate) extern "C" fn Bun__loadMuslCxxRuntimeIfPresent() {
+    #[cfg(all(target_os = "linux", target_env = "musl"))]
+    {
+        static LOAD: std::sync::Once = std::sync::Once::new();
+        LOAD.call_once(|| {
+            // SAFETY: the name is NUL-terminated. The successful handle is
+            // intentionally retained for the lifetime of the process.
+            let handle = unsafe {
+                libc::dlopen(
+                    c"libstdc++.so.6".as_ptr(),
+                    libc::RTLD_NOW | libc::RTLD_GLOBAL,
+                )
+            };
+            if handle.is_null() {
+                // SAFETY: consume the error from this best-effort probe so it
+                // cannot leak into the addon's own loader/error handling.
+                let _ = unsafe { libc::dlerror() };
+            }
+        });
+    }
+}
+
 /// Called from `Process_functionDlopen` (BunProcess.cpp) immediately before
 /// `dlopen`. Returns `1` when the file at `path_ptr[..path_len]` is an ELF
 /// shared object whose `DT_NEEDED` list references glibc and this process is

test/internal/source-lints/musl-static-cxx-runtime.test.ts

@@ -0,0 +1,45 @@
+import { expect, test } from "bun:test";
+import type { Config } from "../../../scripts/build/config.ts";
+import { computeFlags } from "../../../scripts/build/flags.ts";
+
+const config = (abi: "gnu" | "musl", arm64: boolean) =>
+  ({
+    os: "linux",
+    arch: arm64 ? "aarch64" : "x64",
+    linux: true,
+    unix: true,
+    darwin: false,
+    windows: false,
+    freebsd: false,
+    abi,
+    x64: !arm64,
+    arm64,
+    release: true,
+    debug: false,
+    asan: false,
+    valgrind: false,
+    fuzzilli: false,
+    lto: false,
+    canary: true,
+    ci: true,
+    buildkite: false,
+    cwd: "/repo",
+    buildDir: "/repo/build/release",
+    ld: "/usr/bin/ld.lld",
+    rustLld: undefined,
+    crossTarget: undefined,
+  }) as Config;
+
+test.each([
+  ["gnu", "x64", false],
+  ["gnu", "aarch64", true],
+  ["musl", "x64", false],
+  ["musl", "aarch64", true],
+] as const)("linux-%s-%s embeds the C++ runtime", (abi, _, arm64) => {
+  const flags = computeFlags(config(abi, arm64)).ldflags;
+
+  expect(flags).toContain("-static-libstdc++");
+  expect(flags).toContain("-static-libgcc");
+  expect(flags).not.toContain("-lstdc++");
+  expect(flags).not.toContain("-lgcc");
+});

test/regression/issue/29681-cxx-exception-addon.cpp

@@ -0,0 +1,24 @@
+#include "node_api.h"
+
+struct ValidationException {
+  int value;
+};
+
+static volatile int validation_seed = 41;
+
+static int throw_and_catch() {
+  try {
+    throw ValidationException{validation_seed};
+  } catch (const ValidationException &exception) {
+    return exception.value + 1;
+  }
+}
+
+NAPI_MODULE_INIT() {
+  napi_value caught;
+  if (napi_create_int32(env, throw_and_catch(), &caught) != napi_ok)
+    return nullptr;
+  if (napi_set_named_property(env, exports, "caught", caught) != napi_ok)
+    return nullptr;
+  return exports;
+}

test/regression/issue/29681-cxx-runtime-addon.c

@@ -0,0 +1,24 @@
+#include "node_api.h"
+
+/*
+ * Model a legacy native addon which expects the host process to provide the
+ * C++ runtime. The final link intentionally omits libstdc++, leaving these
+ * relocations for process.dlopen() to resolve from the global loader scope.
+ */
+extern void *cxx_operator_new(size_t size) __asm__("_Znwm");
+extern void cxx_operator_delete(void *ptr) __asm__("_ZdlPv");
+
+static void *(*volatile cxx_operator_new_ptr)(size_t) = cxx_operator_new;
+static void (*volatile cxx_operator_delete_ptr)(void *) = cxx_operator_delete;
+
+NAPI_MODULE_INIT() {
+  void *allocation = cxx_operator_new_ptr(32);
+  cxx_operator_delete_ptr(allocation);
+
+  napi_value loaded;
+  if (napi_get_boolean(env, true, &loaded) != napi_ok)
+    return NULL;
+  if (napi_set_named_property(env, exports, "loaded", loaded) != napi_ok)
+    return NULL;
+  return exports;
+}

test/regression/issue/29681.test.ts

@@ -0,0 +1,165 @@
+// https://github.com/oven-sh/bun/issues/29681
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe, isMusl, tempDir } from "harness";
+import { join } from "node:path";
+
+const cc = process.env.CC || Bun.which("cc") || Bun.which("clang") || Bun.which("gcc");
+const cxx = process.env.CXX || Bun.which("c++") || Bun.which("clang++") || Bun.which("g++");
+const readelf = Bun.which("readelf");
+
+function hasOptionalCxxRuntimeProvider(): boolean {
+  if (!isMusl) return false;
+
+  try {
+    const probe = Bun.spawnSync({
+      cmd: [
+        bunExe(),
+        "-e",
+        `import { dlopen } from "bun:ffi"; const library = dlopen("libstdc++.so.6", { _Znwm: { args: ["usize"], returns: "ptr" } }); library.close();`,
+      ],
+      env: bunEnv,
+      stdout: "ignore",
+      stderr: "ignore",
+    });
+    return probe.exitCode === 0;
+  } catch {
+    return false;
+  }
+}
+
+const hasCxxRuntimeProvider = hasOptionalCxxRuntimeProvider();
+
+test.skipIf(!isMusl || !readelf)("bun does not depend on the host C++ runtime", () => {
+  const dynamic = Bun.spawnSync([readelf!, "-d", bunExe()]);
+  const stdout = dynamic.stdout.toString();
+  const forbiddenDependency = stdout.match(/NEEDED.*(?:libstdc\+\+\.so\.6|libgcc_s\.so\.1)/)?.[0] ?? "";
+  expect({ forbiddenDependency, stderr: dynamic.stderr.toString(), exitCode: dynamic.exitCode }).toEqual({
+    forbiddenDependency: "",
+    stderr: "",
+    exitCode: 0,
+  });
+});
+
+interface AddonFixture {
+  compiler: string;
+  compilerFlags?: string[];
+  expectedResult: boolean | number;
+  expectedSymbols: string[];
+  resultProperty: string;
+  source: string;
+}
+
+async function compileAndLoadAddon(fixture: AddonFixture) {
+  using dir = tempDir("issue-29681", {
+    "load.js": `
+      const { readFileSync } = require("node:fs");
+      const runtimeMaps = () => readFileSync("/proc/self/maps", "utf8")
+        .split("\\n")
+        .filter(line => line.includes("libstdc++.so.6") || line.includes("libgcc_s.so.1"));
+
+      const before = runtimeMaps();
+      const addon = require("./addon.node");
+      const result = addon[${JSON.stringify(fixture.resultProperty)}];
+      const after = runtimeMaps();
+      console.log(JSON.stringify({ before, result, after }));
+    `,
+  });
+  const dirPath = String(dir);
+  const addonPath = join(dirPath, "addon.node");
+  const fixturePath = join(import.meta.dir, fixture.source);
+  const napiInclude = join(import.meta.dir, "..", "..", "..", "src", "runtime", "napi");
+
+  await using compile = Bun.spawn({
+    cmd: [
+      fixture.compiler,
+      ...(fixture.compilerFlags ?? []),
+      "-shared",
+      "-fPIC",
+      "-nostdlib",
+      "-Wl,-z,now",
+      "-I",
+      napiInclude,
+      "-o",
+      addonPath,
+      fixturePath,
+    ],
+    env: bunEnv,
+    stderr: "pipe",
+    stdout: "pipe",
+  });
+  const [compileStdout, compileStderr, compileExitCode] = await Promise.all([
+    compile.stdout.text(),
+    compile.stderr.text(),
+    compile.exited,
+  ]);
+  expect({ stdout: compileStdout, stderr: compileStderr, exitCode: compileExitCode }).toEqual({
+    stdout: "",
+    stderr: "",
+    exitCode: 0,
+  });
+
+  const dynamic = Bun.spawnSync([readelf!, "-d", addonPath]);
+  const symbols = Bun.spawnSync([readelf!, "-Ws", addonPath]);
+  expect({ stderr: dynamic.stderr.toString(), exitCode: dynamic.exitCode }).toEqual({ stderr: "", exitCode: 0 });
+  expect({ stderr: symbols.stderr.toString(), exitCode: symbols.exitCode }).toEqual({ stderr: "", exitCode: 0 });
+  expect(dynamic.stdout.toString()).not.toContain("NEEDED");
+  for (const symbol of fixture.expectedSymbols) {
+    expect(symbols.stdout.toString()).toContain(`UND ${symbol}`);
+  }
+
+  await using run = Bun.spawn({
+    cmd: [bunExe(), join(dirPath, "load.js")],
+    cwd: dirPath,
+    env: { ...bunEnv, LD_PRELOAD: undefined },
+    stderr: "pipe",
+    stdout: "pipe",
+  });
+  const [stdout, stderr, exitCode] = await Promise.all([run.stdout.text(), run.stderr.text(), run.exited]);
+  let parsedStdout: unknown = stdout;
+  try {
+    parsedStdout = JSON.parse(stdout);
+  } catch {}
+  expect({ parsedStdout, stderr, exitCode }).toEqual({
+    parsedStdout: {
+      before: [],
+      result: fixture.expectedResult,
+      after: expect.arrayContaining([expect.stringContaining("libstdc++.so.6")]),
+    },
+    stderr: "",
+    exitCode: 0,
+  });
+}
+
+// The compatibility provider is optional, so clean musl developer images may not have it.
+test.skipIf(!isMusl || !cc || !readelf || !hasCxxRuntimeProvider)(
+  "a legacy new/delete addon can use the optional host C++ runtime",
+  () =>
+    compileAndLoadAddon({
+      compiler: cc!,
+      expectedResult: true,
+      expectedSymbols: ["_Znwm", "_ZdlPv"],
+      resultProperty: "loaded",
+      source: "29681-cxx-runtime-addon.c",
+    }),
+);
+
+test.skipIf(!isMusl || !cxx || !readelf || !hasCxxRuntimeProvider)(
+  "a legacy exception addon can use the optional host C++ runtime",
+  () =>
+    compileAndLoadAddon({
+      compiler: cxx!,
+      compilerFlags: ["-O0", "-fexceptions"],
+      expectedResult: 42,
+      expectedSymbols: [
+        "__cxa_allocate_exception",
+        "__cxa_throw",
+        "__cxa_begin_catch",
+        "__cxa_end_catch",
+        "__gxx_personality_v0",
+        "_Unwind_Resume",
+        "_ZTVN10__cxxabiv117__class_type_infoE",
+      ],
+      resultProperty: "caught",
+      source: "29681-cxx-exception-addon.cpp",
+    }),
+);