vfs: load native addons from a mounted file system

A native addon inside a virtual file system could not be require()d:
dlopen() and LoadLibrary() open a shared object by path, and a VFS path
has no inode for them to open.

Read the addon's bytes from the VFS and load them from a private,
self-cleaning image instead, using the smallest on-disk footprint each
platform allows: on Linux an anonymous memfd loaded through
/proc/self/fd, so the bytes never reach the file system; on other POSIX
platforms a file in a 0700 mkdtemp() directory, unlinked right after
loading, with the mapping keeping it alive; on Windows a temp file
opened FILE_FLAG_DELETE_ON_CLOSE whose handle is held until exit.

This is internal. process.dlopen() keeps its documented
(module, filename[, flags]) signature and ignores anything further; the
bytes are passed to a dlopenBinary() reachable only through the
process_methods binding, which the VFS hook calls for VFS paths. Addons
on the real file system are untouched and load directly.

Signed-off-by: Philipp Dunkel <pip@pipobscure.com>
PR-URL: #65680
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: pipobscure

doc/api/cli.md

@@ -268,6 +268,26 @@ process.permission.has('fs.read', 'custom-require.js'); // true
 process.permission.has('fs.read', 'custom-require-2.js'); // true
 ```
 
+### `--allow-fs-vfs`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1.1 - Active development
+
+When using the [Permission Model][], a [virtual file system][] cannot be
+mounted by default: [`vfs.mount()`][] throws `ERR_INVALID_STATE` unless the
+user explicitly passes the `--allow-fs-vfs` flag when starting Node.js.
+
+A mounted VFS serves paths that the file system permissions do not describe,
+so mounting one is gated on its own flag rather than on `--allow-fs-read` or
+`--allow-fs-write`.
+
+```console
+$ node --experimental-vfs --permission --allow-fs-vfs app.js
+```
+
 ### `--allow-fs-write`
 
 <!-- YAML
@@ -4021,6 +4041,7 @@ one is included in the list below.
 * `--allow-child-process`
 * `--allow-ffi`
 * `--allow-fs-read`
+* `--allow-fs-vfs`
 * `--allow-fs-write`
 * `--allow-inspector`
 * `--allow-net`
@@ -4741,6 +4762,7 @@ node --stack-trace-limit=12 -p -e "Error.stackTraceLimit" # prints 12
 [`v8.startupSnapshot.addDeserializeCallback()`]: v8.md#v8startupsnapshotadddeserializecallbackcallback-data
 [`v8.startupSnapshot.setDeserializeMainFunction()`]: v8.md#v8startupsnapshotsetdeserializemainfunctioncallback-data
 [`v8.startupSnapshot` API]: v8.md#startup-snapshot-api
+[`vfs.mount()`]: vfs.md#vfsmount
 [asynchronous module customization hooks]: module.md#asynchronous-customization-hooks
 [benchmark runner]: bench.md#command-line-runner
 [captured by the built-in snapshot of Node.js]: https://github.com/nodejs/node/blob/b19525a33cc84033af4addd0f80acd4dc33ce0cf/test/parallel/test-bootstrap-modules.js#L24
@@ -4773,4 +4795,5 @@ node --stack-trace-limit=12 -p -e "Error.stackTraceLimit" # prints 12
 [test runner execution model]: test.md#test-runner-execution-model
 [timezone IDs]: https://en.wikipedia.org/wiki/List_of_tz_database_time_zones
 [tracking issue for user-land snapshots]: https://github.com/nodejs/node/issues/44014
+[virtual file system]: vfs.md
 [ways that `TZ` is handled in other environments]: https://www.gnu.org/software/libc/manual/html_node/TZ-Variable.html

doc/api/vfs.md

@@ -419,6 +419,12 @@ system, the callers are responsible for avoiding removal or
 invalidation of modules in the virtual file system while they are
 being loaded.
 
+Native addons (`.node` files) stored in a mounted VFS can be `require()`d as
+well. The operating system's dynamic loader cannot open a virtual path, so the
+addon's bytes are read from the VFS and loaded from a private, self-cleaning
+temporary image instead. Addons on the real file system are unaffected and
+load directly.
+
 ## Use with Single Executable Applications
 
 When running as a [Single Executable Application][] built with

doc/node.1

@@ -166,6 +166,17 @@ process.permission.has('fs.read', 'custom-require.js'); // true
 process.permission.has('fs.read', 'custom-require-2.js'); // true
 .Ed
 .
+.It Fl -allow-fs-vfs
+When using the Permission Model, a virtual file system cannot be
+mounted by default: \fBvfs.mount()\fR throws \fBERR_INVALID_STATE\fR unless the
+user explicitly passes the \fB--allow-fs-vfs\fR flag when starting Node.js.
+A mounted VFS serves paths that the file system permissions do not describe,
+so mounting one is gated on its own flag rather than on \fB--allow-fs-read\fR or
+\fB--allow-fs-write\fR.
+.Bd -literal
+$ node --experimental-vfs --permission --allow-fs-vfs app.js
+.Ed
+.
 .It Fl -allow-fs-write
 This flag configures file system write permissions using
 the Permission Model.
@@ -2018,6 +2029,8 @@ one is included in the list below.
 .It
 \fB--allow-fs-read\fR
 .It
+\fB--allow-fs-vfs\fR
+.It
 \fB--allow-fs-write\fR
 .It
 \fB--allow-inspector\fR

lib/internal/vfs/setup.js

@@ -914,6 +914,26 @@ function installModuleLoaderOverrides() {
   });
 }
 
+let originalDlopen;
+
+function installAddonLoader() {
+  originalDlopen = process.dlopen;
+  process.dlopen = function(module, filename, flags) {
+    // dlopen(2) cannot open a native addon that lives in a VFS by path (it has
+    // no real inode). Read its bytes and hand them to the internal
+    // dlopenBinary(), which writes them to a private, self-cleaning temporary
+    // image - an in-memory memfd on Linux - and loads that. Only VFS paths take
+    // this route; everything else loads straight from disk through the
+    // unchanged process.dlopen().
+    if (StringPrototypeStartsWith(filename, normalizedVfsRootPrefix)) {
+      const { readFileSync } = require('fs');
+      const { dlopenBinary } = internalBinding('process_methods');
+      return dlopenBinary(module, filename, flags, readFileSync(filename));
+    }
+    return originalDlopen(module, filename, flags);
+  };
+}
+
 /**
  * Install all VFS hooks: module loader overrides and fs handlers.
  */
@@ -922,6 +942,7 @@ function installHooks() {
   debug('install hooks');
   normalizedVfsRootPrefix = getNormalizedVfsRoot() + sep;
   installModuleLoaderOverrides();
+  installAddonLoader();
   vfsHandlerObj = createVfsHandlers();
   setVfsHandlers(vfsHandlerObj);
   hooksInstalled = true;
@@ -939,6 +960,7 @@ function uninstallHooks() {
   setLoaderOverrides();
   setVfsHandlers(null);
   vfsHandlerObj = undefined;
+  process.dlopen = originalDlopen;
   hooksInstalled = false;
 }