child_process: Add gid/uid flags to spawn config

This is mostly working, but not completely ideal for two reasons.

1. Rather than emitting an error on the ChildProcess object when the
setgid/setuid fails, it is simply printing the error to stderr and
exiting.  The same happens with the cwd, so that's not completely
terrible.

2. I don't have a good test for this.  It fails with an EPERM if you try
to change the uid or gid as a non-root user.

lib/child_process.js

@@ -185,21 +185,27 @@ ChildProcess.prototype.kill = function(sig) {
 ChildProcess.prototype.spawn = function(path, args, options, customFds) {
   args = args || [];
 
-  var cwd, env, setsid;
+  var cwd, env, setsid, uid, gid;
   if (!options || options.cwd === undefined &&
       options.env === undefined &&
-      options.customFds === undefined) {
+      options.customFds === undefined &&
+      options.gid === undefined &&
+      options.uid === undefined) {
     // Deprecated API: (path, args, options, env, customFds)
     cwd = '';
     env = options || process.env;
     customFds = customFds || [-1, -1, -1];
     setsid = false;
+    uid = -1;
+    gid = -1;
   } else {
     // Recommended API: (path, args, options)
     cwd = options.cwd || '';
     env = options.env || process.env;
     customFds = options.customFds || [-1, -1, -1];
     setsid = options.setsid ? true : false;
+    uid = options.hasOwnProperty("uid") ? options.uid : -1;
+    gid = options.hasOwnProperty("gid") ? options.gid : -1;
   }
 
   var envPairs = [];
@@ -214,7 +220,9 @@ ChildProcess.prototype.spawn = function(path, args, options, customFds) {
                                  cwd,
                                  envPairs,
                                  customFds,
-                                 setsid);
+                                 setsid,
+                                 uid,
+                                 gid);
   this.fds = fds;
 
   if (customFds[0] === -1 || customFds[0] === undefined) {

src/node_child_process.cc

@@ -158,7 +158,18 @@ Handle<Value> ChildProcess::Spawn(const Arguments& args) {
 
   int fds[3];
 
-  int r = child->Spawn(argv[0], argv, cwd, env, fds, custom_fds, do_setsid);
+  int uid = args[6]->ToInteger()->Value();
+  int gid = args[7]->ToInteger()->Value();
+
+  int r = child->Spawn(argv[0],
+                       argv,
+                       cwd,
+                       env,
+                       fds,
+                       custom_fds,
+                       do_setsid,
+                       uid,
+                       gid);
 
   for (i = 0; i < argv_length; i++) free(argv[i]);
   delete [] argv;
@@ -233,7 +244,9 @@ int ChildProcess::Spawn(const char *file,
                         char **env,
                         int stdio_fds[3],
                         int custom_fds[3],
-                        bool do_setsid) {
+                        bool do_setsid,
+                        int custom_uid,
+                        int custom_gid) {
   HandleScope scope;
   assert(pid_ == -1);
   assert(!ev_is_active(&child_watcher_));
@@ -276,6 +289,17 @@ int ChildProcess::Spawn(const char *file,
     case 0:  // Child.
       if (do_setsid && setsid() < 0) {
         perror("setsid");
+        _exit(127);
+      }
+
+      if (custom_gid != -1 && setgid(custom_gid)) {
+        perror("setgid()");
+        _exit(127);
+      }
+
+      if (custom_uid != -1 && setuid(custom_uid)) {
+        perror("setuid()");
+        _exit(127);
       }
 
       if (custom_fds[0] == -1) {

src/node_child_process.h

@@ -64,7 +64,9 @@ class ChildProcess : ObjectWrap {
             char **env,
             int stdio_fds[3],
             int custom_fds[3],
-            bool do_setsid);
+            bool do_setsid,
+            int uid,
+            int gid);
 
   // Simple syscall wrapper. Does not disable the watcher. onexit will be
   // called still.

test/disabled/test-child-process-uid-gid.js

@@ -0,0 +1,41 @@
+// must be run as sudo, otherwise the gid/uid setting will fail.
+var child_process = require("child_process"),
+    constants = require("constants"),
+    passwd = require("fs").readFileSync("/etc/passwd", "utf8"),
+    myUid = process.getuid(),
+    myGid = process.getgid();
+
+// get a different user.
+// don't care who it is, as long as it's not root
+passwd = passwd.trim().split(/\n/);
+for (var i = 0, l = passwd.length; i < l; i ++) {
+  if (passwd[i].charAt(0) === "#") continue;
+  passwd[i] = passwd[i].split(":");
+  var otherName = passwd[i][0];
+  var otherUid = +passwd[i][2];
+  var otherGid = +passwd[i][3];
+  if (otherUid && otherUid !== myUid &&
+      otherGid && otherGid !== myGid &&
+      otherUid > 0) {
+    break;
+  }
+}
+if (!otherUid && !otherGid) throw new Error("failed getting passwd info.");
+
+console.error("name, id, gid = %j", [otherName, otherUid, otherGid]);
+
+var whoNumber = child_process.spawn("id",[], {uid:otherUid,gid:otherGid}),
+    assert = require("assert");
+
+whoNumber.stdout.buf = "byNumber:";
+whoNumber.stdout.on("data", onData);
+function onData (c) { this.buf += c; }
+
+whoNumber.on("exit", onExit);
+function onExit (code) {
+  var buf = this.stdout.buf;
+  console.log(buf);
+  var expr = new RegExp("^byNumber:uid="+otherUid+"\\("+
+                        otherName+"\\) gid="+otherGid+"\\(");
+  assert.ok(buf.match(expr), "uid and gid should match "+otherName);
+}