daemon: Fix possible use-after-free.

This is essentially a backport of
<f52b6c944e>
by Eelco Dolstra <eelco.dolstra@logicblox.com>.

The use-after-free bug would typically manifest when building with
GCC 5.1.

nix/libstore/build.cc

@@ -401,18 +401,6 @@ static void commonChildInit(Pipe & logPipe)
 }
 
 
-/* Convert a string list to an array of char pointers.  Careful: the
-   string list should outlive the array. */
-const char * * strings2CharPtrs(const Strings & ss)
-{
-    const char * * arr = new const char * [ss.size() + 1];
-    const char * * p = arr;
-    foreach (Strings::const_iterator, i, ss) *p++ = i->c_str();
-    *p = 0;
-    return arr;
-}
-
-
 /* Restore default handling of SIGPIPE, otherwise some programs will
    randomly say "Broken pipe". */
 static void restoreSIGPIPE()
@@ -2135,11 +2123,7 @@ void DerivationGoal::initChild()
         Strings envStrs;
         foreach (Environment::const_iterator, i, env)
             envStrs.push_back(rewriteHashes(i->first + "=" + i->second, rewritesToTmp));
-        const char * * envArr = strings2CharPtrs(envStrs);
-
-        Path program = drv.builder.c_str();
-        std::vector<const char *> args; /* careful with c_str()! */
-        string user; /* must be here for its c_str()! */
+	std::vector<const char *> envArr = stringsToCharPtrs(envStrs);
 
         /* If we are running in `build-users' mode, then switch to the
            user we allocated above.  Make sure that we drop all root
@@ -2165,17 +2149,18 @@ void DerivationGoal::initChild()
         }
 
         /* Fill in the arguments. */
+	Strings args;
         string builderBasename = baseNameOf(drv.builder);
         args.push_back(builderBasename.c_str());
         foreach (Strings::iterator, i, drv.args)
-            args.push_back(rewriteHashes(*i, rewritesToTmp).c_str());
-        args.push_back(0);
+            args.push_back(rewriteHashes(*i, rewritesToTmp));
+	std::vector<const char *> argArr = stringsToCharPtrs(args);
 
         restoreSIGPIPE();
 
         /* Execute the program.  This should not return. */
         inSetup = false;
-        execve(program.c_str(), (char * *) &args[0], (char * *) envArr);
+        execve(drv.builder.c_str(), (char * *) &argArr[0], (char * *) &envArr[0]);
 
         throw SysError(format("executing `%1%'") % drv.builder);
 
@@ -2778,7 +2763,7 @@ void SubstitutionGoal::tryToRun()
     args.push_back("--substitute");
     args.push_back(storePath);
     args.push_back(destPath);
-    const char * * argArr = strings2CharPtrs(args);
+    std::vector<const char *> argArr = stringsToCharPtrs(args);
 
     /* Fork the substitute program. */
     pid = maybeVfork();
@@ -2796,7 +2781,7 @@ void SubstitutionGoal::tryToRun()
             if (dup2(outPipe.writeSide, STDOUT_FILENO) == -1)
                 throw SysError("cannot dup output pipe into stdout");
 
-            execv(sub.c_str(), (char * *) argArr);
+            execv(sub.c_str(), (char * *) &argArr[0]);
 
             throw SysError(format("executing `%1%'") % sub);
 

nix/libutil/util.cc

@@ -852,16 +852,20 @@ void killUser(uid_t uid)
 //////////////////////////////////////////////////////////////////////
 
 
+std::vector<const char *> stringsToCharPtrs(const Strings & ss)
+{
+    std::vector<const char *> res;
+    foreach (Strings::const_iterator, i, ss)
+        res.push_back(i->c_str());
+    res.push_back(0);
+    return res;
+}
+
+
 string runProgram(Path program, bool searchPath, const Strings & args)
 {
     checkInterrupt();
 
-    std::vector<const char *> cargs; /* careful with c_str()! */
-    cargs.push_back(program.c_str());
-    for (Strings::const_iterator i = args.begin(); i != args.end(); ++i)
-        cargs.push_back(i->c_str());
-    cargs.push_back(0);
-
     /* Create a pipe. */
     Pipe pipe;
     pipe.create();
@@ -880,6 +884,10 @@ string runProgram(Path program, bool searchPath, const Strings & args)
             if (dup2(pipe.writeSide, STDOUT_FILENO) == -1)
                 throw SysError("dupping stdout");
 
+	    Strings args_(args);
+	    args_.push_front(program);
+	    auto cargs = stringsToCharPtrs(args_);
+
             if (searchPath)
                 execvp(program.c_str(), (char * *) &cargs[0]);
             else

nix/libutil/util.hh

@@ -257,6 +257,11 @@ void killUser(uid_t uid);
 string runProgram(Path program, bool searchPath = false,
     const Strings & args = Strings());
 
+/* Convert a list of strings to a null-terminated vector of char
+   *'s. The result must not be accessed beyond the lifetime of the
+   list of strings. */
+std::vector<const char *> stringsToCharPtrs(const Strings & ss);
+
 /* Close all file descriptors except stdin, stdout, stderr, and those
    listed in the given set.  Good practice in child processes. */
 void closeMostFDs(const set<int> & exceptions);