diff --git a/lib/kgrader/assignment.rb b/lib/kgrader/assignment.rb
index 04bf0ce..cf12807 100644
--- a/lib/kgrader/assignment.rb
+++ b/lib/kgrader/assignment.rb
@@ -30,7 +30,8 @@ module KGrader
 
     def tests
       @tests ||= @config['grade'].map do |it|
-        { :name => it.keys.first, :max => it.values.first }
+        script = File.join @root, it.keys.first + ".rb"
+        { :name => it.keys.first, :script => script, :max => it.values.first }
       end
     end
 
diff --git a/lib/kgrader/backend/svn.rb b/lib/kgrader/backend/svn.rb
index 935de1c..4af767f 100644
--- a/lib/kgrader/backend/svn.rb
+++ b/lib/kgrader/backend/svn.rb
@@ -36,6 +36,7 @@ module KGrader::Backend
 
     def commit(repo, message, paths = nil)
       # TODO
+      # run 'commit', '-m', message, *paths.map { |fn| File.join repo, fn }
     end
 
     def commit_date(repo)
diff --git a/lib/kgrader/jail.rb b/lib/kgrader/jail.rb
index 4e7f729..84c841e 100644
--- a/lib/kgrader/jail.rb
+++ b/lib/kgrader/jail.rb
@@ -3,6 +3,7 @@ module KGrader
 
     def initialize(root)
       @root = root
+      @salt = nil
     end
 
     def reset
@@ -11,6 +12,7 @@ module KGrader
 
     def init
       FileUtils.mkdir_p @root
+      @salt = rand 100000000
     end
 
     def stage(source, target)
@@ -18,15 +20,43 @@ module KGrader
     end
 
     def exec(command, logpath)
-      pid = Process.fork do
-        fp = File.open(logpath, 'w+')
-        Dir.chdir @root
-        # TODO: rlimit in exec, umask?
-        Process.exec command, :in => :close, :out => fp, :err => fp,
-          :close_others => true
-      end
+      pid = execute command, logpath
       Process.waitpid pid, 0
       $?.exited? && $?.exitstatus == 0
     end
+
+    def run_test(script, logpath)
+      grade_rd, grade_wr = IO.pipe
+      cmt_rd, cmt_wr = IO.pipe
+
+      command = ['ruby', '-r', '../lib/kgrader/runtime.rb', script, @salt]
+      pid = execute command, logpath do |options|
+        [grade_rd, cmt_rd].each &:close
+        options[3] = grade_wr
+        options[4] = cmt_wr
+      end
+
+      [grade_wr, cmt_wr].each &:close
+      Process.waitpid pid, 0
+
+      cmt_rd.read.split("\n").each { |cmt| yield cmt } if block_given?
+      grade = grade_rd.read.strip.to_i
+      [grade_rd, cmt_rd].each &:close
+      grade
+    end
+
+    private
+    def execute(command, logpath)
+      Process.fork do
+        fp = File.open(logpath, 'a')
+        Dir.chdir @root
+        options = {
+          :in => :close, :out => fp, :err => fp, :close_others => true,
+          :rlimit_nproc => 32
+        }
+        yield options if block_given?
+        Process.exec *command, options
+      end
+    end
   end
 end
diff --git a/lib/kgrader/submission.rb b/lib/kgrader/submission.rb
index bf3ceab..921c890 100644
--- a/lib/kgrader/submission.rb
+++ b/lib/kgrader/submission.rb
@@ -60,10 +60,9 @@ module KGrader
 
     def commit
       if status == :graded && File.exists?(pendingfile)
-        target = File.join(repo, @assignment.report)
         message = @assignment.commit_message @student
-        FileUtils.cp gradefile, target
-        @course.backend.commit repo, message, target
+        FileUtils.cp gradefile, File.join(repo, @assignment.report)
+        @course.backend.commit repo, message, @assignment.report
         FileUtils.rm pendingfile
       end
     end
@@ -113,16 +112,12 @@ module KGrader
       @failure = false
       @comments = []
       @summary = nil
-      @tests = []
+      @tests = @assignment.tests.clone.each { |test| test[:score] = 0 }
 
       self.status = :ungraded
       FileUtils.rm_f [buildlog, testlog]
       @fs.jail.reset
       @fs.jail.init
-
-      @assignment.tests.each do |test|
-        @tests.push({ :name => test[:name], :max => test[:max], :score => 0 })
-      end
     end
 
     def stage
@@ -142,8 +137,10 @@ module KGrader
 
     def test
       return if @failure
-      @assignment.tests.each do |test|
-        # TODO: execute script in jail and update @test/@comments; out testlog
+      @tests.each do |test|
+        test[:score] = @fs.jail.run_test test[:script], testlog do |comment|
+          @comments.push "#{test[:name]}: #{comment}"
+        end
       end
     end
 
@@ -199,7 +196,7 @@ module KGrader
     end
 
     def generate_summary
-      tests = @tests.each do |test|
+      tests = @tests.map do |test|
         "#{test[:score].to_s.rjust get_span(test[:max])}/#{test[:max]}"
       end.join ', '
       "#{format_points score, max_score}: #{tests}"