Initial code for webtry, a web application for allowing users to try out Skia.

experimental/webtry/DESIGN

+Design
+======
+
+
+Overview
+--------
+Allows trying out Skia code in the browser.
+
+
+Security
+--------
+We're putting a C++ compiler on the web, and promising to run the results of
+user submitted code, so security is a large concern. Security is handled in a
+layered approach, using a combination of seccomp-bpf, chroot jail and rlimits.
+
+*seccomp-bpf* - Used to limit the types of system calls that the user code can
+make. Any attempts to make a system call that isn't allowed causes the
+application to terminate immediately.
+
+*chroot jail* - The code is run in a chroot jail, making the rest of the
+operating system files unreachable from the running code.
+
+*rlimits* - Used to limit the resources the running code can get access to,
+for example runtime is limited to 5s of CPU.
+
+User submitted code is also restricted in the following ways:
+  * Limited to 10K of code total.
+  * No preprocessor use is allowed (no lines can begin with \s*#).
+
+
+Architecture
+------------
+
+The server runs on GCE, and consists of a Go Web Server that calls out to the
+c++ compiler and executes code in a chroot jail. See the diagram below:
+
+                           
+   +–––––––––––––+         
+   |             |         
+   |  Browser    |         
+   |             |         
+   +––––––+––––––+         
+          |                
+   +––––––+––––––+         
+   |             |         
+   |             |         
+   | Web Server  |         
+   |             |         
+   |   (Go)      |         
+   |             |         
+   |             |         
+   +–––––––+–––––+         
+           |               
+   +–––––––+––––––––––+    
+   | chroot jail      |    
+   |  +––––––––––––––+|    
+   |  | seccomp      ||    
+   |  |  +––––––––––+||    
+   |  |  |User code |||    
+   |  |  |          |||    
+   |  |  +----------+||    
+   |  +––------------+|    
+   |                  |    
+   +––––––––––––––––––+    
+                           
+                           
+The user code is expanded into a simple template and linked against libskia
+and a couple other .o files that contain main() and the code that sets up the
+seccomp and rlimit restrictions. This code also sets up the SkCanvas that is
+handed to the user code. Any code the user submits is restricted to running in
+a single function that looks like this:
+
+
+    void draw(SkCanvas* canvas) {
+      // User code goes here.
+    }
+
+The user code is tracked by taking an MD5 hash of the code The template is
+expanded out into <hash>.cpp, which is compiled into <hash>.o, which is then
+linked together with all the other libs and object files to create an
+executable named <hash>.  That executable is copied into a directory
+/home/webtry/inout, that is accessible to both the web server and the schroot
+jail. The application is then run in the schroot jail, writing its response,
+<hash>.png, out into the same directory, /home/webtry/inout/, where is it read
+by the web server and returned to the user.
+
+Startup and config
+------------------
+The server is started and stopped via:
+
+    sudo /etc/init.d/webtry [start|stop|restart]
+
+By sysv init only handles starting and stopping a program once, so we use
+Monit to monitor the application and restart it if it crashes. The config
+is in:
+
+    /etc/monit/conf.d/webtry
+
+The chroot jail is implemented using schroot, its configuration
+file is found in:
+
+    /etc/schroot/chroot.d/webtry
+
+The seccomp configuration is in main.cpp and only allows the following system
+calls:
+
+    exit_group
+    exit
+    fstat
+    read
+    write
+    close
+    mmap
+    munmap
+    brk
+
+Installation
+------------
+See the README file.
+
+