Inside the Go playground

A presentation on how the Go playground is built and how it runs safely untrusted Go code.
golang

1. Inside the Go playground Francesc Campoy Flores Developer Advocate, Gopher
2. Agenda What is the Go playground What could go wrong What did we do to avoid it An animated ASCII train
3. The Go playground play.golang.org (http://play.golang.org)
4. De facto pastebin of the Go community play.golang.org/p/bJYnajZ6Kp (http://play.golang.org/p/bJYnajZ6Kp)
5. The Go tour tour.golang.org (http://tour.golang.org)
6. Executable examples on documentation golang.org/pkg/strings (http://golang.org/pkg/strings/#example_Fields)
7. Executable code on blog posts blog.golang.org/slices (http://blog.golang.org/slices)
8. Executable slides These slides are driven by the p r e s e n t Go tool g o g e t c o d e . g o o g l e . c o m / g o . t o o l s / c m d / p r e s e n t p a c k a g e m a i n i m p o r t " f m t " f u n c m a i n ( ) { f m t . P r i n t l n ( " H e l l o , g o p h e r s ! " ) } Run
9. Naive implementation
10. Architecture
11. Backend Let's start with something simple receive code compile it run it
12. What could go wrong?
13. Resource exhaustion
14. Exhausting memory on the stack s t a c k o v e r f l o w The runtime catches the error and panics. p a c k a g e m a i n f u n c f o o ( a [ 1 0 0 0 ] b y t e ) { f o o ( a ) } f u n c m a i n ( ) { f o o ( [ 1 0 0 0 ] b y t e { } ) } Run
15. Too much memory on the heap o u t o f m e m o r y Again the runtime catches the error and panics. p a c k a g e m a i n t y p e l i s t s t r u c t { b u f [ 1 0 0 0 0 0 ] b y t e n e x t * l i s t } f u n c m a i n ( ) { v a r l * l i s t f o r { l = & l i s t { n e x t : l } } } Run
16. Too much CPU time p a c k a g e m a i n f u n c m a i n ( ) { f o r { } } Run
17. Stealing resources by sleeping A sleeping program still consumes resources. Easy way of having a Denial of Service attack. p a c k a g e m a i n i m p o r t ( " f m t " " t i m e " ) f u n c m a i n ( ) { f m t . P r i n t l n ( " G o o d n i g h t " ) t i m e . S l e e p ( 8 * t i m e . H o u r ) f m t . P r i n t l n ( " G o o d m o r n i n g " ) } Run
18. Accessing things you shouldn't
19. File system access User code shouldn't be able to modify the backend's file system. Reading sensitive information Installing backdoors General mayhem f u n c m a i n ( ) { e r r : = o s . R e m o v e A l l ( " / f o o " ) i f e r r ! = n i l { l o g . F a t a l ( e r r ) } } Run
20. Network access f u n c m a i n ( ) { r e s , e r r : = h t t p . G e t ( " h t t p : / / a p i . o p e n w e a t h e r m a p . o r g / d a t a / 2 . 5 / w e a t h e r ? q = P o r t l a n d " ) i f e r r ! = n i l { l o g . F a t a l ( e r r ) } d e f e r r e s . B o d y . C l o s e ( ) v a r w s t r u c t { W e a t h e r [ ] s t r u c t { D e s c s t r i n g ` j s o n : " d e s c r i p t i o n " ` } ` j s o n : " w e a t h e r " ` } i f e r r : = j s o n . N e w D e c o d e r ( r e s . B o d y ) . D e c o d e ( & w ) ; e r r ! = n i l { l o g . F a t a l ( e r r ) } f m t . P r i n t f ( " N o n e e d t o r u s h o u t s i d e , w e h a v e % v . " , w . W e a t h e r [ 0 ] . D e s c ) } Run
21. Use your imagination
22. Countermeasures
23. Restricting resource usage with ulimit Default limits are not safe enough. u l i m i t could solve this. - d m a x i m u m s i z e o f d a t a s e g m e n t o r h e a p ( i n k b y t e s ) - s m a x i m u m s i z e o f s t a c k s e g m e n t ( i n k b y t e s ) - t m a x i m u m C P U t i m e ( i n s e c o n d s ) - v m a x i m u m s i z e o f v i r t u a l m e m o r y ( i n k b y t e s )
24. Native Client Originally designed to execute native code in Chrome safely. NaCl defines restrictions on the binaries being executed. The code runs in a sandbox isolated from the underlying OS. No file access No network access
25. Isolating process execution with NaCl We use NaCl to: limit CPU time limit memory isolate from the filesystem isolate from the network Process can only write to stdout/stderr.
26. Limiting user time "No sleeping in the playground." Custom runtime with a fake t i m e package. f u n c S l e e p ( d t i m e . D u r a t i o n ) { p a n i c ( " N o s l e e p i n g i n t h e p l a y g r o u n d " ) }
27. Restoring functionality
28. Faking the file system The s y s c a l l package is the only link between user code and the OS kernel. The playground runtime has a custom s y s c a l l package. File system operations operate on a fake in-memory file system. f u n c m a i n ( ) { c o n s t f i l e n a m e = " / t m p / f i l e . t x t " e r r : = i o u t i l . W r i t e F i l e ( f i l e n a m e , [ ] b y t e ( " H e l l o , f i l e s y s t e m \ n " ) , 0 6 4 4 ) i f e r r ! = n i l { l o g . F a t a l ( e r r ) } b , e r r : = i o u t i l . R e a d F i l e ( f i l e n a m e ) i f e r r ! = n i l { l o g . F a t a l ( e r r ) } f m t . P r i n t f ( " % s " , b ) } Run
29. Faking the network All network operations also use the s y s c a l l package. The network stack is also faked in-memory. f u n c m a i n ( ) { l , e r r : = n e t . L i s t e n ( " t c p " , " 1 2 7 . 0 . 0 . 1 : 4 0 0 0 " ) i f e r r ! = n i l { l o g . F a t a l ( e r r ) } d e f e r l . C l o s e ( ) g o d i a l ( ) c , e r r : = l . A c c e p t ( ) i f e r r ! = n i l { l o g . F a t a l ( e r r ) } d e f e r c . C l o s e ( ) i o . C o p y ( o s . S t d o u t , c ) } Run
30. Faking the network (continued) f u n c d i a l ( ) { c , e r r : = n e t . D i a l ( " t c p " , " 1 2 7 . 0 . 0 . 1 : 4 0 0 0 " ) i f e r r ! = n i l { l o g . F a t a l ( e r r ) } d e f e r c . C l o s e ( ) c . W r i t e ( [ ] b y t e ( " H e l l o , n e t w o r k \ n " ) ) } Run
31. Sleeping in the playground Go is about concurrency. We need to demonstrate concurrency in blog posts and talks. And demonstrating concurrency without t i m e is hard.
32. What to do if an open source project lacks a feature?
33. File a bug! bug 4280 (https://code.google.com/p/go/issues/detail?id=4280)
34. Normal behavior There's a special goroutine managing timers T . A goroutine G calls t i m e . S l e e p : 1. G adds a timer to the timer heap. 2. G puts itself to sleep. 3. T tells the OS to wake it when the next timer expires and puts itself to sleep. 4. When T is woken up it looks at the timer on the top of the heap, and wakes the corresponding goroutine.
35. Intermission: deadlocks Many flavors of deadlocks. One common property: all goroutines are asleep. p a c k a g e m a i n f u n c m a i n ( ) { c : = m a k e ( c h a n i n t ) < - c } Run
36. New behavior A goroutine G calls t i m e . S l e e p : 1. G adds a timer to the timer heap. 2. G puts itself to sleep. 3. The scheduler detects a deadlock, checks the timer heap for pending timers. 4. The internal clock is advanced to the next timer expiration. 5. The corresponding goroutines are woken up.
37. Sleeping fast Faking time allows precise sleep durations. p a c k a g e m a i n i m p o r t ( " f m t " " t i m e " ) f u n c m a i n ( ) { s t a r t : = t i m e . N o w ( ) f m t . P r i n t l n ( s t a r t ) f o r i : = 0 ; i < 1 0 ; i + + { t i m e . S l e e p ( t i m e . N a n o s e c o n d ) f m t . P r i n t l n ( t i m e . S i n c e ( s t a r t ) ) } } Run
38. So there's no actual sleep? The playground's w r i t e syscall inserts a timestamp before each write. The front end translates that into a series of "events" that the browser can play back. Returns directly { " E r r o r s " : " " , " E v e n t s " : [ { " M e s s a g e " : " G o o d n i g h t \ n " , " D e l a y " : 0 } , { " M e s s a g e " : " G o o d m o r n i n g \ n " , " D e l a y " : 2 8 8 0 0 0 0 0 0 0 0 0 0 0 } ] } f u n c m a i n ( ) { f m t . P r i n t l n ( " G o o d n i g h t " ) t i m e . S l e e p ( 8 * t i m e . H o u r ) f m t . P r i n t l n ( " G o o d m o r n i n g " ) } Run
39. So the bug was fixed play.golang.org/p/3fv0L3-z0s (http://play.golang.org/p/3fv0L3-z0s)
40. And people were happy play.golang.org/p/rX_3WcpUOZ (http://play.golang.org/p/rX_3WcpUOZ)
41. Very happy play.golang.org/p/P-Dk0NH_vf (http://play.golang.org/p/P-Dk0NH_vf) play.golang.org/p/NOycgN2i6b (http://play.golang.org/p/NOycgN2i6b)
42. References These slides: talks.golang.org/2014/playground.slide (http://talks.golang.org/2014/playground.slide) More about the Go tour: Inside the Go playground: blog.golang.org/playground (http://blog.golang.org/playground) The Go tour: tour.golang.org (http://tour.golang.org) More about Go on NaCl: Running Go under Native Client: code.google.com/p/go-wiki/wiki/NativeClient (https://code.google.com/p/go-wiki/wiki/NativeClient) Go 1.3 Native Client Support: golang.org/s/go13nacl (http://golang.org/s/go13nacl)
43. Thank you Francesc Campoy Flores Developer Advocate, Gopher @francesc (http://twitter.com/francesc) campoy@golang.org (mailto:campoy@golang.org) http://campoy.cat (http://campoy.cat)