as far as i understand each android process runs in a "sandbox" environment, what happen if an android application is a multi process application? do all processes use the one single sandbox since all the processes use the same user id?
As Android uses the Linux kernel for sanboxing, the real sandbox is per (unix) user id, rather than per process. And the Dalvik VM itself makes no attempt at sandboxing.
Of course proccesses owned by the same user id are somewhat isolated from each other, but tools like kill(), ptrace() and the /proc filesystem which pierce process isolation are available - and governed primarily by user id isolation.
There are a couple of oddities about how code maps to processes however:
Either an Activity or a Service can be designated in the manifest to run in a distinct process, but this will still be a process owned by the package user id
Distinct application packages with the same signing certificate can use the shared user id feature to share a sandbox; in some cases this can result in their code running in the same process.
The multiprocess attribute of the
<activity> tag says that the activity could run in the process of whatever starts it - it's not entirely clear if this could mean it would end up running under a foreign userid (an idea that seems more risky for the caller than the callee).
The isolatedprocess attribute of the
<service> says that it runs the service in an isolated process with "no permissions of its own" - while it doesn't come out and explicitly say so, I suspect this may mean that this would be a process owned by a distinct, minimally privileged user id.
A more definitive explanation on these last two points would be good. Perhaps I'll look at the implementing code when I have more time.