Okay, that makes sense. Thanks. And I'm looking forward to when we have more failsafe components. On May 13, 2016 3:41 AM, "Norman Feske" <norman.feske@...1...> wrote:
Hello Ben,
On 13.05.2016 06:01, Nobody III wrote:
I'm planning on writing an fs_filter server for the desktop environment I'm developing. The server will combine access to multiple filesystems, much like the vfs server. I want to implement this feature in such a way that a misbehaving filesystem driver can't make the server hang. How should I do this?
this question reminds me of the following issue, where I brought up the same problem for NIC drivers:
https://github.com/genodelabs/genode/issues/1592
In short, rather than developing your fs_filter in a defensive way, I would recommend to develop it assuming that the used file-system servers are trusted. To still use a non-trustworthy file-system server, run it as a child of a dedicated fs_failsafe monitor. This is a runtime environment with the following functionality:
- It runs the real file system as a child component.
- It provides a file-system service to the outside. However, it does not implement the file-system itself but rather forwards all requests to its child. Because the fs_failsafe component is small and trusted, it will never hang. So your fs_filter would be safe to use it at all times.
- It monitors the liveliness of the child. E.g., by using a watchdog thread that looks at the duration of file-system requests. If it detects that the child hangs, it can try to handle this situation (I don't know it restarting a file-system is a reasonable idea or not). In any case, it could still respond to client requests by returning errors instead of hanging. It could also respond to a session-close request by killing the child.
Do you think this approach would work for you?
Norman
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