On Fri, Jun 19, 2015 at 8:08 AM, Ludovic Courtès <l...@gnu.org> wrote: > "Thompson, David" <dthomps...@worcester.edu> skribis: > >> On Sat, Jun 13, 2015 at 4:19 PM, Ludovic Courtès <l...@gnu.org> wrote: >>> "Thompson, David" <dthomps...@worcester.edu> skribis: >>> >>>> On Sat, Jun 13, 2015 at 9:06 AM, Ludovic Courtès <l...@gnu.org> wrote: >>>>> "Thompson, David" <dthomps...@worcester.edu> skribis: >>>>> >>>>>> On Fri, Jun 12, 2015 at 11:12 AM, Ludovic Courtès <l...@gnu.org> wrote: >>>>>>> "Thompson, David" <dthomps...@worcester.edu> skribis: >>>>>>> >>>>>>>> Yeah, our daemon would do the same thing. We could maybe even have a >>>>>>>> little Guile library that allows one to evaluate arbitrary scheme code >>>>>>>> from within the container. :) >>>>>>> >>>>>>> Actually, something quite easily feasible would be this: >>>>>>> >>>>>>> (eval-in-container #~(system* #$evil-program >>>>>>> #$(local-file "important-data.txt")) >>>>>>> #:networking? #f) >>>>>>> >>>>>>> ... where the container’s store would be populated with just >>>>>>> EVIL-PROGRAM and the local file. >>>>>>> >>>>>>> Food for thought... >>>>>> >>>>>> Ooooh yeah! That would be cool. Though I think we should still spawn >>>>>> a dmd process as PID 1 to deal with reaping zombie processes. We >>>>>> could generate a single service that runs the gexp script. How does >>>>>> that sound? >>>>> >>>>> Wouldn’t it be enough to have the Guile process that evaluates the >>>>> expression be PID 1 in the container, as is the case in guix-daemon >>>>> containers? >>>> >>>> Sure, it would work, but my concern is that a long-running process on >>>> a user's machine could create and orphan tons of child processes and >>>> nothing would be able to clean them up until the PID namespace is >>>> garbage collected. >>> >>> My understanding was that killing a container’s PID 1 (from the outside) >>> effectively killed all the processes of that PID name space. Isn’t it >>> the case? >> >> Yes, that is the case. That triggers the "garbage collection" of that >> namespace, if you will. My point is that, without a proper PID 1 that >> can DTRT with orphaned processes, a long running process in a >> container could potentially create a ton of orphaned child processes >> with no way for them to be reaped without killing PID 1. I wouldn't >> be very happy if a program that I was running in a sandbox was >> polluting the process list. I don't think this is a concern for the >> build daemon because the build process is a (relatively) short-lived >> process, but running something like a web browser could go on for >> days, weeks, etc. > > Yes, I understand. This is definitely an important concern for full > GuixSD containers. > > However, ‘eval-in-container’ would be much simpler, synchronous, and > typically for short-lived processes. So I guess the process that runs > ‘eval-in-container’ would clone(2) (via ‘call-with-container’) and > simply waitpid(2) the child process (which is PID 1 in its container). > > When the parent process gets a SIGINT or SIGHUP, it could send SIGKILL > to the child, thereby terminating the container. > > Does that make sense?
Yes, crystal clear now. Thanks for bearing with me. >>> (The daemon works around that by running processes under a separate UID >>> and doing kill(-1, SIGKILL) under that UID.) >> >> So, PID 1 in the build container forks and changes the UID or >> something? > > Yes, with setuid (see build.cc:2180.) Awesome, thank you. My current container work is figuring out how to spawn interactive processes in a container, such as bash or a Guile REPL. Seems I need to learn how to make a pty and maybe do some dup/dup2 calls to pipe stdin in the parent process to the child container process. Any wisdom you have (or anyone else reading this) would be most welcome. :) - Dave