On Wed, Oct 31, 2018 at 12:57 AM, Joel Fernandes <j...@joelfernandes.org> wrote: > On Tue, Oct 30, 2018 at 11:10:47PM +0000, Daniel Colascione wrote: >> On Tue, Oct 30, 2018 at 10:33 PM, Joel Fernandes <j...@joelfernandes.org> >> wrote: >> > On Wed, Oct 31, 2018 at 09:23:39AM +1100, Aleksa Sarai wrote: >> >> On 2018-10-30, Joel Fernandes <j...@joelfernandes.org> wrote: >> >> > On Wed, Oct 31, 2018 at 07:45:01AM +1100, Aleksa Sarai wrote: >> >> > [...] >> >> > > > > (Unfortunately >> >> > > > > there are lots of things that make it a bit difficult to use >> >> > > > > /proc/$pid >> >> > > > > exclusively for introspection of a process -- especially in the >> >> > > > > context >> >> > > > > of containers.) >> >> > > > >> >> > > > Tons of things already break without a working /proc. What do you >> >> > > > have in mind? >> >> > > >> >> > > Heh, if only that was the only blocker. :P >> >> > > >> >> > > The basic problem is that currently container runtimes either depend >> >> > > on >> >> > > some non-transient on-disk state (which becomes invalid on machine >> >> > > reboots or dead processes and so on), or on long-running processes >> >> > > that >> >> > > keep file descriptors required for administration of a container alive >> >> > > (think O_PATH to /dev/pts/ptmx to avoid malicious container filesystem >> >> > > attacks). Usually both. >> >> > > >> >> > > What would be really useful would be having some way of "hiding away" >> >> > > a >> >> > > mount namespace (of the pid1 of the container) that has all of the >> >> > > information and bind-mounts-to-file-descriptors that are necessary for >> >> > > administration. If the container's pid1 dies all of the transient >> >> > > state >> >> > > has disappeared automatically -- because the stashed mount namespace >> >> > > has >> >> > > died. In addition, if this was done the way I'm thinking with (and >> >> > > this >> >> > > is the contentious bit) hierarchical mount namespaces you could make >> >> > > it >> >> > > so that the pid1 could not manipulate its current mount namespace to >> >> > > confuse the administrative process. You would also then create an >> >> > > intermediate user namespace to help with several race conditions (that >> >> > > have caused security bugs like CVE-2016-9962) we've seen when joining >> >> > > containers. >> >> > > >> >> > > Unfortunately this all depends on hierarchical mount namespaces (and >> >> > > note that this would just be that NS_GET_PARENT gives you the mount >> >> > > namespace that it was created in -- I'm not suggesting we redesign >> >> > > peers >> >> > > or anything like that). This makes it basically a non-starter. >> >> > > >> >> > > But if, on top of this ground-work, we then referenced containers >> >> > > entirely via an fd to /proc/$pid then you could also avoid PID reuse >> >> > > races (as well as being able to find out implicitly whether a >> >> > > container >> >> > > has died thanks to the error semantics of /proc/$pid). And that's the >> >> > > way I would suggest doing it (if we had these other things in place). >> >> > >> >> > I didn't fully follow exactly what you mean. If you can explain for the >> >> > layman who doesn't know much experience with containers.. >> >> > >> >> > Are you saying that keeping open a /proc/$pid directory handle is not >> >> > sufficient to prevent PID reuse while the proc entries under /proc/$pid >> >> > are >> >> > being looked into? If its not sufficient, then isn't that a bug? If it >> >> > is >> >> > sufficient, then can we not just keep the handle open while we do >> >> > whatever we >> >> > want under /proc/$pid ? >> >> >> >> Sorry, I went on a bit of a tangent about various internals of container >> >> runtimes. My main point is that I would love to use /proc/$pid because >> >> it makes reuse handling very trivial and is always correct, but that >> >> there are things which stop us from being able to use it for everything >> >> (which is what my incoherent rambling was on about). >> > >> > Ok thanks. So I am guessing if the following sequence works, then Dan's >> > patch is not >> > needed. >> > >> > 1. open /proc/<pid> directory >> > 2. inspect /proc/<pid> or do whatever with <pid> >> > 3. Issue the kill on <pid> >> > 4. Close the /proc/<pid> directory opened in step 1. >> > >> > So unless I missed something, the above sequence will not cause any PID >> > reuse >> > races. >> >> Keeping a /proc/$PID directory file descriptor open does not prevent >> $PID being used to name some other process. If it could, you could >> pretty quickly fill a whole system's process table. See the program >> below, which demonstrates the PID collision. > > I know. We both were not sure about that earlier, that's why I requested you > to write the program when we were privately chatting. Now I'm sure because > Aleska answered that and the you program you wrote showed that too.
I don't think that this behavior was ever in doubt from my side. > I wonder if this cannot be plumbed by just making the /proc/$PID directory > opens hold a reference to task_struct (and a reference to whatever else is > supposed to prevent the PID from getting reused), instead of introducing a > brand new API. That *is* a brand-new API ---- just spelled the same as an old API. Besides, the PID-preserving handle approach has a problem with rlimits. In particular, a user that is otherwise limited by RLIMIT_NPROC could squat on far more entries in the process table than he could otherwise. (And the whole point of RLIMIT_NPROC is to limit process table squatting.) You can't just make procfs directory FDs count against RLIMIT_NPROC, because that'd break existing user code that assumes that procfs FDs *don't* count against the user process limit. >> I think Aleksa's larger point is that it's useful to treat processes >> as other file-descriptor-named, poll-able, wait-able resources. >> Consistency is important. A process is just another system resource, >> and like any other system resource, you should be open to hold a file >> descriptor to it and do things to that process via that file >> descriptor. The precise form of this process-handle FD is up for >> debate. The existing /proc/$PID directory FD is a good candidate for a >> process handle FD, since it does almost all of what's needed. But >> regardless of what form a process handle FD takes, we need it. I don't >> see a case for continuing to treat processes in a non-unixy, >> non-file-descriptor-based manner. > > So wait, how is that supposed to address what you're now saying above > "quickly fill a whole process table"? You either want this, or you don't :) I don't understand what you're getting at.
