On Wed, Oct 31, 2018 at 09:49:08AM +1100, Aleksa Sarai wrote: > On 2018-10-30, Joel Fernandes <j...@joelfernandes.org> 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/$pid because > > > it makes reuse handling very trivial and is always correct, but that > > > there are things which stop us from being able to use it for everything > > > (which is what my incoherent rambling was on about). > > > > Ok thanks. So I am guessing if the following sequence works, then Dan's > > patch is not > > needed. > > > > 1. open /proc/<pid> directory > > 2. inspect /proc/<pid> or do whatever with <pid> > > 3. Issue the kill on <pid> > > 4. Close the /proc/<pid> directory opened in step 1. > > > > So unless I missed something, the above sequence will not cause any PID > > reuse > > races. > > (Sorry, I misunderstood your original question.) > > The problem is that holding /proc/$pid doesn't stop the PID from dying > and being reused. The benefit of holding open /proc/$pid is that you > will get an error if you try to use it *after* the PID has died -- which > means that you don't need to worry about explicitly checking for PID > reuse if you are only operating with the file descriptor and not the > PID. > > So that sequence won't always work. There is a race where the pid might > die and be recycled by the time you call kill(2) -- after you've done > step 2. By tying step 2 and 3 together -- in this patch -- you remove > the race (since in order to resolve the "kill" procfs file VFS must > resolve the PID first -- atomically).
Makes sense, thanks. > Though this race window is likely very tiny, and I wonder how much PID > churn you really need to hit it. Yeah that's what I asked initially how much of a problem it really is. Also, I am wondering why the implementation does not want to keep a reference to the task_struct for the duration of any open proc files/directories. Is there a good reason? - Joel
