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 > $PID being used to name some other process. If it could, you could > pretty quickly fill a whole system's process table. See the program > below, which demonstrates the PID collision.
I know. We both were not sure about that earlier, that's why I requested you to write the program when we were privately chatting. Now I'm sure because Aleska answered that and the you program you wrote showed that too. I wonder if this cannot be plumbed by just making the /proc/$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. > 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 :)
