Pavel Emelianov [xemul@xxxxxxxxxx] wrote: | sukadev@xxxxxxxxxx wrote: | > Pavel Emelianov [xemul@xxxxxxxxxx] wrote: | > | Serge E. Hallyn wrote: | > | > Quoting Pavel Emelyanov (xemul@xxxxxxxxxx): | > | >> [snip] | > | >> | > | >>>> Mmm. I wanted to send one small objection to Cedric's patches with mqns, | > | >>>> but the thread was abandoned by the time I decided to do-it-right-now. | > | >>>> | > | >>>> So I can put it here: forcing the CLONE_NEWNS is not very good, since | > | >>>> this makes impossible to push a bind mount inside a new namespace, which | > | >>>> may operate in some chroot environment. But this ability is heavily | > | >>> Which direction do you want to go? I'm wondering whether mounts | > | >>> propagation can address it. | > | >> Hardly. AFAIS there's no way to let the chroot-ed tasks see parts of | > | >> vfs tree, that left behind them after chroot, unless they are in the | > | >> same mntns as you, and you bind mount this parts to their tree. No? | > | > | > | > Well no, but I suspect I'm just not understanding what you want to do. | > | > But if the chroot is under /jail1, and you've done, say, | > | > | > | > mkdir -p /share/pts | > | > mkdir -p /jail1/share | > | > mount --bind /share /share | > | > mount --make-shared /share | > | > mount --bind /share /jail1/share | > | > mount --make-slave /jail1/share | > | > | > | > before the chroot-ed tasks were cloned with CLONE_NEWNS, then when you | > | > do | > | > | > | > mount --bind /dev/pts /share/pts | > | > | > | > from the parent mntns (not that I know why you'd want to do *that* :) | > | > then the chroot'ed tasks will see the original mntns's /dev/pts under | > | > /jail1/share. | > | | > | I haven't yet tried that, but :( this function | > | | > | static inline int check_mnt(struct vfsmount *mnt) | > | { | > | return mnt->mnt_ns == current->nsproxy->mnt_ns; | > | } | > | | > | and this code in do_loopback | > | | > | if (!check_mnt(nd->mnt) || !check_mnt(old_nd.mnt)) | > | goto out; | > | | > | makes me think that trying to bind a mount from another mntns | > | ot _to_ another is prohibited... Do I miss something? | > | | > | >>> Though really, I think you're right - we shouldn't break the kernel | > | >>> doing CLONE_NEWMQ or CLONE_NEWPTS without CLONE_NEWNS, so we shouldn't | > | >>> force the combination. | > | >>> | > | >>>> exploited in OpenVZ, so if we can somehow avoid forcing the NEWNS flag | > | >>>> that would be very very good :) See my next comment about this issue. | > | >>>> | > | >>>>> Pavel, not long ago you said you were starting to look at tty and pty | > | >>>>> stuff - did you have any different ideas on devpts virtualization, or | > | >>>>> are you ok with this minus your comments thus far? | > | >>>> I have a similar idea of how to implement this, but I didn't thought | > | >>>> about the details. As far as this issue is concerned, I see no reasons | > | >>>> why we need a kern_mount-ed devtpsfs instance. If we don't make such, | > | >>>> we may safely hold the ptsns from the superblock and be happy. The | > | >>>> same seems applicable to the mqns, no? | > | >>> But the current->nsproxy->devpts->mnt is used in several functions in | > | >>> patch 3. | > | >> Indeed. I overlooked this. Then we're in a deep ... problem here. | > | >> | > | >> Breaking this circle was not that easy with pid namespaces, so | > | >> I put the strut in proc_flush_task - when the last task from the | > | >> namespace exits the kern-mount-ed vfsmnt is dropped, but we can't | > | >> do the same stuff with devpts. | > | > | > | > But I still don't see what the problem is with my proposal? So long as | > | > you agree that if there are no tasks remaining in the devptsns, | > | > then any task which has its devpts mounted should see an empty directory | > | > (due to sb->s_info being NULL), I think it works. | > | | > | Well, if we _do_ can handle the races with ns->devpts_mnt switch | > | from not NULL to NULL, then I'm fine with this approach. | > | | > | I just remember, that with pid namespaces this caused a complicated | > | locking and performance degradation. This is the problem I couldn't | > | remember yesterday. | > | > Well, iirc, one problem with pid namespaces was that we need to keep | > the task and pid_namespace association until the task was waited on | > (for instance the wait() call from parent needs the pid_t of the | > child which is tied to the pid ns in struct upid). | > | > For this reason, we don't drop the mnt reference in free_pid_ns() but | > hold the reference till proc_flush_task(). | > | > With devpts, can't we simply drop the reference in free_pts_ns() so | > that when the last task using the pts_ns exits, we can unmount and | > release the mnt ? | | I hope we can. The thing I'm worried about is whether we can correctly | handle race with this pointer switch from NULL to not-NULL. | | > IOW, do you suspect that the circular reference leads to leaking vfsmnts ? | > | | Of course! If the namespace holds the vfsmnt, vfsmnt holds the superblock | and the superblock holds the namespace we won't drop this chain ever, | unless some other object breaks this chain. Of course :-) I had a bug in new_pts_ns() that was masking the problem. I had ns->mnt = kern_mount_data()... ... kref_init(&ns->kref); So the kref_init() would overwrite the reference got by devpts_set_sb() and was preventing the leaking vfsmnt in my test. Thanks Pavel, Sukadev _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/containers