On Mon, Sep 21, 2020 at 04:55:37PM +0200, Michal Hocko wrote: > On Mon 21-09-20 16:43:55, Christian Brauner wrote: > > On Mon, Sep 21, 2020 at 10:38:47AM -0400, Tejun Heo wrote: > > > Hello, > > > > > > On Mon, Sep 21, 2020 at 04:28:34PM +0200, Michal Hocko wrote: > > > > Fundamentaly CLONE_INTO_CGROUP is similar to regular fork + move to the > > > > target cgroup after the child gets executed. So in principle there > > > > shouldn't be any big difference. Except that the move has to be explicit > > > > and the the child has to have enough privileges to move itself. I am not > > > > > > Yeap, they're supposed to be the same operations. We've never clearly > > > defined how the accounting gets split across moves because 1. it's > > > inherently blurry and difficult 2. doesn't make any practical difference for > > > the recommended and vast majority usage pattern which uses migration to seed > > > the new cgroup. CLONE_INTO_CGROUP doesn't change any of that. > > > > > > > completely sure about CLONE_INTO_CGROUP model though. According to man > > > > clone(2) it seems that O_RDONLY for the target cgroup directory is > > > > sufficient. That seems much more relaxed IIUC and it would allow to fork > > > > into a different cgroup while keeping a lot of resources in the parent's > > > > proper. > > > > > > If the man page is documenting that, it's wrong. cgroup_css_set_fork() has > > > an explicit cgroup_may_write() test on the destination cgroup. > > > CLONE_INTO_CGROUP should follow exactly the same rules as regular > > > migrations. > > > > Indeed! > > The O_RDONLY mention on the manpage doesn't make sense but it is > > explained that the semantics are exactly the same for moving via the > > filesystem: > > OK, if the semantic is the same as for the task migration then I do not > see any (new) problems. Care to point me where the actual check is > enforced? For the migration you need a write access to cgroup.procs but > if the API expects directory fd then I am not sure how that would expose > the same behavior. kernel/cgroup/cgroup.c:cgroup_csset_fork() So there's which is the first check for inode_permission() essentially: /* * Verify that we the target cgroup is writable for us. This is * usually done by the vfs layer but since we're not going through * the vfs layer here we need to do it "manually". */ ret = cgroup_may_write(dst_cgrp, sb); if (ret) goto err; and what you're referring to is checked right after in: ret = cgroup_attach_permissions(cset->dfl_cgrp, dst_cgrp, sb, !(kargs->flags & CLONE_THREAD)); if (ret) goto err; which calls: ret = cgroup_procs_write_permission(src_cgrp, dst_cgrp, sb); if (ret) return ret; That should be what you're looking for. I've also added selftests as always that verify this behavior under: tools/testing/selftests/cgroup/ as soon as CLONE_INTO_CGROUP is detected on the kernel than all the usual tests are exercised using CLONE_INTO_CGROUP so we should've seen any regression hopefully. Thanks! Christian
