Re: [PATCH 1/4] mm: Trial do_wp_page() simplification

[Michal Hocko]

On Mon 21-09-20 17:04:50, Christian Brauner wrote:
> 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 a lot for this clarification! So I believe the only existing bug
is in documentation which should be explicit that the cgroup fd read
access is not sufficient because it also requires to have a write access
for cgroup.procs in the same directory at the time of fork. I will send
a patch if I find some time for that.

Thanks!
-- 
Michal Hocko
SUSE Labs