Re: [V2][PATCH] cgroup: fix memory leak caused by missing cgroup_bpf_offline

Ming Lei
2021-10-18T12:59:37Z

On Mon, Oct 18, 2021 at 06:06:28PM +0800, Quanyang Wang wrote:
> Hi Ming,
>
> On 10/18/21 5:02 PM, Ming Lei wrote:
> > On Mon, Oct 18, 2021 at 03:56:23PM +0800, quanyang.wang-CWA4WttNNZF54TAoqtyWWQ@public.gmane.org wrote:
> > > From: Quanyang Wang <quanyang.wang-CWA4WttNNZF54TAoqtyWWQ@public.gmane.org>
> > >
> > > When enabling CONFIG_CGROUP_BPF, kmemleak can be observed by running
> > > the command as below:
> > >
> > > $mount -t cgroup -o none,name=foo cgroup cgroup/
> > > $umount cgroup/
> > >
> > > unreferenced object 0xc3585c40 (size 64):
> > > comm "mount", pid 425, jiffies 4294959825 (age 31.990s)
> > > hex dump (first 32 bytes):
> > > 01 00 00 80 84 8c 28 c0 00 00 00 00 00 00 00 00 ......(.........
> > > 00 00 00 00 00 00 00 00 6c 43 a0 c3 00 00 00 00 ........lC......
> > > backtrace:
> > > [<e95a2f9e>] cgroup_bpf_inherit+0x44/0x24c
> > > [<1f03679c>] cgroup_setup_root+0x174/0x37c
> > > [<ed4b0ac5>] cgroup1_get_tree+0x2c0/0x4a0
> > > [<f85b12fd>] vfs_get_tree+0x24/0x108
> > > [<f55aec5c>] path_mount+0x384/0x988
> > > [<e2d5e9cd>] do_mount+0x64/0x9c
> > > [<208c9cfe>] sys_mount+0xfc/0x1f4
> > > [<06dd06e0>] ret_fast_syscall+0x0/0x48
> > > [<a8308cb3>] 0xbeb4daa8
> > >
> > > This is because that since the commit 2b0d3d3e4fcf ("percpu_ref: reduce
> > > memory footprint of percpu_ref in fast path") root_cgrp->bpf.refcnt.data
> > > is allocated by the function percpu_ref_init in cgroup_bpf_inherit which
> > > is called by cgroup_setup_root when mounting, but not freed along with
> > > root_cgrp when umounting. Adding cgroup_bpf_offline which calls
> > > percpu_ref_kill to cgroup_kill_sb can free root_cgrp->bpf.refcnt.data in
> > > umount path.
> > >
> > > This patch also fixes the commit 4bfc0bb2c60e ("bpf: decouple the lifetime
> > > of cgroup_bpf from cgroup itself"). A cgroup_bpf_offline is needed to do a
> > > cleanup that frees the resources which are allocated by cgroup_bpf_inherit
> > > in cgroup_setup_root.
> > >
> > > And inside cgroup_bpf_offline, cgroup_get() is at the beginning and
> > > cgroup_put is at the end of cgroup_bpf_release which is called by
> > > cgroup_bpf_offline. So cgroup_bpf_offline can keep the balance of
> > > cgroup's refcount.
> > >
> > > Fixes: 2b0d3d3e4fcf ("percpu_ref: reduce memory footprint of percpu_ref in fast path")
> >
> > If I understand correctly, cgroup_bpf_release() won't be called without
> > your patch. So anything allocated in cgroup_bpf_inherit() will be
> > leaked?
> No, for now cgroup_bpf_release is called to release bpf.refcnt.data of the
> cgroup which is not root_cgroup. Only root_cgroup's bpf data is leaked.

You mean that cgroup_bpf_inherit() allocates nothing for root_cgroup?

If yes, I agree you can add 'Fixes: 2b0d3d3e4fcf', otherwise please
remove it.

Thanks,
Ming