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

[Quanyang Wang]

Hi Ming,

On 10/18/21 5:02 PM, Ming Lei wrote:
> On Mon, Oct 18, 2021 at 03:56:23PM +0800, quanyang.wang@xxxxxxxxxxxxx wrote:
> > From: Quanyang Wang <quanyang.wang@xxxxxxxxxxxxx>
> >
> > 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.

For non-root cgroup:
cgroup_mkdir
-> cgroup_create
-->cgroup_bpf_inherit(cgrp_A)
cgroup_rmdir
->cgroup_destroy_locked()
-->cgroup_bpf_offline(cgrp_A)
So for non-root cgroup, there is no memory leak.


For root cgroup:
cgroup_setup_root
->cgroup_bpf_inherit(root_cgrp)
cgroup_kill_sb:
-> (Here should be call cgroup_bpf_offline, or else leak occurs)

Thanks,
Quanyang


> If that is true, 'Fixes: 2b0d3d3e4fcf' looks misleading, cause people has to
> backport your patch if 4bfc0bb2c60e is applied. Meantime, this fix isn't
> needed if 4bfc0bb2c60e isn't merged.
>
>
> Thanks,
> Ming