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

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Hi Roman,

On 10/22/21 9:30 AM, Roman Gushchin 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")
Fixes: 4bfc0bb2c60e ("bpf: decouple the lifetime of cgroup_bpf from cgroup itself")
Signed-off-by: Quanyang Wang <quanyang.wang@xxxxxxxxxxxxx>
---
V1 ---> V2:
1. As per Daniel's suggestion, add description to commit msg about the
balance of cgroup's refcount in cgroup_bpf_offline.
2. As per Michal's suggestion, add tag "Fixes: 4bfc0bb2c60e" and add
description about it.
3. Fix indentation on the percpu_ref_is_dying line.

Acked-by: Roman Gushchin <guro@xxxxxx>

The fix looks correct, two fixes tag are fine too, if only it won't
confuse scripts picking up patches for stable backports.

In fact, it's a very cold path, which is arguably never hit in the real
life. On cgroup v2 it's not an issue. I'm not sure we need a stable
backport at all, only if it creates a noise for some automation tests.

Quanyang, out of curiosity, how did you find it?
I ran ltp testsuite to find this.

./runltp -f controllers -s cgroup

Thanks,
Quanyang

Anyway, thanks for catching and fixing it!

Roman




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