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Subject: Re: [RESEND][PATCH] cgroup: fix memory leak caused by missing
 cgroup_bpf_offline
To:     quanyang.wang@windriver.com, Tejun Heo <tj@kernel.org>,
        Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Ming Lei <ming.lei@redhat.com>, Jens Axboe <axboe@kernel.dk>
Cc:     cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        netdev@vger.kernel.org, bpf@vger.kernel.org,
        Roman Gushchin <guro@fb.com>
References: <20211008004600.1717681-1-quanyang.wang@windriver.com>
From:   Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <c4d12954-aa78-625e-3be7-06d4fc906bf7@iogearbox.net>
Date:   Mon, 11 Oct 2021 15:44:09 +0200
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[ +Roman ]

On 10/8/21 2:46 AM, quanyang.wang@windriver.com wrote:
> From: Quanyang Wang <quanyang.wang@windriver.com>
> 
> 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 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.
> 
> Fixes: 2b0d3d3e4fcfb ("percpu_ref: reduce memory footprint of percpu_ref in fast path")
> Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
> ---
> One of the recipients' email is wrong, so I resend this patch.
> Sorry for any confusion caused.
> ---
>   kernel/cgroup/cgroup.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index 570b0c97392a9..183736ad72f2b 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -2187,8 +2187,10 @@ static void cgroup_kill_sb(struct super_block *sb)
>   	 * And don't kill the default root.
>   	 */
>   	if (list_empty(&root->cgrp.self.children) && root != &cgrp_dfl_root &&
> -	    !percpu_ref_is_dying(&root->cgrp.self.refcnt))
> +			!percpu_ref_is_dying(&root->cgrp.self.refcnt)) {
> +		cgroup_bpf_offline(&root->cgrp);
>   		percpu_ref_kill(&root->cgrp.self.refcnt);
> +	}
>   	cgroup_put(&root->cgrp);

Doesn't cgroup_bpf_offline() internally bump the root cgroup's refcount via cgroup_get()?
How does this relate to the single cgroup_put() in the above line? Would have been nice to
see the commit msg elaborate more on this.

>   	kernfs_kill_sb(sb);
>   }
>