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

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Hi Daniel,
Thank you for your review.

On 10/11/21 9:44 PM, Daniel Borkmann wrote:
[ +Roman ]

On 10/8/21 2:46 AM, 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 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@xxxxxxxxxxxxx>
---
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?
There is cgroup_get/put pair inside cgroup_bpf_offline.
cgroup_get() is at the beginning of cgroup_bpf_offline.
cgroup_put is called at the routine:
cgroup_bpf_offline
->percpu_ref_kill
-->cgroup_bpf_release_fn
--->cgroup_bpf_release (there is a cgroup_put() at the end of it).
So cgroup_bpf_offline can keep the balance of cgroup's refcount.
 Would
have been nice to
see the commit msg elaborate more on this.
>
OK, I will add description about this in the commit msg.

Thanks,
Quanyang

      kernfs_kill_sb(sb);
  }





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