[Devel] [PATCH RHEL8 COMMIT] fs/kernfs: remove bogus kernfs_test_ve() check

[Konstantin Khorenko]

The commit is pushed to "branch-rh8-4.18.0-193.6.3.vz8.4.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh8-4.18.0-193.6.3.vz8.4.4
------>
commit d87e7d21852c8f4b5822a6d1af1fb03c0bfc32e6
Author: Andrey Ryabinin <aryabinin at virtuozzo.com>
Date:   Fri Jul 24 19:09:13 2020 +0300

    fs/kernfs: remove bogus kernfs_test_ve() check
    
    cgroups now also use kernfs subsystem. kernfs_test_ve() check breaks
    it when we try to mount cgroup from ve.
    
    cgroup1_mount() tries to reuse the existing superblock by calling
    kernfs_pin_sb to pin it during mount operation
    
    cgroup1_mount():
                    pinned_sb = kernfs_pin_sb(root->kf_root, NULL);
                    ...
    
    Then it calls cgroup_do_mount() which fails to reuse the existing
    superblock because of failed kernfs_test_ve() check, so instead
    it creates a new one:
    
            dentry = cgroup_do_mount(&cgroup_fs_type, flags, root,
                                     CGROUP_SUPER_MAGIC, ns);
    
    So now we have two superblocks instead of one which may lead to
    the double percpu_ref_kill_and_confirm() call. E.g. :
    
    umount /sys/fs/cgroup/rdma
    vzctl exec CTID umount /sys/fs/cgroup/rdma
    
            ------------[ cut here ]------------
            percpu_ref_kill_and_confirm called more than once on css_release!
            WARNING: CPU: 2 PID: 2664 at lib/percpu-refcount.c:336 percpu_ref_kill_and_confirm+0x7c/0xa0
            CPU: 2 PID: 2664 Comm: kworker/2:5 ve: / Tainted: G                 --------- -t - 4.18.0-80.1.2.vz8.3.12 #1 3.12
            RIP: 0010:percpu_ref_kill_and_confirm+0x7c/0xa0
            Call Trace:
             cgroup_kill_sb+0x64/0x90
             deactivate_locked_super+0x34/0x70
             cleanup_mnt+0x3b/0x70
             delayed_mntput+0x26/0x40
             process_one_work+0x1a7/0x360
             worker_thread+0x30/0x390
             kthread+0x112/0x130
             ret_from_fork+0x35/0x40
            ---[ end trace 0cb674141e1a86e3 ]---
            CT: d3cd4dca-ce04-475d-b5d5-c9ef1f24642a: stopped
    
    Remove the kernfs_test_ve() check to fix this.
    
    As for sysfs which is also uses kernfs, I don't think it gonna cause
    something bad. Sysfs uses net namespaces (info->ns && sb_info->ns) which
    should be enough for us. We enter CT's net ns before mounting sysfs in CT.
    
    Also note that commands like "vzctl set 101 --netif_add eth0 --save"
    calls ip util (which mount sysfs) from ve0 and CT's net namespace.
    So with the removal of kernfs_test_ve() check kernfs_test_super()
    now return success instead of fail as before. Which is totally
    fine IMO, as we reuse the existing CT's sysfs superblock
    instead of creating a new one.
    
    https://jira.sw.ru/browse/PSBM-104902
    Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 fs/kernfs/kernfs-ve.h | 9 ---------
 fs/kernfs/mount.c     | 3 ---
 fs/kernfs/ve.c        | 6 ------
 3 files changed, 18 deletions(-)

diff --git a/fs/kernfs/kernfs-ve.h b/fs/kernfs/kernfs-ve.h
index f1d497b1b1a5..87c480b21d0d 100644
--- a/fs/kernfs/kernfs-ve.h
+++ b/fs/kernfs/kernfs-ve.h
@@ -16,9 +16,6 @@ struct kmapset;
 
 #ifdef CONFIG_VE
 
-int kernfs_test_ve(struct kernfs_super_info *sb_info,
-		   struct kernfs_super_info *info);
-
 void kernfs_get_ve_perms(struct kernfs_node *kn);
 void kernfs_put_ve_perms(struct kernfs_node *kn);
 
@@ -31,12 +28,6 @@ bool kernfs_d_visible(struct kernfs_node *kn, struct kernfs_super_info *info);
 
 #else // CONFIG_VE
 
-static inline int kernfs_test_ve(struct kernfs_super_info *sb_info,
-				 struct kernfs_super_info *info)
-{
-	return 0;
-}
-
 void kernfs_get_ve_perms(struct kernfs_node *kn) { }
 void kernfs_put_ve_perms(struct kernfs_node *kn) { }
 
diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
index 6e759c63a8a6..c8daf9a1fd6d 100644
--- a/fs/kernfs/mount.c
+++ b/fs/kernfs/mount.c
@@ -266,9 +266,6 @@ static int kernfs_test_super(struct super_block *sb, void *data)
 	struct kernfs_super_info *sb_info = kernfs_info(sb);
 	struct kernfs_super_info *info = data;
 
-	if (!kernfs_test_ve(sb_info, info))
-		return 0;
-
 	return sb_info->root == info->root && sb_info->ns == info->ns;
 }
 
diff --git a/fs/kernfs/ve.c b/fs/kernfs/ve.c
index 72e45daa23ae..0c4c28cc745d 100644
--- a/fs/kernfs/ve.c
+++ b/fs/kernfs/ve.c
@@ -49,12 +49,6 @@ int kernfs_ve_allowed(struct kernfs_node *kn)
 	return !kn->ve_perms_map || ve_is_super(get_exec_env());
 }
 
-int kernfs_test_ve(struct kernfs_super_info *sb_info,
-		   struct kernfs_super_info *info)
-{
-	return sb_info->ve == info->ve;
-}
-
 static struct kmapset_key *kernfs_info_perms_key(struct kernfs_super_info *info)
 {
 	return (void *)get_exec_env() + info->ve_perms_off;