[Devel] [PATCH vz10 1/2] fs/kernfs, ve: hide entries from a VE without invalidating the dentry

[Pavel Tikhomirov <ptikhomirov@virtuozzo.com>]

On 8/25/26 15:29, Mirian Shilakadze wrote:
> kernfs_dop_revalidate() ends with a per VE visibility check and answers
> it with the same "return 0" that the staleness checks above it use.
> Those checks are properties of the kernfs node and hold for every
> observer: the node was deactivated, moved, renamed, or retagged.
> Visibility is a property of the calling task's VE, so one host dentry
> answers "valid" to a ve0 task and "stale" to a task inside a Container.
> 
> The VFS reads 0 as a global fact and calls d_invalidate(), which walks
> the subtree and hands every mountpoint it finds to __detach_mounts().
> The mountpoint hash is not scoped to a mount namespace, and m_list holds
> every mount attached at that dentry in any of them, so a Container's
> lookup unmounts the host's mounts.
> 
> One lookup of /sys/fs/bpf from a task that only did setns() into a
> Container's ve namespace, staying in the host mount namespace, both
> hides the entry from the caller and destroys the host's bpffs.  A
> Container start reaches the same path on its own: libvzctl stats every
> mount point in the namespace to collect the mount flags of a bindmount
> source, and does it after CLONE_NEWVE and before pivot_root, so the host
> loses bpffs and tracefs on the way.  libvzctl needs bpffs for the cgroup
> v2 device controller, so no Container on the node can be managed
> afterwards, and the damage outlives the failed start.
> 
> Return -ENOENT instead.  The caller that cannot see the entry is told
> the name is missing, which is what the check is for, and the dentry
> stays valid for everyone else.  No caller of ->d_revalidate() reaches
> d_invalidate() with a negative return: lookup_dcache(), lookup_fast(),
> __lookup_slow() and lookup_open() in fs/namei.c all gate it on exactly
> 0, ovl_revalidate_real() gates it the same way, and
> ecryptfs_d_revalidate() hands the value back without invalidating
> anything itself.  kernfs_iop_lookup() already answers this same
> condition with a plain "not found".
> 
> Feature: kernfs: per-CT entries visibility and permissions configuration
> https://virtuozzo.atlassian.net/browse/VSTOR-142552
> Fixes: 3dd8c2499df6 ("ve/kernfs: hide forbidden entries in container")
> Signed-off-by: Mirian Shilakadze <mirian.shilakadze@virtuozzo.com>
> ---
>  fs/kernfs/dir.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
> index be680eb98ed4..7947c49ed1a6 100644
> --- a/fs/kernfs/dir.c
> +++ b/fs/kernfs/dir.c
> @@ -1199,8 +1199,17 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
>  	    kernfs_info(dentry->d_sb)->ns != kn->ns)
>  		goto out_bad;
>  
> -	if (!kernfs_d_visible(kn, kernfs_info(dentry->d_sb)))
> -		goto out_bad;
> +	if (!kernfs_d_visible(kn, kernfs_info(dentry->d_sb))) {
> +		/*
> +		 * The node is fine, it is only outside this VE's view.
> +		 * Returning 0 would tell the VFS that the dentry is stale, and
> +		 * it answers that with d_invalidate(), which detaches every
> +		 * mount on that dentry in every mount namespace.  Report the
> +		 * name as missing to this caller instead.
> +		 */
> +		up_read(&root->kernfs_rwsem);
> +		return -ENOENT;
> +	}

Not fully sure, but probably worth preserving behavior in CT as much as possible:

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 7947c49ed1a6f..daa4bc264e6bb 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -1201,12 +1201,11 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)

        if (!kernfs_d_visible(kn, kernfs_info(dentry->d_sb))) {
                /*
-                * The node is fine, it is only outside this VE's view.
-                * Returning 0 would tell the VFS that the dentry is stale, and
-                * it answers that with d_invalidate(), which detaches every
-                * mount on that dentry in every mount namespace.  Report the
-                * name as missing to this caller instead.
+                * Invalidate the node in ve owned mount namespace,
+                * or report no entry in other mount namespace.
                 */
+               if (current->nsproxy->mnt_ns->ve_owner == get_exec_env())
+                       return 0;
                up_read(&root->kernfs_rwsem);
                return -ENOENT;
        }

This way we still umount and invalidate dentry in CT filesystem, making it look a bit more clean.

>  
>  	up_read(&root->kernfs_rwsem);
>  	return 1;

-- 
Best regards, Pavel Tikhomirov
Senior Software Developer, Virtuozzo.