Re: [PATCH] kernfs: fix the race in the creation of negative dentry

[Hou Tao]

Hi,

On 9/15/2021 10:09 AM, Ian Kent wrote:
> On Wed, 2021-09-15 at 09:35 +0800, Ian Kent wrote:
>
Sorry for the late reply.
> I think something like this is needed (not even compile tested):
>
> kernfs: dont create a negative dentry if node exists
>
> From: Ian Kent <raven@xxxxxxxxxx>
>
> In kernfs_iop_lookup() a negative dentry is created if associated kernfs
> node is incative which makes it visible to lookups in the VFS path walk.
>
> But inactive kernfs nodes are meant to be invisible to the VFS and
> creating a negative for these can have unexpetced side effects.
>
> Signed-off-by: Ian Kent <raven@xxxxxxxxxx>
> ---
>  fs/kernfs/dir.c |    9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
> index ba581429bf7b..a957c944cf3a 100644
> --- a/fs/kernfs/dir.c
> +++ b/fs/kernfs/dir.c
> @@ -1111,7 +1111,14 @@ static struct dentry *kernfs_iop_lookup(struct inode *dir,
>  
>  	kn = kernfs_find_ns(parent, dentry->d_name.name, ns);
>  	/* attach dentry and inode */
> -	if (kn && kernfs_active(kn)) {
> +	if (kn) {
> +		/* Inactive nodes are invisible to the VFS so don't
> +		 * create a negative.
> +		 */
> +		if (!kernfs_active(kn)) {
> +			up_read(&kernfs_rwsem);
> +			return NULL;
> +		}
>  		inode = kernfs_get_inode(dir->i_sb, kn);
>  		if (!inode)
>  			inode = ERR_PTR(-ENOMEM);
>
>
> Essentially, the definition a kernfs negative dentry, for the
> cases it is meant to cover, is one that has no kernfs node, so
> one that does have a node should not be created as a negative.
>
> Once activated a subsequent ->lookup() will then create a
> positive dentry for the node so that no invalidation is
> necessary.
I'm fine with the fix which is much simpler.
> This distinction is important because we absolutely do not want
> negative dentries created that aren't necessary. We don't want to
> leave any opportunities for negative dentries to accumulate if
> we don't have to.
>     
> I am still thinking about the race you have described.
>
> Given my above comments that race might have (maybe probably)
> been present in the original code before the rwsem change but
> didn't trigger because of the serial nature of the mutex.
I don't think there is such race before the enabling of negative dentry,
but maybe I misunderstanding something.
> So it may be wise (perhaps necessary) to at least move the
> activation under the rwsem (as you have done) which covers most
> of the change your proposing and the remaining hunk shouldn't
> do any harm I think but again I need a little more time on that.
After above fix, doing sibling tree operation and activation atomically
will reduce the unnecessary lookup, but I don't think it is necessary
for the fix of race.

Regards,
Tao
> I'm now a little concerned about the invalidation that should
> occur on deactivation so I want to have a look at that too but
> it's separate to this proposal.
> Greg, Tejun, Hou, any further thoughts on this would be most
> welcome.
>
> Ian
>>
> .