linux-kernel - Re: [RESEND PATCH v7 7/8] kernfs: Replace per-fs rwsem with hashed rwsems.

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Date:   Fri, 18 Mar 2022 00:07:21 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Imran Khan <imran.f.khan@...cle.com>
Cc:     tj@...nel.org, gregkh@...uxfoundation.org,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH v7 7/8] kernfs: Replace per-fs rwsem with hashed
 rwsems.

On Thu, Mar 17, 2022 at 06:26:11PM +1100, Imran Khan wrote:

> diff --git a/fs/kernfs/symlink.c b/fs/kernfs/symlink.c
> index 9d4103602554..cbdd1be5f0a8 100644
> --- a/fs/kernfs/symlink.c
> +++ b/fs/kernfs/symlink.c
> @@ -113,12 +113,19 @@ static int kernfs_getlink(struct inode *inode, char *path)
>  	struct kernfs_node *kn = inode->i_private;
>  	struct kernfs_node *parent = kn->parent;
>  	struct kernfs_node *target = kn->symlink.target_kn;
> -	struct rw_semaphore *rwsem;
> +	struct kernfs_rwsem_token token;
>  	int error;
>  
> -	rwsem = kernfs_down_read(parent);
> +	/**
> +	 * Lock both parent and target, to avoid their movement
> +	 * or removal in the middle of path construction.
> +	 * If a competing remove or rename for parent or target
> +	 * wins, it will be reflected in result returned from
> +	 * kernfs_get_target_path.
> +	 */
> +	kernfs_down_read_double_nodes(target, parent, &token);
>  	error = kernfs_get_target_path(parent, target, path);
> -	kernfs_up_read(rwsem);
> +	kernfs_up_read_double_nodes(target, parent, &token);
>  
>  	return error;
>  }

No.  Read through the kernfs_get_target_path().  Why would locking these
two specific nodes be sufficient for anything useful?  That code relies
upon ->parent of *many* nodes being stable.  Which is not going to be
guaranteed by anything of that sort.

And it's not just "we might get garbage if we race" - it's "we might
walk into kfree'd object and proceed to walk the pointer chain".

Or have this loop
	kn = target;
	while (kn->parent && kn != base) {
		len += strlen(kn->name) + 1;
		kn = kn->parent;
	}
see the names that are not identical to what we see in
	kn = target;
	while (kn->parent && kn != base) {
		int slen = strlen(kn->name);

		len -= slen;
		memcpy(s + len, kn->name, slen);
		if (len)
			s[--len] = '/';

		kn = kn->parent;
	}
done later in the same function.  With obvious unpleasant effects.
Or a different set of nodes, for that matter.

This code really depends upon the tree being stable.  No renames of
any sort allowed during that thing.