linux-kernel - Re: [PATCH] idr: Prevent unintended underflow for the idr index

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Date:   Wed, 18 Sep 2019 04:50:58 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Jordan Crouse <jcrouse@...eaurora.org>
Cc:     freedreno@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] idr: Prevent unintended underflow for the idr index

On Tue, Sep 17, 2019 at 03:48:42PM -0600, Jordan Crouse wrote:
> It is possible for unaware callers of several idr functions to accidentally
> underflow the index by specifying a id that is less than the idr base.

Hi Jordan.  Thanks for the patch, but this seems like a distinction
without a difference.

>  void *idr_remove(struct idr *idr, unsigned long id)
>  {
> +	if (id < idr->idr_base)
> +		return NULL;
> +
>  	return radix_tree_delete_item(&idr->idr_rt, id - idr->idr_base, NULL);

If this underflows, we'll try to delete an index which doesn't exist,
which will return NULL.

>  void *idr_find(const struct idr *idr, unsigned long id)
>  {
> +	if (id < idr->idr_base)
> +		return NULL;
> +
>  	return radix_tree_lookup(&idr->idr_rt, id - idr->idr_base);

If this underflows, we'll look up an entry which doesn't exist, which
will return NULL.

> @@ -302,6 +308,9 @@ void *idr_replace(struct idr *idr, void *ptr, unsigned long id)
>  	void __rcu **slot = NULL;
>  	void *entry;
>  
> +	if (id < idr->idr_base)
> +		return ERR_PTR(-ENOENT);
> +
>  	id -= idr->idr_base;
>  
>  	entry = __radix_tree_lookup(&idr->idr_rt, id, &node, &slot);

... just outside the context is this line:
        if (!slot || radix_tree_tag_get(&idr->idr_rt, id, IDR_FREE))
                return ERR_PTR(-ENOENT);

Looking up an index which doesn't exist gets you a NULL slot, so you get
-ENOENT anyway.

I did think about these possibilities when I was writing the code and
convinced myself I didn't need them.  If you have an example of a case
where I got thast wrong, I'd love to see it.

More generally, the IDR is deprecated; I'm trying to convert users to
the XArray.  If you're adding a new user, can you use the XArray API
instead?