linux-kernel - Re: [PATCH] of: Rework and simplify phandle cache to use a fixed size

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Date:   Thu, 12 Dec 2019 14:05:39 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Rob Herring <robh@...nel.org>
Cc:     devicetree@...r.kernel.org, Frank Rowand <frowand.list@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Segher Boessenkool <segher@...nel.crashing.org>
Subject: Re: [PATCH] of: Rework and simplify phandle cache to use a fixed size

On 2019-12-11 17:48:54 [-0600], Rob Herring wrote:
> > -       if (phandle_cache) {
> > -               if (phandle_cache[masked_handle] &&
> > -                   handle == phandle_cache[masked_handle]->phandle)
> > -                       np = phandle_cache[masked_handle];
> > -               if (np && of_node_check_flag(np, OF_DETACHED)) {
> > -                       WARN_ON(1); /* did not uncache np on node removal */
> > -                       of_node_put(np);
> > -                       phandle_cache[masked_handle] = NULL;
> > -                       np = NULL;
> > -               }
> > +       if (phandle_cache[handle_hash] &&
> > +           handle == phandle_cache[handle_hash]->phandle)
> > +               np = phandle_cache[handle_hash];
> > +       if (np && of_node_check_flag(np, OF_DETACHED)) {
> > +               WARN_ON(1); /* did not uncache np on node removal */
> 
> BTW, I don't think this check is even valid. If we failed to detach
> and remove the node from the cache, then we could be accessing np
> after freeing it.

this is kmalloc()ed memory which is always valid. If the memory is
already re-used then
	handle == phandle_cache[handle_hash]->phandle

will fail (the check, not the memory access itself). If the check
remains valid then you can hope for the OF_DETACHED flag to trigger the
warning.

> Rob

Sebastian