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

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Date:   Thu, 19 Dec 2019 09:33:00 -0600
From:   Frank Rowand <frowand.list@...il.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Rob Herring <robh@...nel.org>
Cc:     devicetree@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Frank Rowand <frowand.list@...il.com>
Subject: Re: [PATCH] of: Rework and simplify phandle cache to use a fixed size

On 12/12/19 7:05 AM, Sebastian Andrzej Siewior wrote:
> 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

It was kmalloc()ed memory _before_ applying Rob's patch.  It no longer
is kmalloc()ed, so the rest of this discussion no longer applies.

-Frank

> 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
>