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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Wed, 21 Feb 2018 14:25:10 +0100
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Subject: Re: [PATCH] cpufreq: powernv: Check negative value returned by cpufreq_table_find_index_dl()
To:     Michael Ellerman <mpe@ellerman.id.au>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Linux PM <linux-pm@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
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On Wed, Feb 21, 2018 at 2:13 PM, Michael Ellerman <mpe@ellerman.id.au> wrote:
> "Rafael J. Wysocki" <rafael@kernel.org> writes:
>
>> On Wed, Feb 21, 2018 at 6:54 AM, Viresh Kumar <viresh.kumar@linaro.org> wrote:
>>> On 21-02-18, 16:39, Michael Ellerman wrote:
>>>> Viresh Kumar <viresh.kumar@linaro.org> writes:
>>>
>>>> > AFAICT, you will get -1 here only if the freq table had no valid
>>>> > frequencies (or the freq table is empty). Why would that happen ?
>>>>
>>>> Bugs?
>>>
>>> The cupfreq driver shouldn't have registered itself in that case (i.e.
>>> if the cpufreq table is empty).
>>
>> To be precise, ->init() should fail as that's where the table is
>> created.  The registration fails as a result then.
>>
>> But what if the bug is that ->init() doesn't fail when it should?
>>
>> I guess the core could double check the frequency table after ->init()
>> if ->target_index is not NULL.
>>
>> The overall point here is that if you get a negative index in
>> ->fast_switch(), that's way too late anyway and we should be able to
>> catch that error much earlier.
>
> OK.
>
> Still it's one thing for the driver to print a warning and bail out,
> it's another to access off the front of an array and keep running using
> some junk values, or oops (though not in this case because the array
> happens to be static).

Well, let me rephrase.  If ->fast_switch() runs, then it must not be
possible to get a negative index in it.  That has to be guaranteed by
the core.