linux-kernel - Re: [PATCH] soc: qcom: icc-bwmon: Fix handling dev_pm_opp_find_bw

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Message-ID: <213ce041-733e-4e3e-87d5-db0b37c410b4@oss.qualcomm.com>
Date: Thu, 14 Aug 2025 13:27:14 +0200
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>,
        Thara Gopinath <thara.gopinath@...il.com>,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: stable@...r.kernel.org, Dan Carpenter <dan.carpenter@...aro.org>
Subject: Re: [PATCH] soc: qcom: icc-bwmon: Fix handling dev_pm_opp_find_bw_*()
 errors

On 8/14/25 1:25 PM, Krzysztof Kozlowski wrote:
> On 14/08/2025 11:15, Konrad Dybcio wrote:
>> On 8/14/25 8:32 AM, Krzysztof Kozlowski wrote:
>>> The ISR calls dev_pm_opp_find_bw_ceil(), which can return EINVAL, ERANGE
>>> or ENODEV, and if that one fails with ERANGE, then it tries again with
>>> floor dev_pm_opp_find_bw_floor().
>>>
>>> Code misses error checks for two cases:
>>> 1. First dev_pm_opp_find_bw_ceil() failed with error different than
>>>    ERANGE,
>>> 2. Any error from second dev_pm_opp_find_bw_floor().
>>>
>>> In an unlikely case these error happened, the code would further
>>> dereference the ERR pointer.  Close that possibility and make the code
>>> more obvious that all errors are correctly handled.
>>>
>>> Reported by Smatch:
>>>   icc-bwmon.c:693 bwmon_intr_thread() error: 'target_opp' dereferencing possible ERR_PTR()
>>>
>>> Fixes: b9c2ae6cac40 ("soc: qcom: icc-bwmon: Add bandwidth monitoring driver")
>>> Cc: <stable@...r.kernel.org>
>>> Reported-by: Dan Carpenter <dan.carpenter@...aro.org>
>>> Closes: https://lore.kernel.org/r/aJTNEQsRFjrFknG9@stanley.mountain/
>>> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
>>>
>>> ---
>>>
>>> Some unreleased smatch, though, because I cannot reproduce the warning,
>>> but I imagine Dan keeps the tastiests reports for later. :)
>>> ---
>>>  drivers/soc/qcom/icc-bwmon.c | 3 +++
>>>  1 file changed, 3 insertions(+)
>>>
>>> diff --git a/drivers/soc/qcom/icc-bwmon.c b/drivers/soc/qcom/icc-bwmon.c
>>> index 3dfa448bf8cf..597f9025e422 100644
>>> --- a/drivers/soc/qcom/icc-bwmon.c
>>> +++ b/drivers/soc/qcom/icc-bwmon.c
>>> @@ -656,6 +656,9 @@ static irqreturn_t bwmon_intr_thread(int irq, void *dev_id)
>>>  	if (IS_ERR(target_opp) && PTR_ERR(target_opp) == -ERANGE)
>>>  		target_opp = dev_pm_opp_find_bw_floor(bwmon->dev, &bw_kbps, 0);
>>>  
>>> +	if (IS_ERR(target_opp))
>>> +		return IRQ_HANDLED;
>>
>> So the thunk above checks for a ceil freq relative to bw_kbps and then
>> if it doesn't exist, for a floor one
>>
>> Meaning essentially if we fall into this branch, there's no OPPs in the
>> table, which would have been caught in probe
> Yes, unless:
> 1. There is a bug in the opp code
> 2. Probe code is anyhow changed in the future
> 
> I think the code should be readable and obviouswithin the function, not
> depend on some pre-checks in the probe. But if you think that's
> defensive coding I can also add a comment to silence future Smatch
> complains.

I ultimately don't *really* mind either, just wanted to point out that
currently it's effectively a false positive

Konrad