linux-kernel - Re: [PATCH v4 06/21] hwmon: (mr75203) fix multi-channel voltage reading

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Date:   Wed, 7 Sep 2022 08:15:36 +0300
From:   "Farber, Eliav" <farbere@...zon.com>
To:     Andy Shevchenko <andriy.shevchenko@...el.com>
CC:     <jdelvare@...e.com>, <linux@...ck-us.net>, <robh+dt@...nel.org>,
        <p.zabel@...gutronix.de>, <rtanwar@...linear.com>,
        <linux-hwmon@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <hhhawa@...zon.com>,
        <jonnyc@...zon.com>, "Farber, Eliav" <farbere@...zon.com>
Subject: Re: [PATCH v4 06/21] hwmon: (mr75203) fix multi-channel voltage reading

On 9/6/2022 5:10 PM, Andy Shevchenko wrote:
> On Tue, Sep 06, 2022 at 08:33:41AM +0000, Eliav Farber wrote:
>> Fix voltage allocation and reading to support all channels in all VMs.
>> Prior to this change allocation and reading were done only for the first
>> channel in each VM.
>> This change counts the total number of channels for allocation, and 
>> takes
>> into account the channel offset when reading the sample data register.
>
> ...
>
>> +             total_ch = ch_num * vm_num;
>> +             in_config = devm_kcalloc(dev, total_ch + 1,
>>                                        sizeof(*in_config), GFP_KERNEL);
>
> Strictly speaking this should be `size_add(size_mul(...) ...)` 
> construction
> from overflow.h.
>
>                total_ch = size_mul(ch_num, vm_num);
>                in_config = devm_kcalloc(dev, size_add(total_ch, 1),
>                                         sizeof(*in_config), GFP_KERNEL);
>
> Alternatively before doing all these, add a check
>
>                if (array3_size(ch_num, vm_num, sizeof(*in_config)) < 
> SIZE_MAX - sizeof(*in_config))
>                        return -EOVERFLOW;
>
> But this is a bit monstrous. Seems like the above looks and feels better.
>
> Also for backporting purposes perhaps it's fine to do without using 
> those macro
> helpers. 
According to the driver code total_ch is a u32 variable while vm_num
and ch_num are both limited to a value of 31:

#define VM_NUM_MSK GENMASK(20, 16)
#define VM_NUM_SFT 16
#define CH_NUM_MSK GENMASK(31, 24)
#define CH_NUM_SFT 24

In addition the PVT Controller Series 3+ Specification mentions that
the actual maximum values are even smaller – 8 for vm_num and 16 for
ch_num.
Therefore we are very far from a scenario of an overflow.
Do you still think overflow protection in necessary?

--
Thanks, Eliav