linux-kernel - Re: [PATCH] ACPI: LPIT: fix u32 multiplication overflow

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Message-ID: <cf813067-9b73-4eca-8c0a-668fc68ca6b0@ancud.ru>
Date:   Wed, 22 Nov 2023 22:40:46 +0300
From:   Nikita Kiryushin <kiryushin@...ud.ru>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Len Brown <lenb@...nel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
        lvc-project@...uxtesting.org
Subject: Re: [PATCH] ACPI: LPIT: fix u32 multiplication overflow

My reasoning was around something like:

1) tsc_khz is declared as unsigned int tsc_khz;

2) tsc_khz * 1000 would overflow, if the result is larger, than an 
unsigned int could hold;

3) given tsc_khz * 1000 > UINT_MAX is bad, tsc_khz > UINT_MAX / 1000 is bad;

4) if UINT_MAX is 4294967295, than tsc_khz > 4294967.295 is bad, for 
example 4294968 would lead to overflow;

5) 4294968 kHz is 4294.968 MHz, which seems realistically high to me.

For me, tsc: Refined TSC clocksource calibration: 3393.624 MHz

(seems like, it is derived from the same value,

pr_info("Refined TSC clocksource calibration: %lu.%03lu MHz\n",
         (unsigned long)tsc_khz / 1000,
         (unsigned long)tsc_khz % 1000);

)

Not sure about the math above, but it seemed reasonable enough to me to 
switch to overflow-resilient arithmetic here.


On 11/21/23 23:14, Rafael J. Wysocki wrote:
> On Tue, Nov 21, 2023 at 8:56 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
> That should be "hundreds of thousands of kHz", so I was mistaken.
>
> But anyway:
>
>> Why is it really a concern?
>>