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

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Date:   Wed, 22 Nov 2023 20:50:20 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Nikita Kiryushin <kiryushin@...ud.ru>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        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

On Wed, Nov 22, 2023 at 8:41 PM Nikita Kiryushin <kiryushin@...ud.ru> wrote:
>
> 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);
>
> )

OK, fair enough.

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