linux-kernel - Re: [PATCH v2] hwmon: (applesmc) fix UB and udelay overflow

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Date:   Wed, 2 Oct 2019 18:17:40 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     clang-built-linux <clang-built-linux@...glegroups.com>,
        jdelvare@...e.com,
        Tomasz Paweł Gajc <tpgxyz@...il.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Henrik Rydberg <rydberg@...math.org>,
        linux-hwmon@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] hwmon: (applesmc) fix UB and udelay overflow

On 10/2/19 2:43 PM, Nick Desaulniers wrote:
> On Mon, Sep 30, 2019 at 5:01 PM Guenter Roeck <linux@...ck-us.net> wrote:
>>
>> Again, I fail to understand why waiting for a multiple of 20 seconds
>> under any circumstances would make any sense. Maybe the idea was
>> to divide us by 1000 before entering the second loop ?
> 
> Yes, that's very clearly a mistake of mine.
> 
>>
>> Looking into the code, there is no need to use udelay() in the first
>> place. It should be possible to replace the longer waits with
>> usleep_range(). Something like
>>
>>                  if (us < some_low_value)        // eg. 0x80
>>                          delay(us)
> 
> Did you mean udelay here?
> 
Yes

>>                  else
>>                          usleep_range(us, us * 2);
>>
>> should do, and at the same time prevent the system from turning
>> into a space heater.
> 
> The issue would persist with the above if udelay remains in a loop
> that gets fully unrolled.  That's while I "peel" the loop into two
> loops over different ranges with different bodies.
> 

Sorry, you lost me. If calls to udelay() with even small delay
parameters for some compiler-related reason no longer work, trying
to fix the problem with some odd driver code is most definitely not
a real solution.

> I think I should iterate in the first loop until the number of `us` is
> greater than 1000 (us per ms)(which is less of a magical constant and
> doesn't expose internal implementation details of udelay), then start
> the second loop (dividing us by 1000).  What do you think, Guenter?
> 

We should have no second loop, period.

Again, a hot delay loop of 128 ms (actually, more like 245 ms,
adding all delays together) is clearly wrong. Those udelay() calls
in the driver should really be replaced with usleep_range().

Guenter