linux-kernel - Re: [PATCH] drm/komeda: use bitmap API to convert U32 to bitmap

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Date:   Tue, 29 Dec 2020 12:22:16 +0000
From:   Carsten Haitzler <Carsten.Haitzler@....com>
To:     Yury Norov <yury.norov@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
CC:     Liviu Dudau <Liviu.Dudau@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Subject: Re: [PATCH] drm/komeda: use bitmap API to convert U32 to bitmap

On 12/28/20 8:10 PM, Yury Norov wrote:
> On Mon, Dec 28, 2020 at 11:49 AM Andy Shevchenko
> <andriy.shevchenko@...ux.intel.com> wrote:
>> On Mon, Dec 28, 2020 at 11:43:43AM -0800, Yury Norov wrote:
>>> The commit be3e477effba636ad25 ("drm/komeda: Fix bit
>>> check to import to value of proper type") fixes possible
>>> out-of-bound issue related to find_first_bit() usage, but
>>> does not address the endianness problem.
>> Hmm... Can you elaborate?
>>
>> ...
>>
>>>                                    u32 comp_mask)
>>> -     unsigned long comp_mask_local = (unsigned long)comp_mask;
>> Here we convert u32 to unsigned long (LSB is kept LSB since it happens in
>> native endianess).
>>
>>> -     id = find_first_bit(&comp_mask_local, 32);
>> Here it takes an address to unsigned long and tries only lower 32 bits.
>>
>> Are you telling that find_first_bit() has an issue?
> It seems you're right, there's no issue with endianness in existing code.
> In fact, the line

Indeed Andy covered this. Take LSB32 with the cast to "local on-stack
long" and possible extend upper 32bits with 0's if needed (64bit longs).

>>> -     unsigned long comp_mask_local = (unsigned long)comp_mask;
> is an opencoded version of bitmap_from_arr32(dst, src, 32).
>
> Maybe it would be better to use the bitmap API here, but existing code is
> correct. Sorry for the noise.
While your code is seemingly also valid (I can check to be sure with
KASAN if you want still), it does seem a little less "nice to read" with
more lines of code for the same work. Is it worth making the code a
little longer here as it's not actually fixing anything to do it this
other way? DECLARE_BITMAP() is a bit of an obscure way to declare a
single unsigned long (in this case) where the compiler does the right
thing anyway with a simple assign+cast making it easier to read/follow IMHO.

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