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

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Message-ID: <CAHp75Vd8PZM5Z5A0bCapeOEbvzqXtfUXUDK=6G1nxD527E3=rA@mail.gmail.com>
Date:   Tue, 29 Dec 2020 15:50:32 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Carsten Haitzler <Carsten.Haitzler@....com>
Cc:     Yury Norov <yury.norov@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        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 Tue, Dec 29, 2020 at 2:24 PM Carsten Haitzler
<Carsten.Haitzler@....com> wrote:
>
> 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.

What we can do is to declare BITS_PER_U32.
Also we can pay attention on these definitions:
https://elixir.bootlin.com/linux/latest/source/kernel/bpf/btf.c#L168
https://elixir.bootlin.com/linux/latest/source/security/selinux/ss/ebitmap.c#L27

And define BITMAP_FROM_U32() macro.

Then It would be written like

DECLARE_BITMAP(comp_mask_local, BITS_PER_U32) = BITMAP_FROM_U32(comp_mask);

But this is a bit verbose.

Also, it can be something like DECLARE_BITMAP_U32(...) = BITMAP_FROM_U32(...);

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Hmm... you probably have to get rid of this footer.

-- 
With Best Regards,
Andy Shevchenko