linux-kernel - Re: [PATCH v4 2/5] lib/test_bitmap: add tests for bitmap_{set,get}

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Date:   Fri, 22 Sep 2023 06:28:14 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     catalin.marinas@....com, will@...nel.org, pcc@...gle.com,
        andreyknvl@...il.com, andriy.shevchenko@...ux.intel.com,
        linux@...musvillemoes.dk, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, eugenis@...gle.com,
        syednwaris@...il.com, william.gray@...aro.org
Subject: Re: [PATCH v4 2/5] lib/test_bitmap: add tests for
 bitmap_{set,get}_value()

On Fri, Sep 22, 2023 at 09:57:32AM +0200, Alexander Potapenko wrote:
> > > +     unsigned long val, bit;
> > > +     int i;
> > > +
> > > +     /* Setting/getting zero bytes should not crash the kernel. */
> > > +     bitmap_write(NULL, 0, 0, zero_bits);
> > > +     val = bitmap_read(NULL, 0, zero_bits);
> > > +     expect_eq_ulong(0, val);
> >
> > No, val is undefined.
> 
> Why? bitmap_read(..., ..., 0) always returns 0.

Because it's unexpected and most likely wrong to pass 0 bits. We
guarantee that bitmap_read() will return immediately, and will not
touch the memory. But we don't guarantee that we return any specific
value.

It's not a hot path, at least now, and we can spend few extra cycles
to clear output register and return 0, but user should not rely on it
in any way, especially in a test that is intended to show an example
of using the new API. 

Consider a less relaxed environment, where we really have to count
cycles. In such environment, we'd return a content of the 1st input
argument, just because it's already in R0, and compiled doesn't have
to:

    mov r0, #0
    ret