Re: [PATCH v12 4/4] gpio: xilinx: Utilize generic bitmap_get_value and _set_value

[Arnd Bergmann]

On Sun, Nov 1, 2020 at 4:00 PM William Breathitt Gray
<vilhelm.gray@xxxxxxxxx> wrote:
>
> On Thu, Oct 29, 2020 at 11:44:47PM +0100, Arnd Bergmann wrote:
> > On Sun, Oct 18, 2020 at 11:44 PM Syed Nayyar Waris <syednwaris@xxxxxxxxx> wrote:
> > >
> > > This patch reimplements the xgpio_set_multiple() function in
> > > drivers/gpio/gpio-xilinx.c to use the new generic functions:
> > > bitmap_get_value() and bitmap_set_value(). The code is now simpler
> > > to read and understand. Moreover, instead of looping for each bit
> > > in xgpio_set_multiple() function, now we can check each channel at
> > > a time and save cycles.
> >
> > This now causes -Wtype-limits warnings in linux-next with gcc-10:
>
> Hi Arnd,
>
> What version of gcc-10 are you running? I'm having trouble generating
> these warnings so I suspect I'm using a different version than you.

I originally saw it with the binaries from
https://mirrors.edge.kernel.org/pub/tools/crosstool/, but I have
also been able to reproduce it with a minimal test case on the
binaries from godbolt.org, see https://godbolt.org/z/Wq8q4n

> Let me first verify that I understand the problem correctly. The issue
> is the possibility of a stack smash in bitmap_set_value() when the value
> of start + nbits is larger than the length of the map bitmap memory
> region. This is because index (or index + 1) could be outside the range
> of the bitmap memory region passed in as map. Is my understanding
> correct here?

Yes, that seems to be the case here.

> In xgpio_set_multiple(), the variables width[0] and width[1] serve as
> possible start and nbits values for the bitmap_set_value() calls.
> Because width[0] and width[1] are unsigned int variables, GCC considers
> the possibility that the value of width[0]/width[1] might exceed the
> length of the bitmap memory region named old and thus result in a stack
> smash.
>
> I don't know if invalid width values are actually possible for the
> Xilinx gpio device, but let's err on the side of safety and assume this
> is actually a possibility. We should verify that the combined value of
> gpio_width[0] + gpio_width[1] does not exceed 64 bits; we can add a
> check for this in xgpio_probe() when we grab the gpio_width values.
>
> However, we're still left with the GCC warnings because GCC is not smart
> enough to know that we've already checked the boundary and width[0] and
> width[1] are valid values. I suspect we can avoid this warning is we
> refactor bitmap_set_value() to increment map seperately and then set it:

As I understand it, part of the problem is that gcc sees the possible
range as being constrained by the operations on 'start' and 'nbits',
in particular the shift in BIT_WORD() that put an upper bound on
the index, but then it sees that the upper bound is higher than the
upper bound of the array, i.e. element zero.

I added a check

      if (start >= 64 || start + size >= 64) return;

in the godbolt.org testcase, which does help limit the start
index appropriately, but it is not sufficient to let the compiler
see that the 'if (space >= nbits) ' condition is guaranteed to
be true for all values here.

> static inline void bitmap_set_value(unsigned long *map,
>                                     unsigned long value,
>                                     unsigned long start, unsigned long nbits)
> {
>         const unsigned long offset = start % BITS_PER_LONG;
>         const unsigned long ceiling = round_up(start + 1, BITS_PER_LONG);
>         const unsigned long space = ceiling - start;
>
>         map += BIT_WORD(start);
>         value &= GENMASK(nbits - 1, 0);
>
>         if (space >= nbits) {
>                 *map &= ~(GENMASK(nbits - 1, 0) << offset);
>                 *map |= value << offset;
>         } else {
>                 *map &= ~BITMAP_FIRST_WORD_MASK(start);
>                 *map |= value << offset;
>                 map++;
>                 *map &= ~BITMAP_LAST_WORD_MASK(start + nbits);
>                 *map |= value >> space;
>         }
> }
>
> This avoids adding a costly conditional check inside bitmap_set_value()
> when almost all bitmap_set_value() calls will have static arguments with
> well-defined and obvious boundaries.
>
> Do you think this would be an acceptable solution to resolve your GCC
> warnings?

Unfortunately, it does not seem to make a difference, as gcc still
knows that this compiles to the same result, and it produces the same
warning as before (see https://godbolt.org/z/rjx34r)

         Arnd