Re: gcc7 log2 compile issues in kernel/time/timekeeping.c

[Markus Trippelsdorf]

On 2017.03.01 at 17:39 +0000, Ard Biesheuvel wrote:
> On 1 March 2017 at 00:00, Laura Abbott <labbott@xxxxxxxxxx> wrote:
> > On 02/25/2017 03:50 AM, Ard Biesheuvel wrote:
> >>
> >>
> >>> On 25 Feb 2017, at 11:23, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
> >>>
> >>> On 25 February 2017 at 11:09, Markus Trippelsdorf
> >>> <markus@xxxxxxxxxxxxxxx> wrote:
> >>>> On 2017.02.25 at 09:11 +0000, Ard Biesheuvel wrote:
> >>>>>> On 25 February 2017 at 08:18, Markus Trippelsdorf <markus@xxxxxxxxxxxxxxx> wrote:
> >>>>>>
> >>>>>> Why not simply get rid of the ____ilog2_NaN thing altogether?
> >>>>>>
> >>>>>
> >>>>> That would remove the issue, sure. But we lose an opportunity to spot
> >>>>> incorrect code at compile time.
> >>>>
> >>>> In the case of kernel/time/timekeeping.c it is clearly a false positive.
> >>>> Was ever incorrect code spotted by ____ilog2_NaN in the past?
> >>>>
> >>>>> My concern is that it by not pushing back on changes to the semantics
> >>>>> of __builtin_constant_p() such as this one, we may start seeing other
> >>>>> issues where we can no longer use it, and we lose a very useful tool.
> >>>>
> >>>> We had a long discussion in:
> >>>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72785
> >>>> As you can see there is no real consensus.
> >>>> But ilog2 seems to be the only place where this ever popped up.
> >>>> (There were several distro-wide mass rebuilds with gcc-7 and no other
> >>>> __builtin_constant_p() issue was found yet.)
> >>>>
> >>>
> >>> Well, given that it is really dead code that is being emitted, and
> >>> that log2(0) is really undefined, perhaps we should simply replace
> >>> ilog2_NaN() with __builtin_unreachable()?
> >>
> >> ... or perhaps it is better to just pass the constant == 0 to the runtime implementation?
> >>
> >> The second ilog2_NaN is really unreachable, given that it deals with unsigned values >0 without a single bit set.
> >>
> >
> > naively throwing in __builtin_unreachable() doesn't seem to
> > work:
> >
> > ./include/linux/log2.h: In function â__order_base_2â:
> > ./include/linux/log2.h:155:10: error: void value not ignored as it ought to be
> >
> > I'm guessing unreachable is treated as void instead of all
> > possible types and therefore gcc assumes that the entire
> > function must be void?
> >
> 
> Something like this perhaps? This will at least prevent incorrect uses
> from being silently ignored, but maybe it is a bit overkill.
> diff --git a/include/linux/log2.h b/include/linux/log2.h
> index ef3d4f67118c..c670b3dfd5ca 100644
> --- a/include/linux/log2.h
> +++ b/include/linux/log2.h
> @@ -18,8 +18,8 @@
>  /*
>   * deal with unrepresentable constant logarithms
>   */
> -extern __attribute__((const, noreturn))
> -int ____ilog2_NaN(void);
> +static noinline __attribute__((noreturn, warning("ilog2(0) is undefined!")))
> +int ____ilog2_NaN(void) { unreachable(); }
> 
>  /*
>   * non-constant log of base 2 calculators

Hmm, this will result in the following warning.

In file included from ./include/linux/kernel.h:11:0,
                 from ./include/linux/list.h:8,
                 from ./include/linux/preempt.h:10,
                 from ./include/linux/spinlock.h:50,
                 from ./include/linux/seqlock.h:35,
                 from ./include/linux/time.h:5,
                 from ./include/uapi/linux/timex.h:56,
                 from ./include/linux/timex.h:56,
                 from ./include/linux/clocksource.h:12,
                 from ./include/linux/timekeeper_internal.h:9,
                 from kernel/time/timekeeping.c:11:
kernel/time/timekeeping.c: In function âupdate_wall_timeâ:
./include/linux/log2.h:88:29: warning: call to â____ilog2_NaNâ declared with attribute warning: ilog2(0) is undefined!
  __builtin_constant_p(n) ? (  \
                            ~~~~
   (n) < 1 ? ____ilog2_NaN() : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
   (n) & (1ULL << 63) ? 63 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 62) ? 62 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 61) ? 61 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 60) ? 60 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 59) ? 59 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 58) ? 58 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 57) ? 57 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 56) ? 56 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 55) ? 55 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 54) ? 54 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 53) ? 53 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 52) ? 52 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 51) ? 51 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 50) ? 50 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 49) ? 49 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 48) ? 48 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 47) ? 47 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 46) ? 46 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 45) ? 45 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 44) ? 44 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 43) ? 43 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 42) ? 42 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 41) ? 41 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 40) ? 40 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 39) ? 39 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 38) ? 38 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 37) ? 37 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 36) ? 36 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 35) ? 35 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 34) ? 34 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 33) ? 33 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 32) ? 32 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 31) ? 31 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 30) ? 30 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 29) ? 29 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 28) ? 28 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 27) ? 27 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 26) ? 26 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 25) ? 25 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 24) ? 24 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 23) ? 23 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 22) ? 22 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 21) ? 21 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 20) ? 20 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 19) ? 19 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 18) ? 18 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 17) ? 17 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 16) ? 16 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 15) ? 15 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 14) ? 14 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 13) ? 13 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 12) ? 12 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 11) ? 11 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL << 10) ? 10 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL <<  9) ?  9 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL <<  8) ?  8 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL <<  7) ?  7 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL <<  6) ?  6 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL <<  5) ?  5 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL <<  4) ?  4 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL <<  3) ?  3 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL <<  2) ?  2 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL <<  1) ?  1 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   (n) & (1ULL <<  0) ?  0 : \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   ____ilog2_NaN()   \
   ~~~~~~~~~~~~~~~~~~~
        ) :  \
        ~
kernel/time/timekeeping.c:2051:10: note: in expansion of macro âilog2â
  shift = ilog2(offset) - ilog2(tk->cycle_interval);
          ^~~~~

-- 
Markus