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

[Ard Biesheuvel]

On 2 March 2017 at 10:11, Markus Trippelsdorf <markus@xxxxxxxxxxxxxxx> wrote:
> 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);
>           ^~~~~
>

It is slightly noisier than I expected, but it emphasizes the fact
that it is GCC that is emitting a const '0' into ilog2() and not the
programmer.