Re: [PATCH 07/14] proc/kcore: hide a harmless warning

[Arnd Bergmann]

On Tue, Jul 18, 2017 at 9:55 PM, Ard Biesheuvel
<ard.biesheuvel@xxxxxxxxxx> wrote:
> On 18 July 2017 at 20:53, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>> On Fri, Jul 14, 2017 at 2:28 PM, Ard Biesheuvel
>> <ard.biesheuvel@xxxxxxxxxx> wrote:
>>> On 14 July 2017 at 10:25, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>>>> gcc warns when MODULES_VADDR/END is defined to the same value as
>>>> VMALLOC_START/VMALLOC_END, e.g. on x86-32:
>>>>
>>>> fs/proc/kcore.c: In function âadd_modules_rangeâ:
>>>> fs/proc/kcore.c:622:161: error: self-comparison always evaluates to false [-Werror=tautological-compare]
>>>>   if (/*MODULES_VADDR != VMALLOC_START && */MODULES_END != VMALLOC_END) {
>>>>
>>>
>>> Does it occur for subtraction as well? Or only for comparison?
>>
>> This replacement patch would also address the warning:
>>
>> diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
>> index 45629f4b5402..35824e986c2c 100644
>> --- a/fs/proc/kcore.c
>> +++ b/fs/proc/kcore.c
>> @@ -623,7 +623,7 @@ static void __init proc_kcore_text_init(void)
>>  struct kcore_list kcore_modules;
>>  static void __init add_modules_range(void)
>>  {
>> -       if (MODULES_VADDR != VMALLOC_START && MODULES_END != VMALLOC_END) {
>> +       if (MODULES_VADDR - VMALLOC_START && MODULES_END - VMALLOC_END) {
>>                 kclist_add(&kcore_modules, (void *)MODULES_VADDR,
>>                         MODULES_END - MODULES_VADDR, KCORE_VMALLOC);
>>         }
>>
>> I have also verified that four of the 14 patches are not needed when building
>> without ccache, this is one of them:
>>
>>  acpi: thermal: fix gcc-6/ccache warning
>>  proc/kcore: hide a harmless warning
>>  SFI: fix tautological-compare warning
>>  [media] fix warning on v4l2_subdev_call() result interpreted as bool
>>
>> Not sure what to do with those, we could either ignore them all and
>> not care about ccache, or we try to address them all in some way.
>>
>
> Any idea why ccache makes a difference here? It is not obvious (not to
> me at least)

When ccache is used, the compilation stage is apparently always done on
the preprocessed source file. So instead of parsing (with the integrated
preprocessor)

          if (MODULES_VADDR != VMALLOC_START ...)

the compiler sees

          if (((unsigned long)high_memory + (8 * 1024 * 1024))  !=
              ((unsigned long)high_memory + (8 * 1024 * 1024))  ...)

and it correctly considers the first expression something that one
would write in source code, while -Wtautological-compare
is intended to warn about the second version being always true,
which makes the 'if()' pointless.

       Arnd