Re: [Xen-devel] [PATCH 2/2] x86/mtrr: fix build with gcc9

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.03.19 at 11:55, <roger.pau@xxxxxxxxxx> wrote:
> On Thu, Mar 07, 2019 at 03:32:12AM -0700, Jan Beulich wrote:
>> generic.c: In function ‘print_mtrr_state’:
>> generic.c:210:11: error: ‘%0*lx’ directive output between 1 and 1073741823 
>> bytes may cause result to exceed
>> ‘INT_MAX’ [-Werror=format-overflow=]
>>   210 |    printk("%s  %u base %0*"PRIx64"000 mask %0*"PRIx64"000 %s\n",
>>       |           ^~~~~~~~~~~~~~~~~
>> generic.c:210:44: note: format string is defined here
>>   210 |    printk("%s  %u base %0*"PRIx64"000 mask %0*"PRIx64"000 %s\n",
>> generic.c:210:11: note: directive argument in the range [0, 4503599627370495]
>>   210 |    printk("%s  %u base %0*"PRIx64"000 mask %0*"PRIx64"000 %s\n",
>>       |           ^~~~~~~~~~~~~~~~~
>> generic.c:210:11: note: assuming directive output of 1 byte
>> 
>> Restrict the width of the variable "width" controlling the number of
>> address digits output.
> 
> I have to admit I'm not sure why gcc complains here, and why switching
> to unsigned char fixes it. unsigned char max value would be 255, which
> when used as a width to print an unsigned long it's also too high?

With

        width = (paddr_bits - PAGE_SHIFT + 3) / 4;

gcc simply can't know that the valid value range is rather small. Hence
it assumes millions of leading zeros _could_ be printed. Their doc
explicitly suggests using a more narrow type, which is why ...

> Does checking that width <= 16 also placate gcc?

... I didn't even try this option (despite having considered it).

Jan


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