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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Mar 07, 2019 at 04:22:13AM -0700, Jan Beulich wrote:
> >>> 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).

Ack, if that's the recommended way. I would prefer using unsigned int
and adding a BUG_ON if > 16. Anyway:

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks.

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