Re: [Xen-devel] [PATCH 2/2] hvmloader: cast to 64bit before multiplication in get_module_entry

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Aug 19, 2016 at 06:00:12AM -0600, Jan Beulich wrote:
> >>> On 19.08.16 at 12:09, <andrew.cooper3@xxxxxxxxxx> wrote:
> > On 19/08/16 09:31, Jan Beulich wrote:
> >>>>> On 19.08.16 at 10:06, <wei.liu2@xxxxxxxxxx> wrote:
> >>> --- a/tools/firmware/hvmloader/hvmloader.c
> >>> +++ b/tools/firmware/hvmloader/hvmloader.c
> >>> @@ -272,8 +272,8 @@ const struct hvm_modlist_entry *get_module_entry(
> >>>  
> >>>      if ( !modlist ||
> >>>           info->modlist_paddr > UINTPTR_MAX ||
> >>> -         (info->modlist_paddr + info->nr_modules * sizeof(*modlist) - 1)
> >>> -            > UINTPTR_MAX )
> >>> +         (info->modlist_paddr +
> >>> +          (uint64_t)info->nr_modules * sizeof(*modlist) - 1) > 
> >>> UINTPTR_MAX )
> >>>          return NULL;
> >> This can be had without resorting to 64-bit multiplication, by bounds
> >> checking
> >>
> >>          (UINTPTR_MAX - (uintptr_t)info->modlist_paddr) / sizeof(*modlist)
> >>
> >> instead. While we would certainly hope that compilers don't resort
> >> to a libgcc helper for 64-bit multiplication, I think we'd better avoid
> >> that risk altogether.
> > 
> > In this case, using libgcc would cause a link error because of
> > -fno-builtin, so I don't think it is too bad.
> 
> And it's this link error which I want to avoid.
> 

What approach should I use? I would like to clear this minor issue as
quick as possible.

Wei.

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