Re: [PATCH 3/4] lib/vsprintf: use int for field_width in vsscanf()

Josh Law
2026-03-31T16:13:25Z

---- On Tue, 31 Mar 2026 17:12:18 +0100 pmladek@suse.com wrote ----

> On Tue 2026-03-31 16:35:22, David Laight wrote:
> > On Tue, 31 Mar 2026 16:31:50 +0200
> > Petr Mladek wrote:
> >
> > > On Wed 2026-03-25 14:00:17, Andy Shevchenko wrote:
> > > > On Tue, Mar 24, 2026 at 10:49:39PM +0000, Josh Law wrote:
> > > > > vsscanf() declares field_width as s16 but assigns it from skip_atoi()
> > > > > which returns int. Values above 32767 silently truncate to negative,
> > > > > causing vsscanf() to abort all remaining parsing. This is inconsistent
> > > > > with struct printf_spec which uses int for field_width.
> > > >
> > > > Is the field_width an acceptable integer range by the specifications?
> > >
> > > I am not sure what is allowed by specification. Anyway, the code is
> > > not ready for a bigger values, for example:
> > >
> > >         case 's':
> > >         {
> > >             char *s = (char *)va_arg(args, char *);
> > >             if (field_width == -1)
> > >                 field_width = SHRT_MAX;
> > >
> > > clearly expects signed short int range.
> > >
> > > I wonder if it might even open some backdoor. The code matching
> > > as sequence of characters expects a defined field width, see
> > >
> > >
> > >         case '[':
> > >         {
> > > [...]
> > >             /* field width is required */
> > >             if (field_width == -1)
> > >                 return num;
> > >
> > > The current code limits valid field width values to positive ones,
>
> I meant this code:
>
>         /* get field width */
>         field_width = -1;
>         if (isdigit(*fmt)) {
>             field_width = skip_atoi(&fmt);
>             if (field_width <= 0)
>                 break;
>         }
>
> If we change the type of the local variable then the above check will
> suddenly accept fied_width <= INT_MAX instead of SHRT_MAX.
>
> As a result, The above mentioned "case '[':" handling will suddely
> allow to iternate over INT_MAX long string instead of SHRT_MAX.
>
> I doubt that there is any kernel code which would be affected
> by this. But I do not want to risk it.
>
> > > aka SHRT_MAX which is clearly much lover than INT_MAX. And it might
> > > prevent some out of bound access.
>
> > > Best Regards,
> > > Petr
> > >
> > >
> >
> > Notwithstanding what the code actually does there is no point defining a
> > local variable as a 'short' unless you really want arithmetic to wrap
> > at 16 bits.
> > All it does is force the compiler to keep adding code to fix the sign
> > extension to 32 bits.
> > Look at the object for anything other than x86 (or m68k).
>
> If you think that it is important enough, feel free to send
> a patch.
>
> I not taking this patch from Josh Law, definitely!
>
> Best Regards,
> Petr
>
> PS: Note that Josh Law seems to be an AI virtual person, see
> https://lore.kernel.org/all/cbd0aafa-bd45-4f4d-a2dd-440473657dba@lucifer.local/[https://lore.kernel.org/all/cbd0aafa-bd45-4f4d-a2dd-440473657dba@lucifer.local/]
>
> I am even not sure what to do with the other 3 patches. They look
> correct. But I should not take patches with an unclear origin, see
> https://lore.kernel.org/all/2f824be3-7b30-41c6-b517-de1086624171@kernel.org/[https://lore.kernel.org/all/2f824be3-7b30-41c6-b517-de1086624171@kernel.org/]

I'm gonna arrange a video call with someone to fix this