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

David Laight
2026-03-31T15:35:25Z

On Tue, 31 Mar 2026 16:31:50 +0200
Petr Mladek <pmladek@suse.com> 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,
> 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).

David