Re: [PATCH v7 2/4] lib: vsprintf: Fix handling of number field widths in vsscanf

[Richard Fitzgerald]

On 29/03/2021 14:36, Andy Shevchenko wrote:
> On Mon, Mar 29, 2021 at 01:08:22PM +0100, Richard Fitzgerald wrote:
> > The existing code attempted to handle numbers by doing a strto[u]l(),
> > ignoring the field width, and then repeatedly dividing to extract the
> > field out of the full converted value. If the string contains a run of
> > valid digits longer than will fit in a long or long long, this would
> > overflow and no amount of dividing can recover the correct value.
> >
> > This patch fixes vsscanf() to obey number field widths when parsing
> > the number.
> >
> > A new _parse_integer_limit() is added that takes a limit for the number
> > of characters to parse. The number field conversion in vsscanf is changed
> > to use this new function.
> >
> > If a number starts with a radix prefix, the field width  must be long
> > enough for at last one digit after the prefix. If not, it will be handled
> > like this:
> >
> >   sscanf("0x4", "%1i", &i): i=0, scanning continues with the 'x'
> >   sscanf("0x4", "%2i", &i): i=0, scanning continues with the '4'
> >
> > This is consistent with the observed behaviour of userland sscanf.
> >
> > Note that this patch does NOT fix the problem of a single field value
> > overflowing the target type. So for example:
> >
> >    sscanf("123456789abcdef", "%x", &i);
> >
> > Will not produce the correct result because the value obviously overflows
> > INT_MAX. But sscanf will report a successful conversion.
> >
> > Note that where a very large number is used to mean "unlimited", the value
> > INT_MAX is used for consistency with the behaviour of vsnprintf().
>
> ...
>
> >   unsigned long simple_strtoul(const char *cp, char **endp, unsigned int base)
> >   {
> > -	return simple_strtoull(cp, endp, base);
> > +	return simple_strntoull(cp, INT_MAX, endp, base);
>
> Why do you need this change?

I agree it's not necessary. I changed it between V1 and V2 but I can't
remember what the reason was.