Re: [PATCH v4 2/4] lib: vsprintf: Fix handling of number field widths in vsscanf

[Richard Fitzgerald]

On 04/02/2021 16:35, Petr Mladek wrote:
> On Wed 2021-02-03 21:45:55, Andy Shevchenko wrote:
> > On Wed, Feb 03, 2021 at 04:50:07PM +0000, 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.
>
> > ...
> >
> > > +	for (; max_chars > 0; max_chars--) {
> >
> > Less fragile is to write
> >
> > 	while (max_chars--)
>
> Except that the original was more obvious at least for me.
> I always prefer more readable code when the compiler might do
> the optimization easily. But this is my personal taste.
> I am fine with both variants.
>
> > This allows max_char to be an unsigned type.
> >
> > Moreover...
> >
> > > +	return _parse_integer_limit(s, base, p, INT_MAX);
> >
> > You have inconsistency with INT_MAX vs, size_t above.
>
> Ah, this was on my request. INT_MAX is already used on many other
> locations in vsnprintf() for this purpose.

I originally had UINT_MAX and changed on Petr's request to be
consistent with other code. (Sorry Andy - my mistake not including
you on the earlier review versions).

But 0 < INT_MAX < UINT_MAX, so ok to pass to an unsigned. And as Petr
said on his original review, INT_MAX is "big enough".

I don't mind either way.

> An alternative is to fix all the other locations. We would need to
> check if it is really safe. Well, I do not want to force Richard
> to fix this historical mess. He already put a lot lot of effort
> into fixing this long term issue.
>
> ...
>
> > > -	unsigned long long result;
> > > +	const char *cp;
> > > +	unsigned long long result = 0ULL;
> > >   	unsigned int rv;
> > >   
> > > -	cp = _parse_integer_fixup_radix(cp, &base);
> > > -	rv = _parse_integer(cp, base, &result);
> >
> > > +	if (max_chars == 0) {
> > > +		cp = startp;
> > > +		goto out;
> > > +	}
> >
> > It's redundant if I'm not mistaken.
>
> Also this is more obvious and less error prone from my POV.
> But I agree that it is redundant. I am not sure if this
> function is used in some fast paths.
>
> Again I am fine with both variants.
>
> > > +	cp = _parse_integer_fixup_radix(startp, &base);
> > > +	if ((cp - startp) >= max_chars) {
> > > +		cp = startp + max_chars;
> > > +		goto out;
> > > +	}
> >
> > This will be exactly the same, no?
>
> Best Regards,
> Petr