Re: [PATCH v3 1/2] lib/vsprintf: Fix to check field_width and precision

[Google]

On Wed, 25 Mar 2026 09:33:20 +0900
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx> wrote:

> On Tue, 24 Mar 2026 17:24:33 +0000
> David Laight <david.laight.linux@xxxxxxxxx> wrote:
> 
> > On Tue, 24 Mar 2026 17:45:49 +0100
> > Petr Mladek <pmladek@xxxxxxxx> wrote:
> > 
> > > On Mon 2026-03-23 13:59:05, David Laight wrote:
> > ...
> > > > I've also just fixed nolibc's handling of %*.*s (which is in 'next' since
> > > > I only wrote it recently), the above is actually broken.
> > > > Negative 'precision' (all values) are fine, they just request the default.  
> > > 
> > > Great catch! We should clamp the precision to (0, PRECISION_MAX). But we
> > > should warn only when it is outside of (-PRECISION_MAX, PRECISION_MAX).
> > 
> > No, you are going to clamp it needs to be to (-1, PRECISION_MAX);
> > but max(precision, PRECISION_MAX) if fine now the value is being saved
> > in an int.
> > And there is no need to warn about changing negative values - they
> > all mean exactly the same thing.
> 
> Ah, indeed.
> 
> "A negative precision is taken as if the precision were omitted."

Hmm, and format_decode() does not accept -1 precision. We need
another patch to fix it.

	spec->precision = -1;
	if (unlikely(*fmt.str == '.')) {
		fmt.str++;
		if (isdigit(*fmt.str)) {  <---- isdigit() accepts '0'-'9', not '-'.
			spec->precision = skip_atoi(&fmt.str); <--- this only returns positive value.
			if (spec->precision < 0)
				spec->precision = 0;
		} else if (*fmt.str == '*') {

Thanks,

-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>