From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Check the field_width and presition correctly. Previously it depends
on the bitfield conversion from int to check out-of-range error.
However, commit 938df695e98d ("vsprintf: associate the format state
with the format pointer") changed those fields to int.
We need to check the out-of-range correctly without bitfield
conversion.

Fixes: 938df695e98d ("vsprintf: associate the format state with the format pointer")
Reported-by: David Laight <david.laight.linux@gmail.com>
Closes: https://lore.kernel.org/all/20260318151250.40fef0ab@pumpkin/
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 Changes in v5:
  - Drop negative precision support.
  - The field and precision passed as string literals are also checked.
 Changes in v4:
  - Do clamp() first.
  - Accept negative precision (this means no precision) .
  - Change the warning message for width.
 Changes in v3:
  - Check and update width and precision before assigning to spec.
 Changes in v2:
  - Fix to use logical split.
---
 lib/vsprintf.c |   39 +++++++++++++++++----------------------
 1 file changed, 17 insertions(+), 22 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 800b8ac49f53..2f6b7179f581 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -2640,6 +2640,21 @@ static unsigned char spec_flag(unsigned char c)
 	return (c < sizeof(spec_flag_array)) ? spec_flag_array[c] : 0;
 }
 
+static void
+set_field_width(struct printf_spec *spec, int width)
+{
+	spec->field_width = clamp(width, -FIELD_WIDTH_MAX, FIELD_WIDTH_MAX);
+	WARN_ONCE(spec->field_width != width, "field width %d out of range",
+		  width);
+}
+
+static void
+set_precision(struct printf_spec *spec, int prec)
+{
+	spec->precision = clamp(prec, 0, PRECISION_MAX);
+	WARN_ONCE(spec->precision < prec, "precision %d too large", prec);
+}
+
 /*
  * Helper function to decode printf style format.
  * Each call decode a token from the format and return the
@@ -2710,7 +2725,7 @@ struct fmt format_decode(struct fmt fmt, struct printf_spec *spec)
 	spec->field_width = -1;
 
 	if (isdigit(*fmt.str))
-		spec->field_width = skip_atoi(&fmt.str);
+		set_field_width(spec, skip_atoi(&fmt.str));
 	else if (unlikely(*fmt.str == '*')) {
 		/* it's the next argument */
 		fmt.state = FORMAT_STATE_WIDTH;
@@ -2724,9 +2739,7 @@ struct fmt format_decode(struct fmt fmt, struct printf_spec *spec)
 	if (unlikely(*fmt.str == '.')) {
 		fmt.str++;
 		if (isdigit(*fmt.str)) {
-			spec->precision = skip_atoi(&fmt.str);
-			if (spec->precision < 0)
-				spec->precision = 0;
+			set_precision(spec, skip_atoi(&fmt.str));
 		} else if (*fmt.str == '*') {
 			/* it's the next argument */
 			fmt.state = FORMAT_STATE_PRECISION;
@@ -2799,24 +2812,6 @@ struct fmt format_decode(struct fmt fmt, struct printf_spec *spec)
 	return fmt;
 }
 
-static void
-set_field_width(struct printf_spec *spec, int width)
-{
-	spec->field_width = width;
-	if (WARN_ONCE(spec->field_width != width, "field width %d too large", width)) {
-		spec->field_width = clamp(width, -FIELD_WIDTH_MAX, FIELD_WIDTH_MAX);
-	}
-}
-
-static void
-set_precision(struct printf_spec *spec, int prec)
-{
-	spec->precision = prec;
-	if (WARN_ONCE(spec->precision != prec, "precision %d too large", prec)) {
-		spec->precision = clamp(prec, 0, PRECISION_MAX);
-	}
-}
-
 /*
  * Turn a 1/2/4-byte value into a 64-bit one for printing: truncate
  * as necessary and deal with signedness.

On Wed, Mar 25, 2026 at 10:27:41PM +0900, Masami Hiramatsu (Google) wrote:

> Check the field_width and presition correctly. Previously it depends
> on the bitfield conversion from int to check out-of-range error.
> However, commit 938df695e98d ("vsprintf: associate the format state
> with the format pointer") changed those fields to int.
> We need to check the out-of-range correctly without bitfield
> conversion.

...

> +static void
> +set_field_width(struct printf_spec *spec, int width)

One line.

static void set_field_width(struct printf_spec *spec, int width)

> +{
> +	spec->field_width = clamp(width, -FIELD_WIDTH_MAX, FIELD_WIDTH_MAX);
> +	WARN_ONCE(spec->field_width != width, "field width %d out of range",
> +		  width);

I would also make it one line for readability.

	WARN_ONCE(spec->field_width != width, "field width %d out of range", width);

> +}
> +
> +static void
> +set_precision(struct printf_spec *spec, int prec)

One line

static void set_precision(struct printf_spec *spec, int prec)

> +{
> +	spec->precision = clamp(prec, 0, PRECISION_MAX);
> +	WARN_ONCE(spec->precision < prec, "precision %d too large", prec);
> +}

-- 
With Best Regards,
Andy Shevchenko

On Thu, 26 Mar 2026 11:57:46 +0200
Andy Shevchenko <andriy.shevchenko@linux.intel.com> wrote:

> On Wed, Mar 25, 2026 at 10:27:41PM +0900, Masami Hiramatsu (Google) wrote:
> 
> > Check the field_width and presition correctly. Previously it depends
> > on the bitfield conversion from int to check out-of-range error.
> > However, commit 938df695e98d ("vsprintf: associate the format state
> > with the format pointer") changed those fields to int.
> > We need to check the out-of-range correctly without bitfield
> > conversion.
> 
> ...
> 
> > +static void
> > +set_field_width(struct printf_spec *spec, int width)
> 
> One line.
> 

Ah, OK.

> static void set_field_width(struct printf_spec *spec, int width)
> 
> > +{
> > +	spec->field_width = clamp(width, -FIELD_WIDTH_MAX, FIELD_WIDTH_MAX);
> > +	WARN_ONCE(spec->field_width != width, "field width %d out of range",
> > +		  width);
> 
> I would also make it one line for readability.
> 
> 	WARN_ONCE(spec->field_width != width, "field width %d out of range", width);

OK.

> 
> > +}
> > +
> > +static void
> > +set_precision(struct printf_spec *spec, int prec)
> 
> One line
> 
> static void set_precision(struct printf_spec *spec, int prec)

OK, let me update it.

Thanks for review!

> 
> > +{
> > +	spec->precision = clamp(prec, 0, PRECISION_MAX);
> > +	WARN_ONCE(spec->precision < prec, "precision %d too large", prec);
> > +}
> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>