[PATCH v4 1/2] lib/vsprintf: Fix to check field_width and precision

[Masami Hiramatsu (Google)]

From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

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@xxxxxxxxx>
Closes: https://lore.kernel.org/all/20260318151250.40fef0ab@pumpkin/
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
---
 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 |   17 ++++++-----------
 1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 800b8ac49f53..5fa8f69030be 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -2679,9 +2679,6 @@ struct fmt format_decode(struct fmt fmt, struct printf_spec *spec)
 
 	/* we finished early by reading the precision */
 	if (unlikely(fmt.state == FORMAT_STATE_PRECISION)) {
-		if (spec->precision < 0)
-			spec->precision = 0;
-
 		fmt.state = FORMAT_STATE_NONE;
 		goto qualifier;
 	}
@@ -2802,19 +2799,17 @@ struct fmt format_decode(struct fmt fmt, struct printf_spec *spec)
 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);
-	}
+	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 = prec;
-	if (WARN_ONCE(spec->precision != prec, "precision %d too large", prec)) {
-		spec->precision = clamp(prec, 0, PRECISION_MAX);
-	}
+	/* We allow negative precision, but treat it as if there was no precision. */
+	spec->precision = clamp(prec, -1, PRECISION_MAX);
+	WARN_ONCE(spec->precision < prec, "precision %d too large", prec);
 }
 
 /*