[PATCH 06/14] lib/vsprintf.c: warn about too large precisions and field widths

[Rasmus Villemoes]

The field width is overloaded to pass some extra information for
some %p extensions (e.g. #bits for %pb). But we might silently
truncate the passed value when we stash it in struct printf_spec (see
e.g. "lib/vsprintf.c: expand field_width to 24 bits").  Hopefully 23
value bits should now be enough for everybody, but if not, let's make
some noise.

Do the same for the precision. In both cases, clamping seems more
sensible than truncating. While, according to POSIX, "A negative
precision is taken as if the precision were omitted.", the kernel's
printf has always treated that case as if the precision was 0, so we
use that as lower bound. For the field width, the smallest
representable value is actually -(1<<23), but a negative field width
means 'set the LEFT flag and use the absolute value', so we want the
absolute value to fit.

Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
---
 lib/vsprintf.c | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index d7e27c54fa00..8af5535fd738 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -386,6 +386,8 @@ struct printf_spec {
 	unsigned int	base:8;		/* number base, 8, 10 or 16 only */
 	signed int	precision:16;	/* # of digits/chars */
 } __packed;
+#define FIELD_WIDTH_MAX ((1 << 23) - 1)
+#define PRECISION_MAX ((1 << 15) - 1)
 extern char __check_printf_spec[1-2*(sizeof(struct printf_spec) != 8)];
 
 static noinline_for_stack
@@ -1815,6 +1817,24 @@ qualifier:
 	return ++fmt - start;
 }
 
+static inline 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 inline 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);
+	}
+}
+
 /**
  * vsnprintf - Format a string and place it in a buffer
  * @buf: The buffer to place the result into
@@ -1882,11 +1902,11 @@ int vsnprintf(char *buf, size_t size, const char *fmt, va_list args)
 		}
 
 		case FORMAT_TYPE_WIDTH:
-			spec.field_width = va_arg(args, int);
+			set_field_width(&spec, va_arg(args, int));
 			break;
 
 		case FORMAT_TYPE_PRECISION:
-			spec.precision = va_arg(args, int);
+			set_precision(&spec, va_arg(args, int));
 			break;
 
 		case FORMAT_TYPE_CHAR: {
-- 
2.6.1

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