linux-kernel - Re: [PATCH] lib/vsprintf.c: increase the size of the field

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Date:	Wed, 09 Sep 2015 12:26:39 -0700
From:	Joe Perches <joe@...ches.com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Maurizio Lombardi <mlombard@...hat.com>, akpm@...ux-foundation.org,
	tj@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib/vsprintf.c: increase the size of the field_width
 variable

On Wed, 2015-09-09 at 20:51 +0200, Rasmus Villemoes wrote:
> On Wed, Sep 09 2015, Joe Perches <joe@...ches.com> wrote:
> > this makes the sizeof struct printf_spec more than
> > 8 bytes which isn't desireable on x86-32.
> 
> I'm pretty sure struct printf_spec
> purposely has sizeof==8 to allow it to be (relatively cheaply) passed
> around by value.

True.  https://lkml.org/lkml/2010/3/6/141

> > %*pb is meant for smallish bitmaps, not big ones.
> 
> Yup. And that leads to my other confusion: Given that the expected
> output is given as "0-15", does the bitmap really consist of > S16_MAX
> bits with only the first 16 set?

No idea.  Tejun?

Perhaps the code in lib/vsprintf.c that sets
spec.field_width and spec.precision from format argument
input should be changed to use a temporary int and
clamped to S16_MIN -> S16_MAX.

Something like:
---
 lib/vsprintf.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 7f0cdd2..2782129 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1913,13 +1913,21 @@ int vsnprintf(char *buf, size_t size, const char *fmt, va_list args)
 			break;
 		}
 
-		case FORMAT_TYPE_WIDTH:
-			spec.field_width = va_arg(args, int);
+		case FORMAT_TYPE_WIDTH: {
+			int tmp = va_arg(args, int);
+
+			spec.field_width = (s16)clamp_t(int, tmp,
+							S16_MIN, S16_MAX);
 			break;
+		}
 
-		case FORMAT_TYPE_PRECISION:
-			spec.precision = va_arg(args, int);
+		case FORMAT_TYPE_PRECISION: {
+			int tmp = va_arg(args, int);
+
+			spec.precision = (s16)clamp_t(int, tmp,
+						      S16_MIN, S16_MAX);
 			break;
+		}
 
 		case FORMAT_TYPE_CHAR: {
 			char c;


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