linux-kernel - [PATCH] fbdev: fix printing in fb_cvt_print

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Message-ID: <20210916132919.GG25094@kili>
Date:   Thu, 16 Sep 2021 16:29:19 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Andrew Morton <akpm@...l.org>, Antonino Daplas <adaplas@....net>,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: [PATCH] fbdev: fix printing in fb_cvt_print_name()

This code has two bugs:
1) "cnt" is 255 but the size of the buffer is 256 so the last byte is
   not used.
2) If we try to print more than 255 characters then "cnt" will be
   negative and that will trigger a WARN() in snprintf(). The fix for
   this is to use scnprintf() instead of snprintf().

We can re-write this code to be cleaner:
1) Rename "offset" to "off" because that's shorter.
2) Get rid of the "cnt" variable and just use "size - off" directly.
3) Get rid of the "read" variable and just increment "off" directly.

Fixes: 96fe6a2109db ("[PATCH] fbdev: Add VESA Coordinated Video Timings (CVT) support")
Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
---
 drivers/video/fbdev/core/fbcvt.c | 53 +++++++++++++-------------------
 1 file changed, 21 insertions(+), 32 deletions(-)

diff --git a/drivers/video/fbdev/core/fbcvt.c b/drivers/video/fbdev/core/fbcvt.c
index 55d2bd0ce5c0..64843464c661 100644
--- a/drivers/video/fbdev/core/fbcvt.c
+++ b/drivers/video/fbdev/core/fbcvt.c
@@ -214,9 +214,11 @@ static u32 fb_cvt_aspect_ratio(struct fb_cvt_data *cvt)
 static void fb_cvt_print_name(struct fb_cvt_data *cvt)
 {
 	u32 pixcount, pixcount_mod;
-	int cnt = 255, offset = 0, read = 0;
-	u8 *buf = kzalloc(256, GFP_KERNEL);
+	int size = 256;
+	int off = 0;
+	u8 *buf;
 
+	buf = kzalloc(size, GFP_KERNEL);
 	if (!buf)
 		return;
 
@@ -224,43 +226,30 @@ static void fb_cvt_print_name(struct fb_cvt_data *cvt)
 	pixcount_mod = (cvt->xres * (cvt->yres/cvt->interlace)) % 1000000;
 	pixcount_mod /= 1000;
 
-	read = snprintf(buf+offset, cnt, "fbcvt: %dx%d@%d: CVT Name - ",
-			cvt->xres, cvt->yres, cvt->refresh);
-	offset += read;
-	cnt -= read;
+	off += scnprintf(buf + off, size - off, "fbcvt: %dx%d@%d: CVT Name - ",
+			    cvt->xres, cvt->yres, cvt->refresh);
 
-	if (cvt->status)
-		snprintf(buf+offset, cnt, "Not a CVT standard - %d.%03d Mega "
-			 "Pixel Image\n", pixcount, pixcount_mod);
+	if (cvt->status) {
+		off += scnprintf(buf + off, size - off,
+				 "Not a CVT standard - %d.%03d Mega Pixel Image\n",
+				 pixcount, pixcount_mod);
-	else {
+	} else {
-		if (pixcount) {
-			read = snprintf(buf+offset, cnt, "%d", pixcount);
-			cnt -= read;
-			offset += read;
-		}
+		if (pixcount)
+			off += scnprintf(buf + off, size - off, "%d", pixcount);
 
-		read = snprintf(buf+offset, cnt, ".%03dM", pixcount_mod);
-		cnt -= read;
-		offset += read;
+		off += scnprintf(buf + off, size - off, ".%03dM", pixcount_mod);
 
 		if (cvt->aspect_ratio == 0)
-			read = snprintf(buf+offset, cnt, "3");
+			off += scnprintf(buf + off, size - off, "3");
 		else if (cvt->aspect_ratio == 3)
-			read = snprintf(buf+offset, cnt, "4");
+			off += scnprintf(buf + off, size - off, "4");
 		else if (cvt->aspect_ratio == 1 || cvt->aspect_ratio == 4)
-			read = snprintf(buf+offset, cnt, "9");
+			off += scnprintf(buf + off, size - off, "9");
 		else if (cvt->aspect_ratio == 2)
-			read = snprintf(buf+offset, cnt, "A");
+			off += scnprintf(buf + off, size - off, "A");
-		else
-			read = 0;
-		cnt -= read;
-		offset += read;
-
-		if (cvt->flags & FB_CVT_FLAG_REDUCED_BLANK) {
-			read = snprintf(buf+offset, cnt, "-R");
-			cnt -= read;
-			offset += read;
-		}
+
+		if (cvt->flags & FB_CVT_FLAG_REDUCED_BLANK)
+			off += scnprintf(buf + off, size - off, "-R");
 	}
 
 	printk(KERN_INFO "%s\n", buf);
-- 
2.20.1