linux-kernel - Re: [PATCH V2] fbcon -- fix race between open and removal of framebuffers

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Message-ID: <4DC9A8C0.9070502@fastmail.fm>
Date:	Tue, 10 May 2011 22:06:08 +0100
From:	Jack Stone <jwjstone@...tmail.fm>
To:	Tim Gardner <tim.gardner@...onical.com>
CC:	linux-fbdev@...r.kernel.org, lethal@...ux-sh.org,
	linux-kernel@...r.kernel.org, Andy Whitcroft <apw@...onical.com>,
	Leann Ogasawara <leann.ogasawara@...onical.com>
Subject: Re: [PATCH V2] fbcon -- fix race between open and removal of framebuffers

Hi Tim,

One more quick question:

On 10/05/2011 13:47, Tim Gardner wrote:
+static struct fb_info *get_framebuffer_info(int idx)
+__acquires(&registered_lock)
+__releases(&registered_lock)
+{
+	struct fb_info *fb_info;
+
+	spin_lock(&registered_lock);
+	fb_info = registered_fb[idx];
+	fb_info->ref_count++;
+	spin_unlock(&registered_lock);
+
+	return fb_info;
+}

[snip]

 static int
 fb_open(struct inode *inode, struct file *file)
 __acquires(&info->lock)
@@ -1363,13 +1421,18 @@ __releases(&info->lock)

 	if (fbidx >= FB_MAX)
 		return -ENODEV;
-	info = registered_fb[fbidx];
-	if (!info)
+	info = get_framebuffer_info(fbidx);
+	if (!info) {
 		request_module("fb%d", fbidx);
-	info = registered_fb[fbidx];
+		info = get_framebuffer_info(fbidx);
+	}
 	if (!info)
 		return -ENODEV;

This section of code implies that get_framebuffer_info can return NULL
but in that case wouldn't the fb_info->ref_count++ have oopsed?

You could add the simple case of

if(fb_info)
	fb_info->ref_count++

to get_framebuffer_info. That should cover it.

Thanks,

Jack
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