[PATCH] fbmem: Convert from atomic_t to refcount_t on fb_info->count

[PATCH] fbmem: Convert from atomic_t to refcount_t on fb_info->count

[Xiyu Yang]

refcount_t type and corresponding API can protect refcounters from
accidental underflow and overflow and further use-after-free situations.

Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
---
 drivers/video/fbdev/core/fbmem.c | 6 +++---
 include/linux/fb.h               | 3 ++-
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 98f193078c05..b7d26b928e1d 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -67,7 +67,7 @@ static struct fb_info *get_fb_info(unsigned int idx)
 	mutex_lock(&registration_lock);
 	fb_info = registered_fb[idx];
 	if (fb_info)
-		atomic_inc(&fb_info->count);
+		refcount_inc(&fb_info->count);
 	mutex_unlock(&registration_lock);
 
 	return fb_info;
@@ -75,7 +75,7 @@ static struct fb_info *get_fb_info(unsigned int idx)
 
 static void put_fb_info(struct fb_info *fb_info)
 {
-	if (!atomic_dec_and_test(&fb_info->count))
+	if (!refcount_dec_and_test(&fb_info->count))
 		return;
 	if (fb_info->fbops->fb_destroy)
 		fb_info->fbops->fb_destroy(fb_info);
@@ -1594,7 +1594,7 @@ static int do_register_framebuffer(struct fb_info *fb_info)
 		if (!registered_fb[i])
 			break;
 	fb_info->node = i;
-	atomic_set(&fb_info->count, 1);
+	refcount_set(&fb_info->count, 1);
 	mutex_init(&fb_info->lock);
 	mutex_init(&fb_info->mm_lock);
 
diff --git a/include/linux/fb.h b/include/linux/fb.h
index ecfbcc0553a5..5950f8f5dc74 100644
--- a/include/linux/fb.h
+++ b/include/linux/fb.h
@@ -2,6 +2,7 @@
 #ifndef _LINUX_FB_H
 #define _LINUX_FB_H
 
+#include <linux/refcount.h>
 #include <linux/kgdb.h>
 #include <uapi/linux/fb.h>
 
@@ -435,7 +436,7 @@ struct fb_tile_ops {
 
 
 struct fb_info {
-	atomic_t count;
+	refcount_t count;
 	int node;
 	int flags;
 	/*
-- 
2.7.4

Re: [PATCH] fbmem: Convert from atomic_t to refcount_t on fb_info->count

[Sam Ravnborg]

Hi Xiyu, Xin,
On Mon, Jul 19, 2021 at 01:59:45PM +0800, Xiyu Yang wrote:
> refcount_t type and corresponding API can protect refcounters from
> accidental underflow and overflow and further use-after-free situations.
> 
> Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
> Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>

Looks like a nice improvement, thanks.
Applied to drm-misc-next, it will appear in -next in a week or so.

	Sam