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

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>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1626674392-55857-1-git-send-email-xiyuyang19@fudan.edu.cn

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);
@@ -1590,7 +1590,7 @@ 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);
 

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;
 	/*