linux-kernel - [PATCH 3/3] debugfs: open_proxy_open(): avoid double fops release

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Date:	Tue, 24 May 2016 13:08:54 +0200
From:	Nicolai Stange <nicstange@...il.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	linux-kernel@...r.kernel.org, Nicolai Stange <nicstange@...il.com>
Subject: [PATCH 3/3] debugfs: open_proxy_open(): avoid double fops release

Debugfs' open_proxy_open(), the ->open() installed at all inodes created
through debugfs_create_file_unsafe(),
- grabs a reference to the original file_operations instance passed to
  debugfs_create_file_unsafe() via fops_get(),
- installs it at the file's ->f_op by means of replace_fops()
- and calls fops_put() on it.

Since the semantics of replace_fops() are such that the reference's
ownership is transferred, the subsequent fops_put() will result in a double
release when the file is eventually closed.

Currently, this is not an issue since fops_put() basically does a
module_put() on the file_operations' ->owner only and there don't exist any
modules calling debugfs_create_file_unsafe() yet. This is expected to
change in the future though, c.f. commit c64688081490 ("debugfs: add
support for self-protecting attribute file fops").

Remove the call to fops_put() from open_proxy_open().

Fixes: 9fd4dcece43a ("debugfs: prevent access to possibly dead
                      file_operations at file open")
Signed-off-by: Nicolai Stange <nicstange@...il.com>
---
 fs/debugfs/file.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index d1ec803..592059f 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -127,7 +127,6 @@ static int open_proxy_open(struct inode *inode, struct file *filp)
 		r = real_fops->open(inode, filp);

 out:
-	fops_put(real_fops);
 	debugfs_use_file_finish(srcu_idx);
 	return r;
 }
-- 
2.8.2