debugfs: implement per-file removal protection

Since commit 49d200deaa68 ("debugfs: prevent access to removed files'
private data"), accesses to a file's private data are protected from
concurrent removal by covering all file_operations with a SRCU read section
and sychronizing with those before returning from debugfs_remove() by means
of synchronize_srcu().

As pointed out by Johannes Berg, there are debugfs files with forever
blocking file_operations. Their corresponding SRCU read side sections would
block any debugfs_remove() forever as well, even unrelated ones. This
results in a livelock. Because a remover can't cancel any indefinite
blocking within foreign files, this is a problem.

Resolve this by introducing support for more granular protection on a
per-file basis.

This is implemented by introducing an  'active_users' refcount_t to the
per-file struct debugfs_fsdata state. At file creation time, it is set to
one and a debugfs_remove() will drop that initial reference. The new
debugfs_file_get() and debugfs_file_put(), intended to be used in place of
former debugfs_use_file_start() and debugfs_use_file_finish(), increment
and decrement it respectively. Once the count drops to zero,
debugfs_file_put() will signal a completion which is possibly being waited
for from debugfs_remove().
Thus, as long as there is a debugfs_file_get() not yet matched by a
corresponding debugfs_file_put() around, debugfs_remove() will block.

Actual users of debugfs_use_file_start() and -finish() will get converted
to the new debugfs_file_get() and debugfs_file_put() by followup patches.

Fixes: 49d200deaa68 ("debugfs: prevent access to removed files' private data")
Reported-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 11 insertions, 0 deletions

diff --git a/include/linux/debugfs.h b/include/linux/debugfs.h
index cbee5f4a02a3..3b914d588148 100644
--- a/include/linux/debugfs.h
+++ b/include/linux/debugfs.h
@@ -98,6 +98,9 @@ void debugfs_use_file_finish(int srcu_idx) __releases(&debugfs_srcu);
 const struct file_operations *debugfs_real_fops(const struct file *filp)
 	__must_hold(&debugfs_srcu);
 
+int debugfs_file_get(struct dentry *dentry);
+void debugfs_file_put(struct dentry *dentry);
+
 ssize_t debugfs_attr_read(struct file *file, char __user *buf,
 			size_t len, loff_t *ppos);
 ssize_t debugfs_attr_write(struct file *file, const char __user *buf,
@@ -236,6 +239,14 @@ static inline void debugfs_use_file_finish(int srcu_idx)
 	__releases(&debugfs_srcu)
 { }
 
+static inline int debugfs_file_get(struct dentry *dentry)
+{
+	return 0;
+}
+
+static inline void debugfs_file_put(struct dentry *dentry)
+{ }
+
 static inline ssize_t debugfs_attr_read(struct file *file, char __user *buf,
 					size_t len, loff_t *ppos)
 {