debugfs: implement per-file removal protection

Since commit 49d200d ("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: 49d200d ("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>

fs/debugfs/file.c

@@ -109,6 +109,54 @@ const struct file_operations *debugfs_real_fops(const struct file *filp)
 }
 EXPORT_SYMBOL_GPL(debugfs_real_fops);
 
+/**
+ * debugfs_file_get - mark the beginning of file data access
+ * @dentry: the dentry object whose data is being accessed.
+ *
+ * Up to a matching call to debugfs_file_put(), any successive call
+ * into the file removing functions debugfs_remove() and
+ * debugfs_remove_recursive() will block. Since associated private
+ * file data may only get freed after a successful return of any of
+ * the removal functions, you may safely access it after a successful
+ * call to debugfs_file_get() without worrying about lifetime issues.
+ *
+ * If -%EIO is returned, the file has already been removed and thus,
+ * it is not safe to access any of its data. If, on the other hand,
+ * it is allowed to access the file data, zero is returned.
+ */
+int debugfs_file_get(struct dentry *dentry)
+{
+	struct debugfs_fsdata *fsd = dentry->d_fsdata;
+
+	/* Avoid starvation of removers. */
+	if (d_unlinked(dentry))
+		return -EIO;
+
+	if (!refcount_inc_not_zero(&fsd->active_users))
+		return -EIO;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(debugfs_file_get);
+
+/**
+ * debugfs_file_put - mark the end of file data access
+ * @dentry: the dentry object formerly passed to
+ *          debugfs_file_get().
+ *
+ * Allow any ongoing concurrent call into debugfs_remove() or
+ * debugfs_remove_recursive() blocked by a former call to
+ * debugfs_file_get() to proceed and return to its caller.
+ */
+void debugfs_file_put(struct dentry *dentry)
+{
+	struct debugfs_fsdata *fsd = dentry->d_fsdata;
+
+	if (refcount_dec_and_test(&fsd->active_users))
+		complete(&fsd->active_users_drained);
+}
+EXPORT_SYMBOL_GPL(debugfs_file_put);
+
 static int open_proxy_open(struct inode *inode, struct file *filp)
 {
 	const struct dentry *dentry = F_DENTRY(filp);

fs/debugfs/inode.c

@@ -374,6 +374,7 @@ static struct dentry *__debugfs_create_file(const char *name, umode_t mode,
 
 	inode->i_fop = proxy_fops;
 	fsd->real_fops = real_fops;
+	refcount_set(&fsd->active_users, 1);
 	dentry->d_fsdata = fsd;
 
 	d_instantiate(dentry, inode);
@@ -631,18 +632,34 @@ struct dentry *debugfs_create_symlink(const char *name, struct dentry *parent,
 }
 EXPORT_SYMBOL_GPL(debugfs_create_symlink);
 
+static void __debugfs_remove_file(struct dentry *dentry, struct dentry *parent)
+{
+	struct debugfs_fsdata *fsd;
+
+	simple_unlink(d_inode(parent), dentry);
+	d_delete(dentry);
+	fsd = dentry->d_fsdata;
+	init_completion(&fsd->active_users_drained);
+	if (!refcount_dec_and_test(&fsd->active_users))
+		wait_for_completion(&fsd->active_users_drained);
+}
+
 static int __debugfs_remove(struct dentry *dentry, struct dentry *parent)
 {
 	int ret = 0;
 
 	if (simple_positive(dentry)) {
 		dget(dentry);
-		if (d_is_dir(dentry))
-			ret = simple_rmdir(d_inode(parent), dentry);
-		else
-			simple_unlink(d_inode(parent), dentry);
-		if (!ret)
-			d_delete(dentry);
+		if (!d_is_reg(dentry)) {
+			if (d_is_dir(dentry))
+				ret = simple_rmdir(d_inode(parent), dentry);
+			else
+				simple_unlink(d_inode(parent), dentry);
+			if (!ret)
+				d_delete(dentry);
+		} else {
+			__debugfs_remove_file(dentry, parent);
+		}
 		dput(dentry);
 	}
 	return ret;

fs/debugfs/internal.h

@@ -21,6 +21,8 @@ extern const struct file_operations debugfs_full_proxy_file_operations;
 
 struct debugfs_fsdata {
 	const struct file_operations *real_fops;
+	refcount_t active_users;
+	struct completion active_users_drained;
 };
 
 #endif /* _DEBUGFS_INTERNAL_H_ */

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)
 {