From e9117a5a4bf65d8e99f060d356a04d27a60b436d Mon Sep 17 00:00:00 2001 From: Nicolai Stange Date: Tue, 31 Oct 2017 00:15:48 +0100 Subject: 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 Signed-off-by: Nicolai Stange Signed-off-by: Greg Kroah-Hartman --- include/linux/debugfs.h | 11 +++++++++++ 1 file changed, 11 insertions(+) (limited to 'include/linux/debugfs.h') 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) { -- cgit v1.2.3