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authorNicolai Stange <nicstange@gmail.com>2017-10-31 02:15:48 +0300
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2017-11-07 22:25:02 +0300
commite9117a5a4bf65d8e99f060d356a04d27a60b436d (patch)
treebd7ad76ed4c2f538f13ab38afb0dda9478ad044e /fs/debugfs/file.c
parent7c8d469877b16d2c1cecf101a0abb7b218db85bc (diff)
downloadlinux-e9117a5a4bf65d8e99f060d356a04d27a60b436d.tar.xz
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>
Diffstat (limited to 'fs/debugfs/file.c')
-rw-r--r--fs/debugfs/file.c48
1 files changed, 48 insertions, 0 deletions
diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index b6f5ddab66bf..6644bfdea2f8 100644
--- a/fs/debugfs/file.c
+++ b/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);