Commit 9fd4dcec authored by Nicolai Stange's avatar Nicolai Stange Committed by Greg Kroah-Hartman
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debugfs: prevent access to possibly dead file_operations at file open

Nothing prevents a dentry found by path lookup before a return of
__debugfs_remove() to actually get opened after that return. Now, after
the return of __debugfs_remove(), there are no guarantees whatsoever
regarding the memory the corresponding inode's file_operations object
had been kept in.

Since __debugfs_remove() is seldomly invoked, usually from module exit
handlers only, the race is hard to trigger and the impact is very low.

A discussion of the problem outlined above as well as a suggested
solution can be found in the (sub-)thread rooted at

  ("Yet another pipe related oops.")

Basically, Greg KH suggests to introduce an intermediate fops and
Al Viro points out that a pointer to the original ones may be stored in

Follow this line of reasoning:
- Add SRCU as a reverse dependency of DEBUG_FS.
- Introduce a srcu_struct object for the debugfs subsystem.
- In debugfs_create_file(), store a pointer to the original
  file_operations object in ->d_fsdata.
- Make debugfs_remove() and debugfs_remove_recursive() wait for a
  SRCU grace period after the dentry has been delete()'d and before they
  return to their callers.
- Introduce an intermediate file_operations object named
  "debugfs_open_proxy_file_operations". It's ->open() functions checks,
  under the protection of a SRCU read lock, whether the dentry is still
  alive, i.e. has not been d_delete()'d and if so, tries to acquire a
  reference on the owning module.
  On success, it sets the file object's ->f_op to the original
  file_operations and forwards the ongoing open() call to the original
- For clarity, rename the former debugfs_file_operations to
  debugfs_noop_file_operations -- they are in no way canonical.

The choice of SRCU over "normal" RCU is justified by the fact, that the
former may also be used to protect ->i_private data from going away
during the execution of a file's readers and writers which may (and do)

Finally, introduce the fs/debugfs/internal.h header containing some
declarations internal to the debugfs implementation.
Signed-off-by: default avatarNicolai Stange <>
Signed-off-by: default avatarGreg Kroah-Hartman <>
parent 3a3a5fec
......@@ -22,6 +22,9 @@
#include <linux/slab.h>
#include <linux/atomic.h>
#include <linux/device.h>
#include <linux/srcu.h>
#include "internal.h"
static ssize_t default_read_file(struct file *file, char __user *buf,
size_t count, loff_t *ppos)
......@@ -35,13 +38,99 @@ static ssize_t default_write_file(struct file *file, const char __user *buf,
return count;
const struct file_operations debugfs_file_operations = {
const struct file_operations debugfs_noop_file_operations = {
.read = default_read_file,
.write = default_write_file,
.open = simple_open,
.llseek = noop_llseek,
* debugfs_use_file_start - mark the beginning of file data access
* @dentry: the dentry object whose data is being accessed.
* @srcu_idx: a pointer to some memory to store a SRCU index in.
* Up to a matching call to debugfs_use_file_finish(), 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_use_file_start() 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.
* Regardless of the return code, any call to
* debugfs_use_file_start() must be followed by a matching call
* to debugfs_use_file_finish().
static int debugfs_use_file_start(const struct dentry *dentry, int *srcu_idx)
*srcu_idx = srcu_read_lock(&debugfs_srcu);
if (d_unlinked(dentry))
return -EIO;
return 0;
* debugfs_use_file_finish - mark the end of file data access
* @srcu_idx: the SRCU index "created" by a former call to
* debugfs_use_file_start().
* Allow any ongoing concurrent call into debugfs_remove() or
* debugfs_remove_recursive() blocked by a former call to
* debugfs_use_file_start() to proceed and return to its caller.
static void debugfs_use_file_finish(int srcu_idx) __releases(&debugfs_srcu)
srcu_read_unlock(&debugfs_srcu, srcu_idx);
#define F_DENTRY(filp) ((filp)->f_path.dentry)
#define REAL_FOPS_DEREF(dentry) \
((const struct file_operations *)(dentry)->d_fsdata)
static int open_proxy_open(struct inode *inode, struct file *filp)
const struct dentry *dentry = F_DENTRY(filp);
const struct file_operations *real_fops = NULL;
int srcu_idx, r;
r = debugfs_use_file_start(dentry, &srcu_idx);
if (r) {
r = -ENOENT;
goto out;
real_fops = REAL_FOPS_DEREF(dentry);
real_fops = fops_get(real_fops);
if (!real_fops) {
/* Huh? Module did not clean up after itself at exit? */
WARN(1, "debugfs file owner did not clean up at exit: %pd",
r = -ENXIO;
goto out;
replace_fops(filp, real_fops);
if (real_fops->open)
r = real_fops->open(inode, filp);
return r;
const struct file_operations debugfs_open_proxy_file_operations = {
.open = open_proxy_open,
static struct dentry *debugfs_create_mode(const char *name, umode_t mode,
struct dentry *parent, void *value,
const struct file_operations *fops,
......@@ -27,9 +27,14 @@
#include <linux/parser.h>
#include <linux/magic.h>
#include <linux/slab.h>
#include <linux/srcu.h>
#include "internal.h"
static struct vfsmount *debugfs_mount;
static int debugfs_mount_count;
static bool debugfs_registered;
......@@ -341,8 +346,12 @@ struct dentry *debugfs_create_file(const char *name, umode_t mode,
return failed_creating(dentry);
inode->i_mode = mode;
inode->i_fop = fops ? fops : &debugfs_file_operations;
inode->i_private = data;
inode->i_fop = fops ? &debugfs_open_proxy_file_operations
: &debugfs_noop_file_operations;
dentry->d_fsdata = (void *)fops;
d_instantiate(dentry, inode);
fsnotify_create(d_inode(dentry->d_parent), dentry);
return end_creating(dentry);
......@@ -570,6 +579,7 @@ void debugfs_remove(struct dentry *dentry)
if (!ret)
simple_release_fs(&debugfs_mount, &debugfs_mount_count);
......@@ -647,6 +657,7 @@ void debugfs_remove_recursive(struct dentry *dentry)
if (!__debugfs_remove(child, parent))
simple_release_fs(&debugfs_mount, &debugfs_mount_count);
* internal.h - declarations internal to debugfs
* Copyright (C) 2016 Nicolai Stange <>
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License version
* 2 as published by the Free Software Foundation.
struct file_operations;
struct srcu_struct;
/* declared over in file.c */
extern const struct file_operations debugfs_noop_file_operations;
extern const struct file_operations debugfs_open_proxy_file_operations;
extern struct srcu_struct debugfs_srcu;
#endif /* _DEBUGFS_INTERNAL_H_ */
......@@ -43,9 +43,6 @@ extern struct dentry *arch_debugfs_dir;
#if defined(CONFIG_DEBUG_FS)
/* declared over in file.c */
extern const struct file_operations debugfs_file_operations;
struct dentry *debugfs_create_file(const char *name, umode_t mode,
struct dentry *parent, void *data,
const struct file_operations *fops);
......@@ -257,6 +257,7 @@ config PAGE_OWNER
config DEBUG_FS
bool "Debug Filesystem"
select SRCU
debugfs is a virtual file system that kernel developers use to put
debugging files into. Enable this option to be able to read and
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