Commit e9117a5a authored by Nicolai Stange's avatar Nicolai Stange Committed by Greg Kroah-Hartman

debugfs: implement per-file removal protection

Since commit 49d200de ("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: 49d200de ("debugfs: prevent access to removed files' private data")
Reported-by: default avatarJohannes Berg <>
Signed-off-by: default avatarNicolai Stange <>
Signed-off-by: default avatarGreg Kroah-Hartman <>
parent 7c8d4698
......@@ -109,6 +109,54 @@ const struct file_operations *debugfs_real_fops(const struct file *filp)
* 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;
* 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))
static int open_proxy_open(struct inode *inode, struct file *filp)
const struct dentry *dentry = F_DENTRY(filp);
......@@ -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,
static void __debugfs_remove_file(struct dentry *dentry, struct dentry *parent)
struct debugfs_fsdata *fsd;
simple_unlink(d_inode(parent), dentry);
fsd = dentry->d_fsdata;
if (!refcount_dec_and_test(&fsd->active_users))
static int __debugfs_remove(struct dentry *dentry, struct dentry *parent)
int ret = 0;
if (simple_positive(dentry)) {
if (d_is_dir(dentry))
ret = simple_rmdir(d_inode(parent), dentry);
simple_unlink(d_inode(parent), dentry);
if (!ret)
if (!d_is_reg(dentry)) {
if (d_is_dir(dentry))
ret = simple_rmdir(d_inode(parent), dentry);
simple_unlink(d_inode(parent), dentry);
if (!ret)
} else {
__debugfs_remove_file(dentry, parent);
return ret;
......@@ -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_ */
......@@ -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)
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)
{ }
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)
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