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Commit 9d70524d authored by Nicolai Stange's avatar Nicolai Stange Committed by Raghavendra Rao Ananta
Browse files

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.

Change-Id: Icfde769b8604854a2e6f91d1070ac72dc2a8f7fc
Fixes: 49d200de ("debugfs: prevent access to removed files' private data")
Reported-by: default avatarJohannes Berg <johannes@sipsolutions.net>
Signed-off-by: default avatarNicolai Stange <nicstange@gmail.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Git-commit: e9117a5a4bf65d8e99f060d356a04d27a60b436d
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: default avatarRaghavendra Rao Ananta <rananta@codeaurora.org>
parent ed5be30b

fs/debugfs/file.c

+48 −0
Original line number Diff line number Diff line
@@ -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

+23 −6
Original line number Diff line number Diff line
@@ -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_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

+2 −0
Original line number Diff line number Diff line
@@ -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

+11 −0
Original line number Diff line number Diff line
@@ -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)

{