path: root/fs/debugfs/file.c

// SPDX-License-Identifier: GPL-2.0
/*
 *  file.c - part of debugfs, a tiny little debug file system
 *
 *  Copyright (C) 2004 Greg Kroah-Hartman <greg@kroah.com>
 *  Copyright (C) 2004 IBM Inc.
 *
 *  debugfs is for people to use instead of /proc or /sys.
 *  See Documentation/filesystems/ for more details.
 */

#include <linux/module.h>
#include <linux/fs.h>
#include <linux/seq_file.h>
#include <linux/pagemap.h>
#include <linux/debugfs.h>
#include <linux/io.h>
#include <linux/slab.h>
#include <linux/atomic.h>
#include <linux/device.h>
#include <linux/pm_runtime.h>
#include <linux/poll.h>
#include <linux/security.h>

#include "internal.h"

struct poll_table_struct;

static ssize_t default_read_file(struct file *file, char __user *buf,
				 size_t count, loff_t *ppos)
{
	return 0;
}

static ssize_t default_write_file(struct file *file, const char __user *buf,
				   size_t count, loff_t *ppos)
{
	return count;
}

const struct file_operations debugfs_noop_file_operations = {
	.read =		default_read_file,
	.write =	default_write_file,
	.open =		simple_open,
	.llseek =	noop_llseek,
};

#define F_DENTRY(filp) ((filp)->f_path.dentry)

const struct file_operations *debugfs_real_fops(const struct file *filp)
{
	struct debugfs_fsdata *fsd = F_DENTRY(filp)->d_fsdata;

	if ((unsigned long)fsd & DEBUGFS_FSDATA_IS_REAL_FOPS_BIT) {
		/*
		 * Urgh, we've been called w/o a protecting
		 * debugfs_file_get().
		 */
		WARN_ON(1);
		return NULL;
	}

	return fsd->real_fops;
}
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;
	void *d_fsd;

	d_fsd = READ_ONCE(dentry->d_fsdata);
	if (!((unsigned long)d_fsd & DEBUGFS_FSDATA_IS_REAL_FOPS_BIT)) {
		fsd = d_fsd;
	} else {
		fsd = kmalloc(sizeof(*fsd), GFP_KERNEL);
		if (!fsd)
			return -ENOMEM;

		fsd->real_fops = (void *)((unsigned long)d_fsd &
					~DEBUGFS_FSDATA_IS_REAL_FOPS_BIT);
		refcount_set(&fsd->active_users, 1);
		init_completion(&fsd->active_users_drained);
		if (cmpxchg(&dentry->d_fsdata, d_fsd, fsd) != d_fsd) {
			kfree(fsd);
			fsd = READ_ONCE(dentry->d_fsdata);
		}
	}

	/*
	 * In case of a successful cmpxchg() above, this check is
	 * strictly necessary and must follow it, see the comment in
	 * __debugfs_remove_file().
	 * OTOH, if the cmpxchg() hasn't been executed or wasn't
	 * successful, this serves the purpose of not starving
	 * 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 = READ_ONCE(dentry->d_fsdata);

	if (refcount_dec_and_test(&fsd->active_users))
		complete(&fsd->active_users_drained);
}
EXPORT_SYMBOL_GPL(debugfs_file_put);

/*
 * Only permit access to world-readable files when the kernel is locked down.
 * We also need to exclude any file that has ways to write or alter it as root
 * can bypass the permissions check.
 */
static int debugfs_locked_down(struct