Commit 387e3746 authored by Amir Goldstein's avatar Amir Goldstein Committed by Jeff Layton
Browse files

locks: eliminate false positive conflicts for write lease 



check_conflicting_open() is checking for existing fd's open for read or
for write before allowing to take a write lease.  The check that was
implemented using i_count and d_count is an approximation that has
several false positives.  For example, overlayfs since v4.19, takes an
extra reference on the dentry; An open with O_PATH takes a reference on
the dentry although the file cannot be read nor written.

Change the implementation to use i_readcount and i_writecount to
eliminate the false positive conflicts and allow a write lease to be
taken on an overlayfs file.

The change of behavior with existing fd's open with O_PATH is symmetric
w.r.t. current behavior of lease breakers - an open with O_PATH currently
does not break a write lease.

This increases the size of struct inode by 4 bytes on 32bit archs when
CONFIG_FILE_LOCKING is defined and CONFIG_IMA was not already
defined.

Signed-off-by: default avatarAmir Goldstein <amir73il@gmail.com>
Signed-off-by: default avatarJeff Layton <jlayton@kernel.org>
parent d51f527f

fs/locks.c

+27 −15
Original line number Diff line number Diff line
@@ -1763,10 +1763,10 @@ int fcntl_getlease(struct file *filp) 
}



/**

 * check_conflicting_open - see if the given dentry points to a file that has

 * check_conflicting_open - see if the given file points to an inode that has

 *			    an existing open that would conflict with the

 *			    desired lease.

 * @dentry:	dentry to check

 * @filp:	file to check

 * @arg:	type of lease that we're trying to acquire

 * @flags:	current lock flags

 *
@@ -1774,30 +1774,42 @@ int fcntl_getlease(struct file *filp) 
 * conflict with the lease we're trying to set.

 */

static int

check_conflicting_open(const struct dentry *dentry, const long arg, int flags)

check_conflicting_open(struct file *filp, const long arg, int flags)

{

	int ret = 0;

	struct inode *inode = dentry->d_inode;

	struct inode *inode = locks_inode(filp);

	int self_wcount = 0, self_rcount = 0;



	if (flags & FL_LAYOUT)

		return 0;



	if ((arg == F_RDLCK) && inode_is_open_for_write(inode))

		return -EAGAIN;

	if (arg == F_RDLCK)

		return inode_is_open_for_write(inode) ? -EAGAIN : 0;

	else if (arg != F_WRLCK)

		return 0;



	if ((arg == F_WRLCK) && ((d_count(dentry) > 1) ||

	    (atomic_read(&inode->i_count) > 1)))

		ret = -EAGAIN;

	/*

	 * Make sure that only read/write count is from lease requestor.

	 * Note that this will result in denying write leases when i_writecount

	 * is negative, which is what we want.  (We shouldn't grant write leases

	 * on files open for execution.)

	 */

	if (filp->f_mode & FMODE_WRITE)

		self_wcount = 1;

	else if (filp->f_mode & FMODE_READ)

		self_rcount = 1;



	if (atomic_read(&inode->i_writecount) != self_wcount ||

	    atomic_read(&inode->i_readcount) != self_rcount)

		return -EAGAIN;



	return ret;

	return 0;

}



static int

generic_add_lease(struct file *filp, long arg, struct file_lock **flp, void **priv)

{

	struct file_lock *fl, *my_fl = NULL, *lease;

	struct dentry *dentry = filp->f_path.dentry;

	struct inode *inode = dentry->d_inode;

	struct inode *inode = locks_inode(filp);

	struct file_lock_context *ctx;

	bool is_deleg = (*flp)->fl_flags & FL_DELEG;

	int error;
@@ -1832,7 +1844,7 @@ generic_add_lease(struct file *filp, long arg, struct file_lock **flp, void **pr 
	percpu_down_read(&file_rwsem);

	spin_lock(&ctx->flc_lock);

	time_out_leases(inode, &dispose);

	error = check_conflicting_open(dentry, arg, lease->fl_flags);

	error = check_conflicting_open(filp, arg, lease->fl_flags);

	if (error)

		goto out;
@@ -1889,7 +1901,7 @@ generic_add_lease(struct file *filp, long arg, struct file_lock **flp, void **pr 
	 * precedes these checks.

	 */

	smp_mb();

	error = check_conflicting_open(dentry, arg, lease->fl_flags);

	error = check_conflicting_open(filp, arg, lease->fl_flags);

	if (error) {

		locks_unlink_lock_ctx(lease);

		goto out;

include/linux/fs.h

+2 −2
Original line number Diff line number Diff line
@@ -694,7 +694,7 @@ struct inode { 
	atomic_t		i_count;

	atomic_t		i_dio_count;

	atomic_t		i_writecount;

#ifdef CONFIG_IMA

#if defined(CONFIG_IMA) || defined(CONFIG_FILE_LOCKING)

	atomic_t		i_readcount; /* struct files open RO */

#endif

	union {
@@ -2890,7 +2890,7 @@ static inline bool inode_is_open_for_write(const struct inode *inode) 
	return atomic_read(&inode->i_writecount) > 0;

}



#ifdef CONFIG_IMA

#if defined(CONFIG_IMA) || defined(CONFIG_FILE_LOCKING)

static inline void i_readcount_dec(struct inode *inode)

{

	BUG_ON(!atomic_read(&inode->i_readcount));

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