[PATCH v2] locks: eliminate false positive conflicts for write lease

[Amir Goldstein <amir73il@gmail.com>]

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.

Cc: <stable@vger.kernel.org> # v4.19
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---

Miklos, Jeff and Bruce,

This patch fixes a v4.19 overlayfs regression with taking write
leases. It also provides correct semantics w.r.t RDONLY open counter
that Bruce also needed for nfsd.

Since this is locks code that fixes an overlayfs regression which
is also needed for nfsd, it could go via either of your trees.
I didn't want to pick sides, so first one to grab the patch wins ;-)

I verified the changes using modified LTP F_SETLEASE tests [1],
which I ran over xfs and overlayfs.

Thanks,
Amir.

[1] https://github.com/amir73il/ltp/commits/overlayfs-devel

Changes since v1:
- Drop patch to fold i_readcount into i_count
- Make i_readcount depend on CONFIG_FILE_LOCKING

 fs/locks.c         | 33 ++++++++++++++++++++++-----------
 include/linux/fs.h |  4 ++--
 2 files changed, 24 insertions(+), 13 deletions(-)

diff --git a/fs/locks.c b/fs/locks.c
index ec1e4a5df629..28528b4fc53b 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -1753,10 +1753,10 @@ 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
  *
@@ -1764,19 +1764,31 @@ 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 wcount = atomic_read(&inode->i_writecount);
+	int self_wcount = 0, self_rcount = 0;
 
 	if (flags & FL_LAYOUT)
 		return 0;
 
-	if ((arg == F_RDLCK) && inode_is_open_for_write(inode))
+	if (arg == F_RDLCK && wcount > 0)
 		return -EAGAIN;
 
-	if ((arg == F_WRLCK) && ((d_count(dentry) > 1) ||
-	    (atomic_read(&inode->i_count) > 1)))
+	/* Eliminate deny writes from actual writers count */
+	if (wcount < 0)
+		wcount = 0;
+
+	/* Make sure that only read/write count is from lease requestor */
+	if (filp->f_mode & FMODE_WRITE)
+		self_wcount = 1;
+	else if ((filp->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
+		self_rcount = 1;
+
+	if (arg == F_WRLCK && (wcount != self_wcount ||
+	    atomic_read(&inode->i_readcount) != self_rcount))
 		ret = -EAGAIN;
 
 	return ret;
@@ -1786,8 +1798,7 @@ 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;
@@ -1822,7 +1833,7 @@ 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;
 
@@ -1879,7 +1890,7 @@ 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;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 79ffa2958bd8..2d55f1b64014 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -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 {
@@ -2895,7 +2895,7 @@ 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));
-- 
2.17.1