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: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index f7fdfe93e25d..419e327022de 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 {
@@ -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));