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authorAmir Goldstein2019-06-07 17:24:38 +0300
committerJeff Layton2019-06-19 08:49:38 -0400
commit387e3746d01c34457d6a73688acd90428725070b (patch)
tree575796428aac6a85321113d53a75abe9cfdccbcf
parentd51f527f44f96276a94c191bc160de051f64aeea (diff)
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>
-rw-r--r--fs/locks.c42
-rw-r--r--include/linux/fs.h4
2 files changed, 29 insertions, 17 deletions
diff --git a/fs/locks.c b/fs/locks.c
index 0cc2b9f30e22..de87a3231789 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -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;
+
+ /*
+ * 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 ((arg == F_WRLCK) && ((d_count(dentry) > 1) ||
- (atomic_read(&inode->i_count) > 1)))
- ret = -EAGAIN;
+ 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;
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));