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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>
alistair/sunxi64-5.4-dsi
Amir Goldstein 2019-06-07 17:24:38 +03:00 committed by Jeff Layton
parent d51f527f44
commit 387e3746d0
2 changed files with 29 additions and 17 deletions
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42
fs/locks.c
View File

@ -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))
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 (atomic_read(&inode->i_writecount) != self_wcount ||
atomic_read(&inode->i_readcount) != self_rcount)
return -EAGAIN;
if ((arg == F_WRLCK) && ((d_count(dentry) > 1) ||
(atomic_read(&inode->i_count) > 1)))
ret = -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;

4
include/linux/fs.h
View File

@ -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));