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

["J . Bruce Fields" <bfields@fieldses.org>]

How do opens for execute work?  I guess they create a struct file with
FMODE_EXEC and FMODE_RDONLY set and they decrement i_writecount.  Do
they also increment i_readcount?  Reading do_open_execat and alloc_file,
looks like it does, so, good, they should conflict with write leases,
which sounds right.

Looks good to me.--b.

On Wed, Jun 12, 2019 at 08:24:08PM +0300, Amir Goldstein wrote:
> 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