Re: [PATCH 2/6] vfs: flush and wait for io when setting the immutable flag via SETFLAGS

[Jan Kara]

On Mon 10-06-19 21:46:25, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> 
> When we're using FS_IOC_SETFLAGS to set the immutable flag on a file, we
> need to ensure that userspace can't continue to write the file after the
> file becomes immutable.  To make that happen, we have to flush all the
> dirty pagecache pages to disk to ensure that we can fail a page fault on
> a mmap'd region, wait for pending directio to complete, and hope the
> caller locked out any new writes by holding the inode lock.
> 
> Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>

...

> diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
> index 6aa1df1918f7..a05341b94d98 100644
> --- a/fs/ext4/ioctl.c
> +++ b/fs/ext4/ioctl.c
> @@ -290,6 +290,9 @@ static int ext4_ioctl_setflags(struct inode *inode,
>  	jflag = flags & EXT4_JOURNAL_DATA_FL;
>  
>  	err = vfs_ioc_setflags_check(inode, oldflags, flags);
> +	if (err)
> +		goto flags_out;
> +	err = vfs_ioc_setflags_flush_data(inode, flags);
>  	if (err)
>  		goto flags_out;
>  

...

> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 8dad3c80b611..9c899c63957e 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -3548,7 +3548,41 @@ static inline struct sock *io_uring_get_socket(struct file *file)
>  
>  int vfs_ioc_setflags_check(struct inode *inode, int oldflags, int flags);
>  
> +/*
> + * Do we need to flush the file data before changing attributes?  When we're
> + * setting the immutable flag we must stop all directio writes and flush the
> + * dirty pages so that we can fail the page fault on the next write attempt.
> + */
> +static inline bool vfs_ioc_setflags_need_flush(struct inode *inode, int flags)
> +{
> +	if (S_ISREG(inode->i_mode) && !IS_IMMUTABLE(inode) &&
> +	    (flags & FS_IMMUTABLE_FL))
> +		return true;
> +
> +	return false;
> +}
> +
> +/*
> + * Flush file data before changing attributes.  Caller must hold any locks
> + * required to prevent further writes to this file until we're done setting
> + * flags.
> + */
> +static inline int inode_flush_data(struct inode *inode)
> +{
> +	inode_dio_wait(inode);
> +	return filemap_write_and_wait(inode->i_mapping);
> +}
> +
> +/* Flush file data before changing attributes, if necessary. */
> +static inline int vfs_ioc_setflags_flush_data(struct inode *inode, int flags)
> +{
> +	if (vfs_ioc_setflags_need_flush(inode, flags))
> +		return inode_flush_data(inode);
> +	return 0;
> +}
> +

But this is racy at least for page faults, isn't it? What protects you
against write faults just after filemap_write_and_wait() has finished?
So either you need to set FS_IMMUTABLE_FL before flushing data or you need
to get more protection from the fs than just i_rwsem. In the case of ext4
that would be i_mmap_rwsem but other filesystems don't have equivalent
protection...

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR