Re: [RFC PATCH V2 09/12] fs: Prevent mode change if file is mmap'ed

[Darrick J. Wong]

On Fri, Jan 10, 2020 at 11:29:39AM -0800, ira.weiny@xxxxxxxxx wrote:
> From: Ira Weiny <ira.weiny@xxxxxxxxx>
> 
> Page faults need to ensure the inode mode is correct and consistent with
> the vmf information at the time of the fault.  There is no easy way to
> ensure the vmf information is correct if a mode change is in progress.
> Furthermore, there is no good use case to require a mode change while
> the file is mmap'ed.
> 
> Track mmap's of the file and fail the mode change if the file is
> mmap'ed.
> 
> Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> ---
>  fs/inode.c         |  2 ++
>  fs/xfs/xfs_ioctl.c |  8 ++++++++
>  include/linux/fs.h |  1 +
>  mm/mmap.c          | 19 +++++++++++++++++--
>  4 files changed, 28 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/inode.c b/fs/inode.c
> index 2b0f51161918..944711aed6f8 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -245,6 +245,8 @@ static struct inode *alloc_inode(struct super_block *sb)
>  		return NULL;
>  	}
>  
> +	atomic64_set(&inode->i_mapped, 0);
> +
>  	return inode;
>  }
>  
> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> index bc3654fe3b5d..1ab0906c6c7f 100644
> --- a/fs/xfs/xfs_ioctl.c
> +++ b/fs/xfs/xfs_ioctl.c
> @@ -1200,6 +1200,14 @@ xfs_ioctl_setattr_dax_invalidate(
>  		goto out_unlock;
>  	}
>  
> +	/*
> +	 * If there is a mapping in place we must remain in our current mode.
> +	 */
> +	if (atomic64_read(&inode->i_mapped)) {

Urk, should we really be messing around with the address space
internals?

> +		error = -EBUSY;
> +		goto out_unlock;
> +	}
> +
>  	error = filemap_write_and_wait(inode->i_mapping);
>  	if (error)
>  		goto out_unlock;
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 631f11d6246e..6e7dc626b657 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -740,6 +740,7 @@ struct inode {
>  #endif
>  
>  	void			*i_private; /* fs or device private pointer */
> +	atomic64_t               i_mapped;

I would have expected to find this in struct address_space since the
mapping count is a function of the address space, right?

--D

>  } __randomize_layout;
>  
>  struct timespec64 timestamp_truncate(struct timespec64 t, struct inode *inode);
> diff --git a/mm/mmap.c b/mm/mmap.c
> index dfaf1130e706..e6b68924b7ca 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -171,12 +171,17 @@ void unlink_file_vma(struct vm_area_struct *vma)
>  static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
>  {
>  	struct vm_area_struct *next = vma->vm_next;
> +	struct file *f = vma->vm_file;
>  
>  	might_sleep();
>  	if (vma->vm_ops && vma->vm_ops->close)
>  		vma->vm_ops->close(vma);
> -	if (vma->vm_file)
> -		fput(vma->vm_file);
> +	if (f) {
> +		struct inode *inode = file_inode(f);
> +		if (inode)
> +			atomic64_dec(&inode->i_mapped);
> +		fput(f);
> +	}
>  	mpol_put(vma_policy(vma));
>  	vm_area_free(vma);
>  	return next;
> @@ -1837,6 +1842,16 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  
>  	vma_set_page_prot(vma);
>  
> +	/*
> +	 * Track if there is mapping in place such that a mode change
> +	 * does not occur on a file which is mapped
> +	 */
> +	if (file) {
> +		struct inode		*inode = file_inode(file);
> +
> +		atomic64_inc(&inode->i_mapped);
> +	}
> +
>  	return addr;
>  
>  unmap_and_free_vma:
> -- 
> 2.21.0
>