[PATCH v2] ovl: fix null pointer when filesystem doesn't support direct IO

[Chengguang Xu]

在 2021/9/22 11:47, Huang Jianan 写道:
> At present, overlayfs provides overlayfs inode to users. Overlayfs
> inode provides ovl_aops with noop_direct_IO to avoid open failure
> with O_DIRECT. But some compressed filesystems, such as erofs and
> squashfs, don't support direct_IO.
>
> Users who use f_mapping->a_ops->direct_IO to check O_DIRECT support,
> will read file through this way. This will cause overlayfs to access
> a non-existent direct_IO function and cause panic due to null pointer:
>
> Kernel panic - not syncing: CFI failure (target: 0x0)
> CPU: 6 PID: 247 Comm: loop0
> Call Trace:
>   panic+0x188/0x45c
>   __cfi_slowpath+0x0/0x254
>   __cfi_slowpath+0x200/0x254
>   generic_file_read_iter+0x14c/0x150
>   vfs_iocb_iter_read+0xac/0x164
>   ovl_read_iter+0x13c/0x2fc
>   lo_rw_aio+0x2bc/0x458
>   loop_queue_work+0x4a4/0xbc0
>   kthread_worker_fn+0xf8/0x1d0
>   loop_kthread_worker_fn+0x24/0x38
>   kthread+0x29c/0x310
>   ret_from_fork+0x10/0x30
>
> The filesystem may only support direct_IO for some file types. For
> example, erofs supports direct_IO for uncompressed files. So reset
> f_mapping->a_ops to NULL when the file doesn't support direct_IO to
> fix this problem.
>
> Fixes: 5b910bd615ba ("ovl: fix GPF in swapfile_activate of file from overlayfs over xfs")
> Signed-off-by: Huang Jianan <huangjianan at oppo.com>
> ---
> Change since v1:
>   - Return error to user rather than fall back to buffered io. (Chengguang Xu)
>
>   fs/overlayfs/file.c | 4 ++++
>   1 file changed, 4 insertions(+)
>
> diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> index d081faa55e83..38118d3b46f8 100644
> --- a/fs/overlayfs/file.c
> +++ b/fs/overlayfs/file.c
> @@ -157,6 +157,10 @@ static int ovl_open(struct inode *inode, struct file *file)
>   	if (IS_ERR(realfile))
>   		return PTR_ERR(realfile);
>   
> +	if ((f->f_flags & O_DIRECT) && (!realfile->f_mapping->a_ops ||
> +		!realfile->f_mapping->a_ops->direct_IO))
> +		file->f_mapping->a_ops = NULL;


There are many other functions in a_ops and also address_space struct 
will be shared

between files which belong to same inode. Although overlayfs currently 
only defines

->direct_IO in a_ops, it will be extended in the future. (like 
containerized sycnfs [1])


It seems the simplest solution is directly return error to upper layer.


Thanks,

Chengguang


[1] https://www.spinics.net/lists/linux-unionfs/msg08569.html



> +
>   	file->private_data = realfile;
>   
>   	return 0;