[PATCH 4/5] erofs-utils: mount: gracefully exit when `erofsmount_nbd()` encounts an error

[Gao Xiang]

On 2025/12/23 18:04, Yifan Zhao wrote:
> If the main process of `erofsmount_nbd()` encounters an error after the
> nbd device has been successfully set up, it fails to disconnect it
> before exiting, resulting in the subprocess not being cleaned up and
> keeping its connection with NBD device.
> 
> This patch resolves the issue by disconnecting NBD device before exiting
> on error.
> 
> Signed-off-by: Yifan Zhao <zhaoyifan28 at huawei.com>
> ---
> Note:
> - I believe directly killing the child process is unsafe, as it may leave
> in-flight NBD requests from the kernel unhandled, causing soft lockup.
> - And I believe using nbdpath here is safe, as the child process maintains
> the NBD device connection throughout, preventing concurrent access by other
> actors.

What if the child process is already exited earlier, and the current NBD
device is reused for others?

How about keeping the previous nbdfd for ioctl interfaces instead to
avoid nbd device reuse.

Thanks,
Gao Xiang

> 
>   mount/main.c | 17 +++++++++++++++--
>   1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/mount/main.c b/mount/main.c
> index 2a21979..d2d4815 100644
> --- a/mount/main.c
> +++ b/mount/main.c
> @@ -1287,10 +1287,23 @@ static int erofsmount_nbd(struct erofs_nbd_source *source,
>   
>   	if (!err) {
>   		err = mount(nbdpath, mountpoint, fstype, flags, options);
> -		if (err < 0)
> +		if (err < 0) {
>   			err = -errno;
> +			if (msg.is_netlink) {
> +				erofs_nbd_nl_disconnect(msg.nbdnum);
> +			} else {
> +				int nbdfd;
> +
> +				nbdfd = open(nbdpath, O_RDWR);
> +				if (nbdfd > 0) {
> +					erofs_nbd_disconnect(nbdfd);
> +					close(nbdfd);
> +				}
> +			}
> +			return err;
> +		}
>   
> -		if (!err && msg.is_netlink) {
> +		if (msg.is_netlink) {
>   			id = erofs_nbd_get_identifier(msg.nbdnum);
>   
>   			err = IS_ERR(id) ? PTR_ERR(id) :