[CRIU] [PATCH v2] Add inherit fd support

[Pavel Emelyanov]

On 12/09/2014 11:20 PM, Saied Kazemi wrote:
> There are cases where a process's file descriptor cannot be restored
> from the checkpoint images.  For example, a pipe file descriptor with
> one end in the checkpointed process and the other end in a separate
> process (that was not part of the checkpointed process tree) cannot be
> restored because after checkpoint the pipe will be broken.
> 
> There are also cases where the user wants to use a new file during
> restore instead of the original file at checkpoint time.  For example,
> the user wants to change the log file of a process from /path/to/oldlog
> to /path/to/newlog.
> 
> In these cases, criu's caller should set up a new file descriptor to be
> inherited by the restored process and specify the file descriptor with the
> --inherit-fd command line option.  The argument of --inherit-fd has the
> format fd[%d]:%s, where %d tells criu which of its own file descriptors
> to use for restoring the file identified by %s.
> 
> As a debugging aid, if the argument has the format debug[%d]:%s, it tells
> criu to write out the string after colon to the file descriptor %d.  This
> can be used, for example, as an easy way to leave a "restore marker"
> in the output stream of the process.
> 
> It's important to note that inherit fd support breaks applications
> that depend on the state of the file descriptor being inherited.  So,
> consider inherit fd only for specific use cases that you know for sure
> won't break the application.
> 
> For examples please visit http://criu.org/Category:HOWTO.
> 
> Signed-off-by: Saied Kazemi <saied at google.com>

Patch applied, thanks a lot! I have only one question, would you clarify
it please:


> +int inherit_fd_fini()
> +{
> +	int reused;
> +	struct inherit_fd *inh;
> +
> +	list_for_each_entry(inh, &opts.inherit_fds, inh_list) {
> +		if (inh->inh_fd < 0) {
> +			pr_err("File %s in inherit fd list has invalid fd %d\n",
> +				inh->inh_id, inh->inh_fd);
> +			return -1;
> +		}
> +
> +		reused = inherit_fd_reused(inh);

How can the fd become reused? I've found no places where the fd sitting
in the inh object may become such.

> +		if (reused < 0)
> +			return -1;
> +
> +		if (!reused) {
> +			pr_debug("Closing inherit fd %d -> %s\n", inh->inh_fd,
> +				inh->inh_id);
> +			if (close_safe(&inh->inh_fd) < 0)
> +				return -1;
> +		}
> +	}
> +	return 0;
> +}