[CRIU] [PATCH] Add inherit fd support

[Andrew Vagin]

On Thu, Dec 04, 2014 at 01:29:53PM +0300, Andrew Vagin wrote:
> On Wed, Dec 03, 2014 at 01:04:18PM -0800, Saied Kazemi wrote:
> > There are cases where a process's file descriptor cannot be restored
> > from the checkpointed image.  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 would be broken and removed.
> > 
> > There are also cases where the user wants to use a new file during
> > restore instead of the original file in the checkpointed image.  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 it 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 descriptor to use
> > for restoring 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 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>
> ...
> > @@ -302,6 +314,20 @@ static int open_pipe(struct file_desc *d)
> >  
> >  	pi = container_of(d, struct pipe_info, d);
> >  
> > +	/*
> > +	 * If the pipe is in the inherit fd list,
> > +	 * it should be inherited rather than created.
> > +	 */
> > +	pipe_name = pipe_id_string(pi->pe->pipe_id);
> > +	if (pipe_name == NULL)
> > +		return -1;
> > +	tmp = inherit_fd_lookup_id(pipe_name);
> > +	if (tmp >= 0) {
> > +		pr_info("Pipe %s will be restored from inherit fd %d\n",
> > +			pipe_name, tmp);
> > +		return tmp;
> 
> A process can have more than one descriptor associated with a pipe. This
> code doesn't handle this case.
> 
> I think inherit_fd_lookup_id must return dup(inh->inh_fd).

Actually Pavel already said about this problem.

> 
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