[CRIU] [PATCH] remap: add a dead pid /proc remap

[Pavel Emelyanov]

On 09/04/2014 01:13 AM, Tycho Andersen wrote:
> From: Tycho Andersen <tycho at tycho.ws>
> 
> If a file like /proc/20/mountinfo is open, but 20 is a zombie (or doesn't exist
> any more), we can't read this file at all, so a link remap won't work. Instead,
> we add a new remap, called the dead process remap, which forks a TASK_HELPER as
> that dead pid so that the restore task can open the new /proc/20/mountinfo
> instead.
> 
> Previously, TASK_HELPERs died before the restore stage took place. However, the
> fds are set up during the restore stage, so we need TASK_HELPERs to live at
> least until then. This patch also rearranges some of the synchronization to
> keep TASK_HELPERs alive until the restore stage ends.

Can you fix the helpers lifetime with separate patch, please?
And a couple of more comments inline.


> @@ -539,6 +562,38 @@ static int check_path_remap(char *rpath, int plen, const struct fd_parms *parms,
>  	struct stat pst;
>  	const struct stat *ost = &parms->stat;
>  
> +	if (is_path_prefix(rpath, "./proc")) {

I would suggest checking that the file is procfs's one by looking at the
fd_parms.fs_type field.

> +		/* The file points to /proc/pid/<foo> where pid is a dead
> +		 * process. We remap this file by adding this pid to be
> +		 * fork()ed into a TASK_HELPER state so that we can point to it
> +		 * on restore.
> +		 */
> +		pid_t pid;
> +		char *start, *end;
> +
> +		/* skip "./proc/" */
> +		start = strstr(rpath, "/") + 1;
> +		if (!start)
> +			return -1;
> +		start = strstr(start, "/") + 1;
> +		if (!start)
> +			return -1;
> +		pid = strtol(start, &end, 10);
> +
> +		/* if we didn't find another /, this path something
> +		 * like ./proc/kmsg, which we shouldn't mess with. */

...

>  static inline int shared_fdtable(struct pstree_item *item) {
> diff --git a/protobuf/pstree.proto b/protobuf/pstree.proto
> index 6cbcfd3..6cc3953 100644
> --- a/protobuf/pstree.proto
> +++ b/protobuf/pstree.proto
> @@ -4,4 +4,5 @@ message pstree_entry {
>  	required uint32			pgid		= 3;
>  	required uint32			sid		= 4;
>  	repeated uint32			threads		= 5;
> +	repeated uint32			dead_pids	= 6;

Can we have the "dead" tasks encoded in remap-fpath image instead of
the pstree one? I.e. -- the remap_id should be the pid of the process
we need and there should be new optional remap_type field set to
PROCFS constant.

>  }

Thanks,
Pavel