[CRIU] [PATCH 2/2] remap: add a dead pid /proc remap

[Tycho Andersen]

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.

Signed-off-by: Tycho Andersen <tycho.andersen at canonical.com>
---
 files-reg.c                    | 73 +++++++++++++++++++++++++++++++++++++++++-
 include/fs-magic.h             |  4 +++
 protobuf/remap-file-path.proto |  5 +++
 3 files changed, 81 insertions(+), 1 deletion(-)

diff --git a/files-reg.c b/files-reg.c
index 0ffce0e..eadd15f 100644
--- a/files-reg.c
+++ b/files-reg.c
@@ -24,6 +24,7 @@
 #include "asm/atomic.h"
 #include "namespaces.h"
 #include "proc_parse.h"
+#include "pstree.h"
 
 #include "protobuf.h"
 #include "protobuf/regfile.pb-c.h"
@@ -227,6 +228,28 @@ static int open_remap_linked(struct reg_file_info *rfi,
 	return 0;
 }
 
+static int open_remap_dead_process(struct reg_file_info *rfi,
+		RemapFilePathEntry *rfe)
+{
+	struct pstree_item *helper;
+
+	helper = alloc_pstree_item_with_rst();
+	if (!helper)
+		return -1;
+
+	helper->sid = root_item->sid;
+	helper->pgid = root_item->sid;
+	helper->pid.virt = rfe->remap_id;
+	helper->state = TASK_HELPER;
+	helper->parent = root_item;
+	list_add_tail(&helper->sibling, &root_item->children);
+	task_entries->nr_helpers++;
+
+	pr_info("Added a helper for restoring /proc/%d\n", helper->pid.virt);
+
+	return 0;
+}
+
 static int collect_one_remap(void *obj, ProtobufCMessage *msg)
 {
 	int ret = -1;
@@ -248,8 +271,10 @@ static int collect_one_remap(void *obj, ProtobufCMessage *msg)
 
 	if (rfe->remap_id & REMAP_GHOST)
 		ret = open_remap_ghost(rfi, rfe);
-	else
+	else if (!rfe->has_remap_type)
 		ret = open_remap_linked(rfi, rfe);
+	else if (rfe->remap_type == REMAP_TYPE__PROCFS)
+		ret = open_remap_dead_process(rfi, rfe);
 out:
 	return ret;
 }
@@ -498,6 +523,20 @@ static int dump_linked_remap(char *path, int len, const struct stat *ost,
 			&rpe, PB_REMAP_FPATH);
 }
 
+static int dump_dead_process_remap(pid_t pid, char *path, int len, const struct stat *ost,
+				int lfd, u32 id, struct ns_id *nsid)
+{
+	RemapFilePathEntry rpe = REMAP_FILE_PATH_ENTRY__INIT;
+
+	rpe.orig_id = id;
+	rpe.remap_id = pid;
+	rpe.has_remap_type = true;
+	rpe.remap_type = REMAP_TYPE__PROCFS;
+
+	return pb_write_one(fdset_fd(glob_fdset, CR_FD_REMAP_FPATH),
+			&rpe, PB_REMAP_FPATH);
+}
+
 static bool is_sillyrename_name(char *name)
 {
 	int i;
@@ -539,6 +578,38 @@ static int check_path_remap(char *rpath, int plen, const struct fd_parms *parms,
 	struct stat pst;
 	const struct stat *ost = &parms->stat;
 
+	if (parms->fs_type == PROC_SUPER_MAGIC) {
+		/* 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. */
+		if (*end == '/') {
+			*end = 0;
+			ret = access(rpath, F_OK);
+			*end = '/';
+
+			if (ret) {
+				pr_info("Dumping dead process remap of %d\n", pid);
+				return dump_dead_process_remap(pid, rpath + 1, plen - 1, ost, lfd, id, nsid);
+			}
+		}
+	}
+
 	if (ost->st_nlink == 0)
 		/*
 		 * Unpleasant, but easy case. File is completely invisible
diff --git a/include/fs-magic.h b/include/fs-magic.h
index 13c4961..777d736 100644
--- a/include/fs-magic.h
+++ b/include/fs-magic.h
@@ -41,4 +41,8 @@
 #define AUFS_SUPER_MAGIC	0x61756673
 #endif
 
+#ifndef PROC_SUPER_MAGIC
+#define PROC_SUPER_MAGIC	0x9fa0
+#endif
+
 #endif /* __CR_FS_MAGIC_H__ */
diff --git a/protobuf/remap-file-path.proto b/protobuf/remap-file-path.proto
index faeb745..294cdec 100644
--- a/protobuf/remap-file-path.proto
+++ b/protobuf/remap-file-path.proto
@@ -1,4 +1,9 @@
+enum remap_type {
+	PROCFS			= 0;
+};
+
 message remap_file_path_entry {
 	required uint32		orig_id		= 1;
 	required uint32		remap_id	= 2;
+	optional uint32		remap_type	= 3;
 }
-- 
1.9.1