[user-cr][PATCH 1/2] restart: remount /proc for new tasks created with CLONE_NEWPID

[Oren Laadan]

Not doing this can be a pain for restarted software which relies
on /proc...

This builds on a patch by Serge Hallyn, but also aims to address the
future cases of hierarchical pid-ns:

1) Before mounting the new /proc, first umount the old one, which
isn't necessary anymore.

2) Perform the unshare() together with the remount of /proc, so it
will occur for every new pid-ns and not only for the first one.

Signed-off-by: Oren Laadan <orenl at cs.columbia.edu>
---
 restart.c |   32 ++++++++++++++++++++++++++++++++
 1 files changed, 32 insertions(+), 0 deletions(-)

diff --git a/restart.c b/restart.c
index f3d33de..f42b456 100644
--- a/restart.c
+++ b/restart.c
@@ -30,6 +30,7 @@
 #include <asm/unistd.h>
 #include <sys/syscall.h>
 #include <sys/prctl.h>
+#include <sys/mount.h>
 
 #include <linux/sched.h>
 #include <linux/checkpoint.h>
@@ -273,6 +274,8 @@ int global_child_collected;
 int global_send_sigint = -1;
 int global_sent_sigint;
 
+static int ckpt_remount_proc(void);
+
 static int ckpt_build_tree(struct ckpt_ctx *ctx);
 static int ckpt_init_tree(struct ckpt_ctx *ctx);
 static int ckpt_set_creator(struct ckpt_ctx *ctx, struct task *task);
@@ -981,11 +984,36 @@ static int ckpt_probe_child(pid_t pid, char *str)
 	return 0;
 }
 
+/*
+ * Remount the /proc with a new instance: tasks that start a new
+ * pid-ns need a fresh mount of /proc to reflect their pid-ns.
+ */
+static int ckpt_remount_proc(void)
+{
+	if (unshare(CLONE_NEWNS | CLONE_FS) < 0) {
+		perror("unshare");
+		return -1;
+	}
+	if (umount2("/proc", MNT_DETACH) < 0) {
+		perror("umount -l /proc");
+		return -1;
+	}
+	if (mount("proc", "/proc", "proc", 0, NULL) < 0) {
+		perror("mount -t proc");
+		return -1;
+	}
+
+	return 0;
+}
+
 #ifdef CLONE_NEWPID
 static int __ckpt_coordinator(void *arg)
 {
 	struct ckpt_ctx *ctx = (struct ckpt_ctx *) arg;
 
+	if (ckpt_remount_proc() < 0)
+		return -1;
+
 	if (!ctx->args->wait)
 		close(ctx->pipe_coord[0]);
 
@@ -1850,6 +1878,10 @@ int ckpt_fork_stub(void *data)
 	struct task *task = (struct task *) data;
 	struct ckpt_ctx *ctx = task->ctx;
 
+	/* tasks with new pid-ns need new /proc mount */
+	if ((task->flags & TASK_NEWPID) && ckpt_remount_proc() < 0)
+		return -1;
+
 	/*
 	 * In restart into a new pid namespace (--pidns), coordinator
 	 * is the container init, hence if it terminated permatutely
-- 
1.6.3.3