rst: Open sys.ns_last_pid before diving into restorer

We restore chroot before doing this, so if we might need to
open one, we may have no access to the /proc/... paths.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

cr-restore.c

@@ -2545,6 +2545,16 @@ static int sigreturn_restore(pid_t pid, CoreEntry *core)
 	if (ret < 0)
 		goto err;
 
+	/*
+	 * Open the last_pid syscl early, since restorer (maybe) lives
+	 * in chroot and has no access to "/proc/..." paths.
+	 */
+	task_args->fd_last_pid = open(LAST_PID_PATH, O_RDWR);
+	if (task_args->fd_last_pid < 0) {
+		pr_perror("Can't open sys.ns_last_pid");
+		goto err;
+	}
+
 	/*
 	 * Now prepare run-time data for threads restore.
 	 */

include/crtools.h

@@ -92,7 +92,6 @@ int open_pages_image_at(int dfd, unsigned long flags, int pm_fd);
 void up_page_ids_base(void);
 
 #define LAST_PID_PATH		"/proc/sys/kernel/ns_last_pid"
-#define LAST_PID_PERM		0666
 
 struct cr_fdset {
 	int fd_off;

include/restorer.h

@@ -151,6 +151,8 @@ struct task_restore_core_args {
 	struct rst_tcp_sock		*tcp_socks;
 	int				tcp_socks_nr;
 
+	int				fd_last_pid; /* sys.ns_last_pid for threads rst */
+
 	struct vdso_symtable		vdso_sym_rt;		/* runtime vdso symbols */
 	unsigned long			vdso_rt_parked_at;	/* safe place to keep vdso */
 } __aligned(sizeof(long));

pie/restorer.c

@@ -810,12 +810,7 @@ long __export_restore_task(struct task_restore_core_args *args)
 		long parent_tid;
 		int i, fd;
 
-		fd = sys_open(LAST_PID_PATH, O_RDWR, LAST_PID_PERM);
-		if (fd < 0) {
-			pr_err("Can't open last_pid %d\n", fd);
-			goto core_restore_end;
-		}
-
+		fd = args->fd_last_pid;
 		ret = sys_flock(fd, LOCK_EX);
 		if (ret) {
 			pr_err("Can't lock last_pid %d\n", fd);
@@ -856,9 +851,10 @@ long __export_restore_task(struct task_restore_core_args *args)
 			goto core_restore_end;
 		}
 
-		sys_close(fd);
 	}
 
+	sys_close(args->fd_last_pid);
+
 	restore_rlims(args);
 
 	pr_info("%ld: Restored\n", sys_getpid());