restore: use breakpoints instead of tracing syscalls

Currently CRIU traces syscalls to catch a moment, when sigreturn() is
called. Now we trace recv(cmd), close(logfd), close(cmdfd), sigreturn().

We can reduce a number of steps by using hw breakpoints. A breakpoint is
set before sigreturn, so we will need to trace only it.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

cr-restore.c

@@ -1546,7 +1546,7 @@ static int attach_to_tasks(bool root_seized)
 
 	for_each_pstree_item(item) {
 		pid_t pid = item->pid.real;
-		int status, i;
+		int status, i, ret;
 
 		if (!task_alive(item))
 			continue;
@@ -1579,7 +1579,12 @@ static int attach_to_tasks(bool root_seized)
 				return -1;
 			}
 
-			if (ptrace(PTRACE_SYSCALL, pid, NULL, NULL)) {
+			ret = ptrace_set_breakpoint(pid, item->rst->breakpoint);
+			if (ret < 0)
+				return -1;
+
+			/* A breakpoint was not set */
+			if (ret == 0 && ptrace(PTRACE_SYSCALL, pid, NULL, NULL)) {
 				pr_perror("Unable to start %d", pid);
 				return -1;
 			}
@@ -2691,6 +2696,7 @@ static int sigreturn_restore(pid_t pid, CoreEntry *core)
 	if (rst_mem_remap(mem))
 		goto err;
 
+	task_args->breakpoint = &current->rst->breakpoint;
 	task_args->task_entries = rst_mem_remap_ptr(task_entries_pos, RM_SHREMAP);
 
 	task_args->rst_mem = mem;

include/restorer.h

@@ -158,6 +158,7 @@ struct task_restore_args {
 	struct vdso_symtable		vdso_sym_rt;		/* runtime vdso symbols */
 	unsigned long			vdso_rt_parked_at;	/* safe place to keep vdso */
 #endif
+	void				**breakpoint;
 } __aligned(64);
 
 #define RESTORE_ALIGN_STACK(start, size)	\

include/rst_info.h

@@ -54,6 +54,8 @@ struct rst_info {
 	struct file_desc	*root;
 	bool			has_umask;
 	u32			umask;
+
+	void			*breakpoint;
 };
 
 #endif /* __CR_RST_INFO_H__ */

pie/restorer.c

@@ -330,6 +330,11 @@ static int restore_thread_common(struct rt_sigframe *sigframe,
 	return 0;
 }
 
+static void noinline rst_sigreturn(unsigned long new_sp)
+{
+	ARCH_RT_SIGRETURN(new_sp);
+}
+
 /*
  * Threads restoration via sigreturn. Note it's locked
  * routine and calls for unlock at the end.
@@ -381,7 +386,7 @@ long __export_restore_thread(struct thread_restore_args *args)
 	futex_dec_and_wake(&thread_inprogress);
 
 	new_sp = (long)rt_sigframe + SIGFRAME_OFFSET;
-	ARCH_RT_SIGRETURN(new_sp);
+	rst_sigreturn(new_sp);
 
 core_restore_end:
 	pr_err("Restorer abnormal termination for %ld\n", sys_getpid());
@@ -764,6 +769,7 @@ long __export_restore_task(struct task_restore_args *args)
 	task_entries = args->task_entries;
 	helpers = args->helpers;
 	n_helpers = args->n_helpers;
+	*args->breakpoint = rst_sigreturn;
 
 	ksigfillset(&act.rt_sa_mask);
 	act.rt_sa_handler = sigchld_handler;
@@ -1130,7 +1136,7 @@ long __export_restore_task(struct task_restore_args *args)
 	 * pure assembly since we don't need any additional
 	 * code insns from gcc.
 	 */
-	ARCH_RT_SIGRETURN(new_sp);
+	rst_sigreturn(new_sp);
 
 core_restore_end:
 	futex_abort_and_wake(&task_entries->nr_in_progress);