restorer: Don't memcpy restorer blob in each task on restore

Instead -- mmap it once in root task and then mremap it later.
No mremap of original restorer can be done, since in that case
the restorer vma would be tied to crtools binary which in turn
will make set-exe-file prctl to fail with EBUSY.

Note -- after mremap the original vmas list becomes non relevant,
but it's OK. Only new holes appear inside which is OK for munmap.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

cr-restore.c

@@ -62,6 +62,7 @@ static struct pstree_item *current;
 
 static int restore_task_with_children(void *);
 static int sigreturn_restore(pid_t pid, CoreEntry *core, struct list_head *vmas, int nr_vmas);
+static int prepare_restorer_blob(void);
 
 static int shmem_remap(void *old_addr, void *new_addr, unsigned long size)
 {
@@ -153,6 +154,10 @@ static int prepare_shared(void)
 	if (ret)
 		goto err;
 
+	ret = prepare_restorer_blob();
+	if (ret)
+		goto err;
+
 	show_saved_shmems();
 	show_saved_files();
 err:
@@ -1189,9 +1194,47 @@ out:
 	return ret;
 }
 
+static void *restorer;
+static unsigned long restorer_len;
+
+static int prepare_restorer_blob(void)
+{
+	/*
+	 * We map anonymous mapping, not mremap the restorer itself later.
+	 * Otherwise the resoter vma would be tied to crtools binary which 
+	 * in turn will lead to set-exe-file prctl to fail with EBUSY.
+	 */
+
+	restorer_len = round_up(sizeof(restorer_blob), PAGE_SIZE);
+	restorer = mmap(NULL, restorer_len,
+			PROT_READ | PROT_WRITE | PROT_EXEC,
+			MAP_PRIVATE | MAP_ANON, 0, 0);
+	if (restorer == MAP_FAILED) {
+		pr_err("Can't map restorer code");
+		return -1;
+	}
+
+	memcpy(restorer, &restorer_blob, sizeof(restorer_blob));
+	return 0;
+}
+
+static int remap_restorer_blob(void *addr)
+{
+	void *mem;
+
+	mem = mremap(restorer, restorer_len, restorer_len,
+			MREMAP_FIXED | MREMAP_MAYMOVE, addr);
+	if (mem != addr) {
+		pr_perror("Can't remap restorer blob");
+		return -1;
+	}
+
+	return 0;
+}
+
 static int sigreturn_restore(pid_t pid, CoreEntry *core, struct list_head *tgt_vmas, int nr_vmas)
 {
-	long restore_code_len, restore_task_vma_len;
+	long restore_task_vma_len;
 	long restore_thread_vma_len, self_vmas_len, vmas_len;
 
 	void *mem = MAP_FAILED;
@@ -1211,7 +1254,6 @@ static int sigreturn_restore(pid_t pid, CoreEntry *core, struct list_head *tgt_v
 
 	pr_info("Restore via sigreturn\n");
 
-	restore_code_len	= 0;
 	restore_task_vma_len	= 0;
 	restore_thread_vma_len	= 0;
 
@@ -1239,9 +1281,6 @@ static int sigreturn_restore(pid_t pid, CoreEntry *core, struct list_head *tgt_v
 		goto err;
 	}
 
-	restore_code_len	= sizeof(restorer_blob);
-	restore_code_len	= round_up(restore_code_len, PAGE_SIZE);
-
 	restore_task_vma_len   = round_up(sizeof(*task_args), PAGE_SIZE);
 	restore_thread_vma_len = round_up(sizeof(*thread_args) * current->nr_threads, PAGE_SIZE);
 
@@ -1252,7 +1291,7 @@ static int sigreturn_restore(pid_t pid, CoreEntry *core, struct list_head *tgt_v
 	restore_thread_vma_len = round_up(restore_thread_vma_len, PAGE_SIZE);
 
 	exec_mem_hint = restorer_get_vma_hint(pid, tgt_vmas, &self_vma_list,
-					      restore_code_len +
+					      restorer_len +
 					      restore_task_vma_len +
 					      restore_thread_vma_len +
 					      self_vmas_len + vmas_len +
@@ -1266,24 +1305,19 @@ static int sigreturn_restore(pid_t pid, CoreEntry *core, struct list_head *tgt_v
 	pr_info("Found bootstrap VMA hint at: 0x%lx (needs ~%ldK)\n", exec_mem_hint,
 			KBYTES(restore_task_vma_len + restore_thread_vma_len));
 
-	mem = mmap((void *)exec_mem_hint, restore_code_len,
-			PROT_READ | PROT_WRITE | PROT_EXEC,
-			MAP_PRIVATE | MAP_ANON | MAP_FIXED, 0, 0);
-	if (mem != (void *)exec_mem_hint) {
-		pr_err("Can't map restorer code");
+	ret = remap_restorer_blob((void *)exec_mem_hint);
+	if (ret < 0)
 		goto err;
-	}
-	memcpy(mem, &restorer_blob, sizeof(restorer_blob));
 
 	/*
 	 * Prepare a memory map for restorer. Note a thread space
 	 * might be completely unused so it's here just for convenience.
 	 */
-	restore_code_start		= mem;
+	restore_code_start		= (void *)exec_mem_hint;
 	restore_thread_exec_start	= restore_code_start + restorer_blob_offset____export_restore_thread;
 	restore_task_exec_start		= restore_code_start + restorer_blob_offset____export_restore_task;
 
-	exec_mem_hint += restore_code_len;
+	exec_mem_hint += restorer_len;
 
 	/* VMA we need to run task_restore code */
 	mem = mmap((void *)exec_mem_hint,