1// SPDX-License-Identifier: GPL-2.0
2/*
3 * cgroup_event_listener.c - Simple listener of cgroup events
4 *
5 * Copyright (C) Kirill A. Shutemov <kirill@shutemov.name>
6 */
7
8#include <assert.h>
9#include <err.h>
10#include <errno.h>
11#include <fcntl.h>
12#include <libgen.h>
13#include <limits.h>
14#include <stdio.h>
15#include <string.h>
16#include <unistd.h>
17
18#include <sys/eventfd.h>
19
20#define USAGE_STR "Usage: cgroup_event_listener <path-to-control-file> <args>"
21
22int main(int argc, char **argv)
23{
24	int efd = -1;
25	int cfd = -1;
26	int event_control = -1;
27	char event_control_path[PATH_MAX];
28	char line[LINE_MAX];
29	int ret;
30
31	if (argc != 3)
32		errx(1, "%s", USAGE_STR);
33
34	cfd = open(argv[1], O_RDONLY);
35	if (cfd == -1)
36		err(1, "Cannot open %s", argv[1]);
37
38	ret = snprintf(event_control_path, PATH_MAX, "%s/cgroup.event_control",
39			dirname(argv[1]));
40	if (ret >= PATH_MAX)
41		errx(1, "Path to cgroup.event_control is too long");
42
43	event_control = open(event_control_path, O_WRONLY);
44	if (event_control == -1)
45		err(1, "Cannot open %s", event_control_path);
46
47	efd = eventfd(0, 0);
48	if (efd == -1)
49		err(1, "eventfd() failed");
50
51	ret = snprintf(line, LINE_MAX, "%d %d %s", efd, cfd, argv[2]);
52	if (ret >= LINE_MAX)
53		errx(1, "Arguments string is too long");
54
55	ret = write(event_control, line, strlen(line) + 1);
56	if (ret == -1)
57		err(1, "Cannot write to cgroup.event_control");
58
59	while (1) {
60		uint64_t result;
61
62		ret = read(efd, &result, sizeof(result));
63		if (ret == -1) {
64			if (errno == EINTR)
65				continue;
66			err(1, "Cannot read from eventfd");
67		}
68		assert(ret == sizeof(result));
69
70		ret = access(event_control_path, W_OK);
71		if ((ret == -1) && (errno == ENOENT)) {
72			puts("The cgroup seems to have removed.");
73			break;
74		}
75
76		if (ret == -1)
77			err(1, "cgroup.event_control is not accessible any more");
78
79		printf("%s %s: crossed\n", argv[1], argv[2]);
80	}
81
82	return 0;
83}
84