path: root/util-linux/uevent.c (plain)
blob: c22216a8a11717913980c3755646b3a5b62c06fa

1	/*
2	* Copyright 2015 Denys Vlasenko
3	*
4	* Licensed under GPLv2, see file LICENSE in this source tree.
5	*/
6	//config:config UEVENT
7	//config: bool "uevent"
8	//config: default y
9	//config: select PLATFORM_LINUX
10	//config: help
11	//config: uevent is a netlink listener for kernel uevent notifications
12	//config: sent via netlink. It is usually used for dynamic device creation.
13
14	//applet:IF_UEVENT(APPLET(uevent, BB_DIR_SBIN, BB_SUID_DROP))
15
16	//kbuild:lib-$(CONFIG_UEVENT) += uevent.o
17
18	//usage:#define uevent_trivial_usage
19	//usage: "[PROG [ARGS]]"
20	//usage:#define uevent_full_usage "\n\n"
21	//usage: "uevent runs PROG for every netlink notification."
22	//usage: "\n""PROG's environment contains data passed from the kernel."
23	//usage: "\n""Typical usage (daemon for dynamic device node creation):"
24	//usage: "\n"" # uevent mdev & mdev -s"
25
26	#include "libbb.h"
27	#include "common_bufsiz.h"
28	#include <linux/netlink.h>
29
30	#define BUFFER_SIZE 16*1024
31
32	#define env ((char **)bb_common_bufsiz1)
33	#define INIT_G() do { setup_common_bufsiz(); } while (0)
34	enum {
35	MAX_ENV = COMMON_BUFSIZE / sizeof(env[0]) - 1,
36	};
37
38	#ifndef SO_RCVBUFFORCE
39	#define SO_RCVBUFFORCE 33
40	#endif
41	enum { RCVBUF = 2 * 1024 * 1024 };
42
43	int uevent_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
44	int uevent_main(int argc UNUSED_PARAM, char **argv)
45	{
46	struct sockaddr_nl sa;
47	int fd;
48
49	INIT_G();
50
51	argv++;
52
53	// Subscribe for UEVENT kernel messages
54	sa.nl_family = AF_NETLINK;
55	sa.nl_pad = 0;
56	sa.nl_pid = getpid();
57	sa.nl_groups = 1 << 0;
58	fd = xsocket(AF_NETLINK, SOCK_DGRAM, NETLINK_KOBJECT_UEVENT);
59	xbind(fd, (struct sockaddr *) &sa, sizeof(sa));
60	close_on_exec_on(fd);
61
62	// Without a sufficiently big RCVBUF, a ton of simultaneous events
63	// can trigger ENOBUFS on read, which is unrecoverable.
64	// Reproducer:
65	// uevent mdev &
66	// find /sys -name uevent -exec sh -c 'echo add >"{}"' ';'
67	//
68	// SO_RCVBUFFORCE (root only) can go above net.core.rmem_max sysctl
69	setsockopt_SOL_SOCKET_int(fd, SO_RCVBUF, RCVBUF);
70	setsockopt_SOL_SOCKET_int(fd, SO_RCVBUFFORCE, RCVBUF);
71	if (0) {
72	int z;
73	socklen_t zl = sizeof(z);
74	getsockopt(fd, SOL_SOCKET, SO_RCVBUF, &z, &zl);
75	bb_error_msg("SO_RCVBUF:%d", z);
76	}
77
78	for (;;) {
79	char *netbuf;
80	char *s, *end;
81	ssize_t len;
82	int idx;
83
84	// In many cases, a system sits for *days* waiting
85	// for a new uevent notification to come in.
86	// We use a fresh mmap so that buffer is not allocated
87	// until kernel actually starts filling it.
88	netbuf = mmap(NULL, BUFFER_SIZE,
89	PROT_READ | PROT_WRITE,
90	MAP_PRIVATE | MAP_ANON,
91	/* ignored: */ -1, 0);
92	if (netbuf == MAP_FAILED)
93	bb_perror_msg_and_die("mmap");
94
95	// Here we block, possibly for a very long time
96	len = safe_read(fd, netbuf, BUFFER_SIZE - 1);
97	if (len < 0)
98	bb_perror_msg_and_die("read");
99	end = netbuf + len;
100	*end = '\0';
101
102	// Each netlink message starts with "ACTION@/path"
103	// (which we currently ignore),
104	// followed by environment variables.
105	if (!argv[0])
106	putchar('\n');
107	idx = 0;
108	s = netbuf;
109	while (s < end) {
110	if (!argv[0])
111	puts(s);
112	if (strchr(s, '=') && idx < MAX_ENV)
113	env[idx++] = s;
114	s += strlen(s) + 1;
115	}
116	env[idx] = NULL;
117
118	idx = 0;
119	while (env[idx])
120	putenv(env[idx++]);
121	if (argv[0])
122	spawn_and_wait(argv);
123	idx = 0;
124	while (env[idx])
125	bb_unsetenv(env[idx++]);
126	munmap(netbuf, BUFFER_SIZE);
127	}
128
129	return 0; // not reached
130	}
131