util-linux/uevent.c

   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 (3.1 kb)"
   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(char*) - 1,
  36         /* sizeof(env[0]) instead of sizeof(char*)
  37          * makes gcc-6.3.0 emit "strict-aliasing" warning.
  38          */
  39 };
  40
  41 #ifndef SO_RCVBUFFORCE
  42 #define SO_RCVBUFFORCE 33
  43 #endif
  44 enum { RCVBUF = 2 * 1024 * 1024 };
  45
  46 int uevent_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
  47 int uevent_main(int argc UNUSED_PARAM, char **argv)
  48 {
  49         int fd;
  50
  51         INIT_G();
  52
  53         argv++;
  54
  55         // Subscribe for UEVENT kernel messages.
  56         // Without a sufficiently big RCVBUF, a ton of simultaneous events
  57         // can trigger ENOBUFS on read, which is unrecoverable.
  58         // Reproducer:
  59         //      uevent mdev &
  60         //      find /sys -name uevent -exec sh -c 'echo add >"{}"' ';'
  61         fd = create_and_bind_to_netlink(NETLINK_KOBJECT_UEVENT, /*groups:*/ 1 << 0, RCVBUF);
  62
  63         for (;;) {
  64                 char *netbuf;
  65                 char *s, *end;
  66                 ssize_t len;
  67                 int idx;
  68
  69                 // In many cases, a system sits for *days* waiting
  70                 // for a new uevent notification to come in.
  71                 // We use a fresh mmap so that buffer is not allocated
  72                 // until kernel actually starts filling it.
  73                 netbuf = mmap(NULL, BUFFER_SIZE,
  74                                         PROT_READ | PROT_WRITE,
  75                                         MAP_PRIVATE | MAP_ANON,
  76                                         /* ignored: */ -1, 0);
  77                 if (netbuf == MAP_FAILED)
  78                         bb_simple_perror_msg_and_die("mmap");
  79
  80                 // Here we block, possibly for a very long time
  81                 len = safe_read(fd, netbuf, BUFFER_SIZE - 1);
  82                 if (len < 0)
  83                         bb_simple_perror_msg_and_die("read");
  84                 end = netbuf + len;
  85                 *end = '\0';
  86
  87                 // Each netlink message starts with "ACTION@/path"
  88                 // (which we currently ignore),
  89                 // followed by environment variables.
  90                 if (!argv[0])
  91                         putchar('\n');
  92                 idx = 0;
  93                 s = netbuf;
  94                 while (s < end) {
  95                         if (!argv[0])
  96                                 puts(s);
  97                         if (strchr(s, '=') && idx < MAX_ENV)
  98                                 env[idx++] = s;
  99                         s += strlen(s) + 1;
 100                 }
 101                 env[idx] = NULL;
 102
 103                 if (argv[0]) {
 104                         idx = 0;
 105                         while (env[idx])
 106                                 putenv(env[idx++]);
 107                         spawn_and_wait(argv);
 108                         idx = 0;
 109                         while (env[idx])
 110                                 bb_unsetenv(env[idx++]);
 111                 }
 112                 munmap(netbuf, BUFFER_SIZE);
 113         }
 114
 115         return 0; // not reached
 116 }