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/*
* Copyright 2015 Denys Vlasenko
*
* Licensed under GPLv2, see file LICENSE in this source tree.
*/
//config:config UEVENT
//config: bool "uevent (3.1 kb)"
//config: default y
//config: help
//config: uevent is a netlink listener for kernel uevent notifications
//config: sent via netlink. It is usually used for dynamic device creation.
//applet:IF_UEVENT(APPLET(uevent, BB_DIR_SBIN, BB_SUID_DROP))
//kbuild:lib-$(CONFIG_UEVENT) += uevent.o
//usage:#define uevent_trivial_usage
//usage: "[PROG [ARGS]]"
//usage:#define uevent_full_usage "\n\n"
//usage: "uevent runs PROG for every netlink notification."
//usage: "\n""PROG's environment contains data passed from the kernel."
//usage: "\n""Typical usage (daemon for dynamic device node creation):"
//usage: "\n"" # uevent mdev & mdev -s"
#include "libbb.h"
#include "common_bufsiz.h"
#include <linux/netlink.h>
#define env ((char **)bb_common_bufsiz1)
#define INIT_G() do { setup_common_bufsiz(); } while (0)
enum {
MAX_ENV = COMMON_BUFSIZE / sizeof(char*) - 1,
// ^^^sizeof(env[0]) instead of sizeof(char*)
// makes gcc-6.3.0 emit "strict-aliasing" warning.
// socket receive buffer of 2MiB proved to be too small:
// http://lists.busybox.net/pipermail/busybox/2019-December/087665.html
// udevd seems to use a whooping 128MiB.
// The socket receive buffer size is just a resource limit.
// The buffers are allocated lazily so the memory is not wasted.
KERN_RCVBUF = 128 * 1024 * 1024,
// Might be made smaller: the kernel v5.4 passes up to 32 environment
// variables with a total of 2kb on each event.
// On top of that the action string and device path are added.
USER_RCVBUF = 16 * 1024,
};
int uevent_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
int uevent_main(int argc UNUSED_PARAM, char **argv)
{
int fd;
INIT_G();
argv++;
// Subscribe for UEVENT kernel messages.
// Without a sufficiently big RCVBUF, a ton of simultaneous events
// can trigger ENOBUFS on read, which is unrecoverable.
// Reproducer:
// uevent mdev &
// find /sys -name uevent -exec sh -c 'echo add >"{}"' ';'
reopen:
fd = create_and_bind_to_netlink(NETLINK_KOBJECT_UEVENT, /*groups:*/ 1 << 0, KERN_RCVBUF);
for (;;) {
char *netbuf;
char *s, *end;
ssize_t len;
int idx;
// In many cases, a system sits for *days* waiting
// for a new uevent notification to come in.
// We use a fresh mmap so that buffer is not allocated
// until kernel actually starts filling it.
netbuf = xmmap_anon(USER_RCVBUF);
// Here we block, possibly for a very long time
len = safe_read(fd, netbuf, USER_RCVBUF - 1);
if (len < 0) {
if (errno == ENOBUFS) {
// Ran out of socket receive buffer
bb_simple_error_msg("uevent overrun");
close(fd);
munmap(netbuf, USER_RCVBUF);
goto reopen;
}
bb_simple_perror_msg_and_die("read");
}
end = netbuf + len;
*end = '\0';
// Each netlink message starts with "ACTION@/path"
// (which we currently ignore),
// followed by environment variables.
if (!argv[0])
putchar('\n');
idx = 0;
s = netbuf;
while (s < end) {
if (!argv[0])
puts(s);
if (strchr(s, '=') && idx < MAX_ENV)
env[idx++] = s;
s += strlen(s) + 1;
}
env[idx] = NULL;
if (argv[0]) {
idx = 0;
while (env[idx])
putenv(env[idx++]);
spawn_and_wait(argv);
idx = 0;
while (env[idx])
bb_unsetenv(env[idx++]);
}
munmap(netbuf, USER_RCVBUF);
}
return 0; // not reached
}
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