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124 lines
3.4 KiB
C
124 lines
3.4 KiB
C
/*
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* Copyright 2015 Denys Vlasenko
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*
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* Licensed under GPLv2, see file LICENSE in this source tree.
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*/
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//config:config UEVENT
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//config: bool "uevent (3.5 kb)"
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//config: default y
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//config: help
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//config: uevent is a netlink listener for kernel uevent notifications
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//config: sent via netlink. It is usually used for dynamic device creation.
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//applet:IF_UEVENT(APPLET(uevent, BB_DIR_SBIN, BB_SUID_DROP))
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//kbuild:lib-$(CONFIG_UEVENT) += uevent.o
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//usage:#define uevent_trivial_usage
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//usage: "[PROG ARGS]"
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//usage:#define uevent_full_usage "\n\n"
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//usage: "uevent runs PROG for every netlink notification."
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//usage: "\n""PROG's environment contains data passed from the kernel."
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//usage: "\n""Typical usage (daemon for dynamic device node creation):"
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//usage: "\n"" # uevent mdev & mdev -s"
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#include "libbb.h"
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#include "common_bufsiz.h"
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#include <linux/netlink.h>
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#define env ((char **)bb_common_bufsiz1)
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#define INIT_G() do { setup_common_bufsiz(); } while (0)
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enum {
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MAX_ENV = COMMON_BUFSIZE / sizeof(char*) - 1,
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// ^^^sizeof(env[0]) instead of sizeof(char*)
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// makes gcc-6.3.0 emit "strict-aliasing" warning.
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// socket receive buffer of 2MiB proved to be too small:
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// http://lists.busybox.net/pipermail/busybox/2019-December/087665.html
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// udevd seems to use a whooping 128MiB.
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// The socket receive buffer size is just a resource limit.
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// The buffers are allocated lazily so the memory is not wasted.
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KERN_RCVBUF = 128 * 1024 * 1024,
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// Might be made smaller: the kernel v5.4 passes up to 32 environment
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// variables with a total of 2kb on each event.
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// On top of that the action string and device path are added.
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USER_RCVBUF = 16 * 1024,
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};
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int uevent_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
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int uevent_main(int argc UNUSED_PARAM, char **argv)
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{
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int fd;
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INIT_G();
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argv++;
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// Subscribe for UEVENT kernel messages.
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// Without a sufficiently big RCVBUF, a ton of simultaneous events
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// can trigger ENOBUFS on read, which is unrecoverable.
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// Reproducer:
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// uevent mdev &
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// find /sys -name uevent -exec sh -c 'echo add >"{}"' ';'
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reopen:
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fd = create_and_bind_to_netlink(NETLINK_KOBJECT_UEVENT, /*groups:*/ 1 << 0, KERN_RCVBUF);
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for (;;) {
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char *netbuf;
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char *s, *end;
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ssize_t len;
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int idx;
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// In many cases, a system sits for *days* waiting
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// for a new uevent notification to come in.
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// We use a fresh mmap so that buffer is not allocated
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// until kernel actually starts filling it.
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netbuf = xmmap_anon(USER_RCVBUF);
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// Here we block, possibly for a very long time
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len = safe_read(fd, netbuf, USER_RCVBUF - 1);
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if (len < 0) {
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if (errno == ENOBUFS) {
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// Ran out of socket receive buffer
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bb_simple_error_msg("uevent overrun");
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close(fd);
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munmap(netbuf, USER_RCVBUF);
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goto reopen;
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}
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bb_simple_perror_msg_and_die("read");
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}
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end = netbuf + len;
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*end = '\0';
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// Each netlink message starts with "ACTION@/path"
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// (which we currently ignore),
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// followed by environment variables.
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if (!argv[0])
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putchar('\n');
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idx = 0;
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s = netbuf;
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while (s < end) {
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if (!argv[0])
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puts(s);
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if (strchr(s, '=') && idx < MAX_ENV)
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env[idx++] = s;
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s += strlen(s) + 1;
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}
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env[idx] = NULL;
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if (argv[0]) {
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idx = 0;
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while (env[idx])
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putenv(env[idx++]);
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spawn_and_wait(argv);
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idx = 0;
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while (env[idx])
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bb_unsetenv(env[idx++]);
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}
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munmap(netbuf, USER_RCVBUF);
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}
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return 0; // not reached
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}
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