3293bc1469
Fixes:
commit 52a515d187
"udhcp: use poll() instead of select()"
Feb 16 2017
udhcp_sp_read() is meant to check whether signal pipe indeed has some data to read.
In the above commit, it was changed as follows:
- if (!FD_ISSET(signal_pipe.rd, rfds))
+ if (!pfds[0].revents)
return 0;
The problem is, the check was working for select() purely by accident.
Caught signal interrupts select()/poll() syscalls, they return with EINTR
(regardless of SA_RESTART flag in sigaction). _Then_ signal handler is invoked.
IOW: they can't see any changes to fd state caused by signal haldler
(in our case, signal handler makes signal pipe ready to be read).
For select(), it means that rfds[] bit array is unmodified, bit of signal
pipe's read fd is still set, and the above check "works": it thinks select()
says there is data to read.
This accident does not work for poll(): .revents stays clear, and we do not
try reading signal pipe as we should. In udhcpd, we fall through and block
in socket read. Further SIGTERM signals simply cause socket read to be
interrupted and then restarted (since SIGTERM handler has SA_RESTART=1).
Fixing this as follows: remove the check altogether. Set signal pipe read fd
to nonblocking mode. Always read it in udhcp_sp_read().
If read fails, assume it's EAGAIN and return 0 ("no signal seen").
udhcpd avoids reading signal pipe on every recvd packet by looping if EINTR
(using safe_poll()) - thus ensuring we have correct .revents for all fds -
and calling udhcp_sp_read() only if pfds[0].revents!=0.
udhcpc performs much fewer reads (typically it sleeps >99.999% of the time),
there is no need to optimize it: can call udhcp_sp_read() after each poll
unconditionally.
To robustify socket reads, unconditionally set pfds[1].revents=0
in udhcp_sp_fd_set() (which is before poll), and check it before reading
network socket in udhcpd.
TODO:
This might still fail: if pfds[1].revents=POLLIN, socket read may still block.
There are rare cases when select/poll indicates that data can be read,
but then actual read still blocks (one such case is UDP packets with
wrong checksum). General advise is, if you use a poll/select loop,
keep all your fds nonblocking.
Maybe we should also do that to our network sockets?
function old new delta
udhcp_sp_setup 55 65 +10
udhcp_sp_fd_set 54 60 +6
udhcp_sp_read 46 36 -10
udhcpd_main 1451 1437 -14
udhcpc_main 2723 2708 -15
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/3 up/down: 16/-39) Total: -23 bytes
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
82 lines
2.3 KiB
C
82 lines
2.3 KiB
C
/* vi: set sw=4 ts=4: */
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/*
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* Signal pipe infrastructure. A reliable way of delivering signals.
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*
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* Russ Dill <Russ.Dill@asu.edu> December 2003
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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*/
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#include "common.h"
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/* Global variable: we access it from signal handler */
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static struct fd_pair signal_pipe;
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static void signal_handler(int sig)
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{
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int sv = errno;
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unsigned char ch = sig; /* use char, avoid dealing with partial writes */
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if (write(signal_pipe.wr, &ch, 1) != 1)
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bb_perror_msg("can't send signal");
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errno = sv;
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}
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/* Call this before doing anything else. Sets up the socket pair
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* and installs the signal handler */
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void FAST_FUNC udhcp_sp_setup(void)
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{
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/* was socketpair, but it needs AF_UNIX in kernel */
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xpiped_pair(signal_pipe);
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close_on_exec_on(signal_pipe.rd);
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close_on_exec_on(signal_pipe.wr);
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ndelay_on(signal_pipe.rd);
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ndelay_on(signal_pipe.wr);
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bb_signals(0
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+ (1 << SIGUSR1)
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+ (1 << SIGUSR2)
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+ (1 << SIGTERM)
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, signal_handler);
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}
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/* Quick little function to setup the pfds.
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* Limited in that you can only pass one extra fd.
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*/
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void FAST_FUNC udhcp_sp_fd_set(struct pollfd pfds[2], int extra_fd)
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{
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pfds[0].fd = signal_pipe.rd;
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pfds[0].events = POLLIN;
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pfds[1].fd = -1;
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if (extra_fd >= 0) {
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close_on_exec_on(extra_fd);
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pfds[1].fd = extra_fd;
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pfds[1].events = POLLIN;
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}
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/* this simplifies "is extra_fd ready?" tests elsewhere: */
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pfds[1].revents = 0;
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}
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/* Read a signal from the signal pipe. Returns 0 if there is
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* no signal, -1 on error (and sets errno appropriately), and
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* your signal on success */
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int FAST_FUNC udhcp_sp_read(void)
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{
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unsigned char sig;
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/* Can't block here, fd is in nonblocking mode */
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if (safe_read(signal_pipe.rd, &sig, 1) != 1)
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return 0;
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return sig;
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}
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