线上使用zabbix的host update来监测监控值是否完整（关于host update的实现请参考：
http://caiguangguang.blog.51cto.com/1652935/1345789）
一直发现有机器过一段时间update值就会莫名其妙变低，之前一直没有找到rc，只是简单通过重启agent来进行修复，最近同事细心地发现可能是和sudo的bug有关系。
回过头再来验证下整个的排查过程。
1.通过zabbix 数据库获取丢失数据的item，拿出缺失的(20分钟没有更新的)值的item列表
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select
b.key_,b.lastvalue,from_unixtime(b.lastclock)
from
hosts a,
items b
where
a.hostid=b.hostid
and
a.host=
'xxxxxx'
and
b.lastclock < (unix_timestamp() - 1200) limit 10;
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比如这里看agent.ping:
观察监控图，发现在18点20分之后数据丢失：
2.分析zabbix agent端的日志
发现在18点24粉左右出现下面的日志，没有看到正常的获取值和发送值的情况，只有大量的update_cpustats状态，同时发现有一行kill command 失败的日志:
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27589:20141021:182442.143 In zbx_popen()
command
:
'sudo hadoop_stats.sh nodemanager StopContainerAvgTime'
27589:20141021:182442.143 End of zbx_popen():5
48430:20141021:182442.143 zbx_popen(): executing script
27585:20141021:182442.284 In update_cpustats()
27585:20141021:182442.285 End of update_cpustats()
27585:20141021:182443.285 In update_cpustats()
27585:20141021:182443.286 End of update_cpustats()
27585:20141021:182444.286 In update_cpustats()
27585:20141021:182444.287 End of update_cpustats()
27585:20141021:182445.287 In update_cpustats()
27585:20141021:182445.287 End of update_cpustats()
27585:20141021:182446.288 In update_cpustats()
27585:20141021:182446.288 End of update_cpustats()
..........
27585:20141021:182508.305 In update_cpustats()
27585:20141021:182508.305 End of update_cpustats()
27585:20141021:182509.306 In update_cpustats()
27585:20141021:182509.306 End of update_cpustats()
27585:20141021:182510.306 In update_cpustats()
27585:20141021:182510.307 End of update_cpustats()
27585:20141021:182511.307 In update_cpustats()
27585:20141021:182511.308 End of update_cpustats()
27589:20141021:182512.154 failed to
kill
[
sudo
hadoop_stats.sh nodemanager StopContainerAvgTime]: [1] Operation not permitted
27589:20141021:182512.155 In zbx_waitpid()
27585:20141021:182512.308 In update_cpustats()
27585:20141021:182512.309 End of update_cpustats()
27585:20141021:182513.309 In update_cpustats()
27585:20141021:182513.309 End of update_cpustats()
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对比正常的日志：
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27589:20141021:180054.376 In zbx_popen()
command
:
'sudo hadoop_stats.sh nodemanager StopContainerAvgTime'
27589:20141021:180054.377 End of zbx_popen():5
18798:20141021:180054.377 zbx_popen(): executing script
27589:20141021:180054.384 In zbx_waitpid()
27589:20141021:180054.384 zbx_waitpid() exited, status:1
27589:20141021:180054.384 End of zbx_waitpid():18798
27589:20141021:180054.384 Run remote
command
[
sudo
hadoop_stats.sh nodemanager StopContainerAvgTime] Result [2] [-1]...
27589:20141021:180054.384 For key [hadoop_stats[nodemanager,StopContainerAvgTime]] received value [-1]
27589:20141021:180054.384 In process_value() key:
'gd6g203s80-hadoop-datanode.idc.vipshop.com:hadoop_stats[nodemanager,StopContainerAvgTime]'
value:
'-1'
27589:20141021:180054.384 In send_buffer() host:
'10.200.100.28'
port:10051 values:37
/50
27589:20141021:180054.384 Will not send now. Now 1413885654 lastsent 1413885654 < 1
27589:20141021:180054.385 End of send_buffer():SUCCEED
27589:20141021:180054.385 buffer: new element 37
27589:20141021:180054.385 End of process_value():SUCCEED
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可以看到正常情况下脚本会有返回值，而出问题的时候，脚本是没有返回值的，并且由于是使用sudo 运行脚本，导致以普通用户启动的zabbix在超时时没有办法杀掉这个command(Operation not permitted 错误)
3.假设这里启动zabbix agent的普通用户为apps用户，我们看下这个脚本目前的状态
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ps
-ef|
grep
hadoop_stats.sh
root 34494 31429 0 12:54 pts
/0
00:00:00
grep
48430
root 48430 27589 0 Oct21 ? 00:00:00
sudo
hadoop_stats.sh nodemanager StopContainerAvgTime
root 48431 48430 0 Oct21 ? 00:00:00 [hadoop_stats.sh] <defunct>
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可以看到，这里产生了一个僵尸进程（关于僵尸进程可以参考：http://en.wikipedia.org/wiki/Zombie_process）
僵尸进程是由于子进程运行完毕之后，发送SIGCHLD到父进程，而父进程没有正常处理这个信号导致。
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You have killed the process, but a dead process doesn't disappear from the process table
until
its parent process performs a task called
"reaping"
(essentially calling wait(3)
for
that process to
read
its
exit
status). Dead processes that haven't been reaped are
called
"zombie processes."
The parent process
id
you see
for
31756 is process
id
1,
which
always belongs to init.
That process should reap its zombie processes periodically, but
if
it can't, they will
remain zombies
in
the process table
until
you reboot.
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正常的进程情况下，我们使用strace attach到父进程，然后杀掉子进程后可以看到如下信息：
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Process 3036 attached - interrupt to quit
select
(6, [5], [], NULL, NULL
) = ? ERESTARTNOHAND (To be restarted)
--- SIGCHLD (Child exited) @ 0 (0) ---
rt_sigreturn(0x11) = -1 EINTR (Interrupted system call)
wait4(3037, [{WIFSIGNALED(s) && WTERMSIG(s) == SIGTERM}], WNOHANG|WSTOPPED, NULL) = 3037
exit_group(143) = ?
Process 3036 detached
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产生僵尸进程之后，可以通过杀掉父进程把僵尸进程变成孤儿进程（父进程为init进程）
但是这里因为是用sudo启动的脚本，导致启动用户都是root，apps用户就没有权限杀掉启动的命令，进而导致子进程一直是僵尸进程的状态存在
4.来看一下zabbix agent端启动的相关进程情况
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ps -ef|grep zabbix
apps
27583
1
0
Sep09 ?
00
:
00
:
00
/apps/svr/zabbix/sbin/zabbix_agentd -c /apps/conf/zabbix_agentd.conf
apps
27585
27583
0
Sep09 ?
00
:
33
:
25
/apps/svr/zabbix/sbin/zabbix_agentd -c /apps/conf/zabbix_agentd.conf
apps
27586
27583
0
Sep09 ?
00
:
00
:
14
/apps/svr/zabbix/sbin/zabbix_agentd -c /apps/conf/zabbix_agentd.conf
apps
27587
27583
0
Sep09 ?
00
:
00
:
14
/apps/svr/zabbix/sbin/zabbix_agentd -c /apps/conf/zabbix_agentd.conf
apps
27588
27583
0
Sep09 ?
00
:
00
:
14
/apps/svr/zabbix/sbin/zabbix_agentd -c /apps/conf/zabbix_agentd.conf
apps
27589
27583
0
Sep09 ?
02
:
28
:
12
/apps/svr/zabbix/sbin/zabbix_agentd -c /apps/conf/zabbix_agentd.conf
root
34207
31429
0
12
:
54
pts/
0
00
:
00
:
00
grep zabbix
root
48430
27589
0
Oct21 ?
00
:
00
:
00
sudo /apps/sh/zabbix_scripts/hadoop/hadoop_stats.sh nodemanager StopContainerAvgTime
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通过strace我们发现27589的进程一直在等待48430的进程
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strace -p
27589
Process
27589
attached - interrupt to quit
wait4(
48430
, ^C <unfinished ...>
Process
27589
detached
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而48430的进程即为僵尸进程的父进程，通过strace attach上去，可以看到在等待#5的fd
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strace -p
48430
Process
48430
attached - interrupt to quit
select(
6
, [
5
], [], NULL, NULL^C <unfinished ...>
Process
48430
detached
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通过lsof可以看到#5的fd其实是一个socket
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lsof -p
48430
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
sudo
48430
root cwd DIR
8
,
2
4096
2
/
sudo
48430
root rtd DIR
8
,
2
4096
2
/
sudo
48430
root txt REG
8
,
2
212904
1578739
/usr/bin/sudo
sudo
48430
root mem REG
8
,
2
65928
1441822
/lib64/libnss_files-
2.12
.so
sudo
48430
root mem REG
8
,
2
99158704
1573509
/usr/lib/locale/locale-archive
sudo
48430
root mem REG
8
,
2
91096
1441832
/lib64/libz.so.
1.2
.
3
sudo
48430
root mem REG
8
,
2
141576
1442145
/lib64/libpthread-
2.12
.so
sudo
48430
root mem REG
8
,
2
386040
1442172
/lib64/libfreebl3.so
sudo
48430
root mem REG
8
,
2
108728
1575924
/usr/lib64/libsasl2.so.
2.0
.
23
sudo
48430
root mem REG
8
,
2
243064
1441896
/lib64/libnspr4.so
sudo
48430
root mem REG
8
,
2
21256
1442186
/lib64/libplc4.so
sudo
48430
root mem REG
8
,
2
17096
1442187
/lib64/libplds4.so
sudo
48430
root mem REG
8
,
2
128368
1577789
/usr/lib64/libnssutil3.so
sudo
48430
root mem REG
8
,
2
1290648
1582418
/usr/lib64/libnss3.so
sudo
48430
root mem REG
8
,
2
188072
1575925
/usr/lib64/libsmime3.so
sudo
48430
root mem REG
8
,
2
220200
1587191
/usr/lib64/libssl3.so
sudo
48430
root mem REG
8
,
2
113952
1442182
/lib64/libresolv-
2.12
.so
sudo
48430
root mem REG
8
,
2
43392
1442173
/lib64/libcrypt-
2.12
.so
sudo
48430
root mem REG
8
,
2
63304
1442180
/lib64/liblber-
2.4
.so.
2.5
.
6
sudo
48430
root mem REG
8
,
2
1979000
1442169
/lib64/libc-
2.12
.so
sudo
48430
root mem REG
8
,
2
308912
1442181
/lib64/libldap-
2.4
.so.
2.5
.
6
sudo
48430
root mem REG
8
,
2
22536
1442171
/lib64/libdl-
2.12
.so
sudo
48430
root mem REG
8
,
2
58480
1442174
/lib64/libpam.so.
0.82
.
2
sudo
48430
root mem REG
8
,
2
17520
1441884
/lib64/libutil-
2.12
.so
sudo
48430
root mem REG
8
,
2
124624
1441798
/lib64/libselinux.so.
1
sudo
48430
root mem REG
8
,
2
99112
1442170
/lib64/libaudit.so.
1.0
.
0
sudo
48430
root mem REG
8
,
2
156872
1442168
/lib64/ld-
2.12
.so
sudo
48430
root 0r CHR
1
,
3
0t0
3916
/dev/
null
sudo
48430
root 1w FIFO
0
,
8
0t0
1429910151
pipe
sudo
48430
root 2w REG
8
,
3
376639626
524292
/apps/logs/zabbix/zabbix_agentd.log
sudo
48430
root 3u sock
0
,
6
0t0
1429910161
can't identify protocol
sudo
48430
root 4r REG
8
,
2
764
2240617
/etc/group
sudo
48430
root 5u unix
0xffff880179ee4680
0t0
1429910162
socket
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这里通过查看/proc/pid/fd下的文件描述符的状态，发现这个fd其实是已经关闭的。
这里就有可能是子进程已经运行完成，而父进程没有正确处理子进程的返回信息导致父进程一直认为子进程还在运行，最终产生了僵尸进程。
这其实是sudo的一个bug，相关的bug id :
http://www.gratisoft.us/bugzilla/show_bug.cgi?id=447
关于bug的描述：
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If the parent process gets re-scheduled after the “
if
” was executed, and at
this
very
time the child process finishes and SIGCHLD is sent to the parent process, sudo gets
in trouble. The SIGCHLD handler accounts in the variable “recvsig[]” that the signal
was received, and then the parent process calls select(). This select will never be
interrupted, as the author had it in mind. In
99
% of the cases, the parent process
will enter in the select() blocking state before the child process ended.
The child would then send SIGCHLD, which will be accounted in the handler procedure,
and will also interrupt select() which will
return
-
1
in “nready”, and “errno”
will be set to EINTR.
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问题出在sudo的代码sudo/file/tip/src/exec.c，小于 1.7.5或1.8.0 之前的版本都有问题，当子进程恰好在select()这个系统调用前退出的时候，句柄已经被退出，所以sudo会卡在select这里
patch:
http://www.sudo.ws/repos/sudo/rev/99adc5ea7f0a
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Avoid a potential race condition
if
SIGCHLD is received immediately before we call select().
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网上有人遇到了同样地问题：
http://blog.famzah.net/2010/11/01/sudo-hangs-and-leaves-the-executed-program-as-zombie/
回过头总结，这个问题其实是多个潜在问题同时造成：
1.zabbix agent的自定义监控配置中使用了sudo，导致僵尸进程的父进程不能正常关闭(如果需要sudo，写在脚本里面即可)
2.sudo的bug导致产生了僵死进程（升级sudo即可）
3.zabbix agent端的实现也有问题，某一个进程成为僵死进程后会影响其他的监控项获取（zabbix agent的进程被阻塞导致）
本文转自菜菜光 51CTO博客，原文链接：http://blog.51cto.com/caiguangguang/1566952，如需转载请自行联系原作者
