问题

我们有一台机器使用 tail -f file，查看日志时，使用 ctrl+c 无法中断退出。导致每次使用 ctrl+z 挂起再杀掉，十分麻烦。

分析

正常情况下使用 ctrl+c 会终止前台运行的程序，因为 ctrl+c 会向进程发送一个终止信号。联想到 trap 是不是设置有问题。

问题主机：

[root@zabbix_a5 ~]# lsb_release -a LSB Version: :core-4.1-amd64:core-4.1-noarch Distributor ID: CentOS Description: CentOS Linux release 7.2.1511 (Core) Release: 7.2.1511 Codename: Core [root@zabbix_a5 ~]# trap trap -- '' SIGINT trap -- '' SIGQUIT trap -- '' SIGTSTP trap -- '' SIGTTIN trap -- '' SIGTTOU

正常主机：

[root@VM_85_116_centos test]# lsb_release -a LSB Version: :core-4.1-amd64:core-4.1-noarch Distributor ID: CentOS Description: CentOS Linux release 7.6.1810 (Core) Release: 7.6.1810 Codename: Core [root@VM_85_116_centos test]# trap trap -- '' SIGTSTP trap -- '' SIGTTIN trap -- '' SIGTTOU

问题主机有下面的设置，导致 SIGINT （ ctrl+c ）、SIGQUIT 信号会被忽略。

trap -- '' SIGINT trap -- '' SIGQUIT

尝试解决

问题主机执行

[root@zabbix_a5 ~]# trap trap -- '' SIGINT trap -- '' SIGQUIT trap -- '' SIGTSTP trap -- '' SIGTTIN trap -- '' SIGTTOU [root@zabbix_a5 ~]# trap - SIGINT [root@zabbix_a5 ~]# trap trap -- '' SIGINT trap -- '' SIGQUIT trap -- '' SIGTSTP trap -- '' SIGTTIN trap -- '' SIGTTOU

正常主机执行

[root@VM_85_116_centos ~]# trap -- '' SIGINT [root@VM_85_116_centos ~]# trap trap -- '' SIGINT trap -- '' SIGTSTP trap -- '' SIGTTIN trap -- '' SIGTTOU [root@VM_85_116_centos ~]# trap - SIGINT [root@VM_85_116_centos ~]# trap trap -- '' SIGTSTP trap -- '' SIGTTIN trap -- '' SIGTTOU

问题主机 trap - SIGINT 无法清楚 trap 设置

strace 分析

编写脚本 test.sh

echo '111' trap 'ls' SIGINT echo '222' trap 'ls' SIGINT echo '333' trap - SIGINT

分别在正常与问题主机上执行

strace -o output.log -v sh test.sh

问题主机摘要：

write(1, "111\n", 4) = 4 rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0 rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0 write(1, "222\n", 4) = 4 rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0 rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0 write(1, "333\n", 4) = 4 rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0 rt_sigaction(SIGINT, {0x456a90, [], SA_RESTORER, 0x7f76eedc7250}, {SIG_IGN, [], SA_RESTORER, 0x7f76eedc7250}, 8) = 0 rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0 read(255, "", 81) = 0 exit_group(0) = ? +++ exited with 0 +++

正常主机摘要：

write(1, "111\n", 4) = 4 rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0 rt_sigprocmask(SIG_BLOCK, [INT], [], 8) = 0 rt_sigaction(SIGINT, {0x454380, [], SA_RESTORER, 0x7fed821c4280}, {SIG_DFL, [], SA_RESTORER, 0x7fed821c4280}, 8) = 0 rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0 rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0 write(1, "222\n", 4) = 4 rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0 rt_sigprocmask(SIG_BLOCK, [INT], [], 8) = 0 rt_sigaction(SIGINT, {0x454380, [], SA_RESTORER, 0x7fed821c4280}, {0x454380, [], SA_RESTORER, 0x7fed821c4280}, 8) = 0 rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0 rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0 write(1, "333\n", 4) = 4 rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0 rt_sigaction(SIGINT, {SIG_DFL, [], SA_RESTORER, 0x7fed821c4280}, {0x454380, [], SA_RESTORER, 0x7fed821c4280}, 8) = 0 rt_sigaction(SIGINT, {0x456be0, [], SA_RESTORER, 0x7fed821c4280}, {SIG_DFL, [], SA_RESTORER, 0x7fed821c4280}, 8) = 0 rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0 read(255, "", 81) = 0 exit_group(0) = ? +++ exited with 0 +++

相同命令系统调用差异对比

执行trap 'ls' SIGINT

问题主机：

rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0 rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0

正常主机：

rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0 rt_sigprocmask(SIG_BLOCK, [INT], [], 8) = 0 rt_sigaction(SIGINT, {0x454380, [], SA_RESTORER, 0x7fed821c4280}, {0x454380, [], SA_RESTORER, 0x7fed821c4280}, 8) = 0 rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0 rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0

执行trap - SIGINT

问题主机：

rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0 rt_sigaction(SIGINT, {0x456a90, [], SA_RESTORER, 0x7f76eedc7250}, {SIG_IGN, [], SA_RESTORER, 0x7f76eedc7250}, 8) = 0 rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0

正常主机：

rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0 rt_sigaction(SIGINT, {SIG_DFL, [], SA_RESTORER, 0x7fed821c4280}, {0x454380, [], SA_RESTORER, 0x7fed821c4280}, 8) = 0 rt_sigaction(SIGINT, {0x456be0, [], SA_RESTORER, 0x7fed821c4280}, {SIG_DFL, [], SA_RESTORER, 0x7fed821c4280}, 8) = 0 rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0

貌似是问题主机少了一次 rt_sigaction 系统调用