无法获取指向控制台的文件描述符 (couldn’t get a file descriptor referring to the console)
- 2021 年 10 月 19 日
- 筆記
- permission, RaspberryPi, tty, 学以致用
背景
最近收拾东西，从一堆杂物里翻出来尘封四年多的树莓派 3B 主机来，打扫打扫灰尘，接上电源，居然还能通过之前设置好的 VNC 连上。欣慰之余，开始 clone 我的 git 项目，为它们拓展一个新的平台。在执行 cnblogs 项目 (参考《博客园排名预测 》) 对应的绘图命令时，趋势图、预测图是生成了，但没有自动打开图片，这个问题经过一番探索居然解决了，这篇文章就来分享一下解决问题的过程。
问题
第一眼看到的错误信息：
$ open ./fit.png 无法获取指向控制台的文件描述符
这里我设置了控制台 locale 为中文，如果是英文的话，得到下面的结果：
Couldn't get a file descriptor referring to the console
果断在网上搜索了这个错误，得到的结果比较少，根据解决方案主要分以下几种：
- setfont 命令：
- loadkeys 命令：
这里我并没有调用 setfont 或 loadkeys 命令，直接执行它俩也会报一样的错误，难道需要在登录脚本里执行一下？抱着试试看的态度，我在 ~/.bashrc 中加了这么一句：
loadkeys
重启树莓派，这回用 ssh 登录，结果一上来就报错：
The programs included with the Debian GNU/Linux system are free software; the exact distribution terms for each program are described in the individual files in /usr/share/doc/*/copyright. Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent permitted by applicable law. Last login: Sun Aug 8 23:07:46 2021 from 192.168.1.118 无法获取指向控制台的文件描述符
看来网上的说法和我遇到的不一样，只能自己探索了。好在遇事不决、量子力学，哦 no，strace，看看底层调用哪个环节出问题了：
$ strace open ./fit.png
execve("/bin/open", ["open", "fit.png"], [/* 36 vars */]) = 0
brk(0) = 0x822000
uname({sys="Linux", node="raspberrypi", ...}) = 0
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
mmap2(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x76f91000
access("/etc/ld.so.preload", R_OK) = 0
open("/etc/ld.so.preload", O_RDONLY|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=42, ...}) = 0
mmap2(NULL, 42, PROT_READ|PROT_WRITE, MAP_PRIVATE, 3, 0) = 0x76f90000
close(3) = 0
open("/usr/lib/arm-linux-gnueabihf/libarmmem.so", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0(\0\1\0\0\0h\5\0\0004\0\0\0"..., 512) = 512
lseek(3, 17960, SEEK_SET) = 17960
read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 960) = 960
lseek(3, 17696, SEEK_SET) = 17696
read(3, "A.\0\0\0aeabi\0\1$\0\0\0\0056\0\6\6\10\1\t\1\n\3\f\1\22\4\24"..., 47) = 47
fstat64(3, {st_mode=S_IFREG|0644, st_size=18920, ...}) = 0
mmap2(NULL, 83236, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x76f4f000
mprotect(0x76f54000, 61440, PROT_NONE) = 0
mmap2(0x76f63000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x4000) = 0x76f63000
mprotect(0x7eb5f000, 4096, PROT_READ|PROT_WRITE|PROT_EXEC|PROT_GROWSDOWN) = 0
close(3) = 0
munmap(0x76f90000, 42) = 0
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=76981, ...}) = 0
mmap2(NULL, 76981, PROT_READ, MAP_PRIVATE, 3, 0) = 0x76f3c000
close(3) = 0
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
open("/lib/arm-linux-gnueabihf/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0(\0\1\0\0\0L\204\1\0004\0\0\0"..., 512) = 512
lseek(3, 1239936, SEEK_SET) = 1239936
read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 2840) = 2840
lseek(3, 1236500, SEEK_SET) = 1236500
read(3, "A.\0\0\0aeabi\0\1$\0\0\0\0056\0\6\6\10\1\t\1\n\2\22\4\23\1\24"..., 47) = 47
fstat64(3, {st_mode=S_IFREG|0755, st_size=1242776, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x76f90000
mmap2(NULL, 1312152, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x76dfb000
mprotect(0x76f26000, 65536, PROT_NONE) = 0
mmap2(0x76f36000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x12b000) = 0x76f36000
mmap2(0x76f39000, 9624, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x76f39000
close(3) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x76f8f000
set_tls(0x76f8f4c0, 0x76f8fba8, 0x76f94050, 0x76f8f4c0, 0x76f94050) = 0
mprotect(0x76f36000, 8192, PROT_READ) = 0
mprotect(0x76f4f000, 20480, PROT_READ|PROT_WRITE) = 0
mprotect(0x76f4f000, 20480, PROT_READ|PROT_EXEC) = 0
cacheflush(0x76f4f000, 0x76f54000, 0, 0x15, 0x7eb5f0b0) = 0
mprotect(0x22000, 4096, PROT_READ) = 0
mprotect(0x76f93000, 4096, PROT_READ) = 0
munmap(0x76f3c000, 76981) = 0
brk(0) = 0x822000
brk(0x843000) = 0x843000
open("/usr/lib/locale/locale-archive", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=4395328, ...}) = 0
mmap2(NULL, 2097152, PROT_READ, MAP_PRIVATE, 3, 0) = 0x76bfb000
mmap2(NULL, 1720320, PROT_READ, MAP_PRIVATE, 3, 0x28e000) = 0x76a57000
close(3) = 0
fstat64(0, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 1), ...}) = 0
fstat64(1, {st_mode=S_IFREG|0644, st_size=3753, ...}) = 0
fstat64(2, {st_mode=S_IFREG|0644, st_size=3811, ...}) = 0
open("/proc/self/fd/0", O_RDWR) = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(3, KDGKBTYPE, 0x7eb5f14b) = -1 ENOTTY (Inappropriate ioctl for device)
close(3) = 0
open("/dev/tty", O_RDWR) = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(3, KDGKBTYPE, 0x7eb5f14b) = -1 ENOTTY (Inappropriate ioctl for device)
close(3) = 0
open("/dev/tty0", O_RDWR) = -1 EACCES (Permission denied)
open("/dev/tty0", O_WRONLY) = -1 EACCES (Permission denied)
open("/dev/tty0", O_RDONLY) = -1 EACCES (Permission denied)
open("/dev/vc/0", O_RDWR) = -1 ENOENT (No such file or directory)
open("/dev/vc/0", O_WRONLY) = -1 ENOENT (No such file or directory)
open("/dev/vc/0", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/dev/systty", O_RDWR) = -1 ENOENT (No such file or directory)
open("/dev/systty", O_WRONLY) = -1 ENOENT (No such file or directory)
open("/dev/systty", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/dev/console", O_RDWR) = -1 EACCES (Permission denied)
open("/dev/console", O_WRONLY) = -1 EACCES (Permission denied)
open("/dev/console", O_RDONLY) = -1 EACCES (Permission denied)
ioctl(0, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(0, KDGKBTYPE, 0x7eb5f14b) = -1 ENOTTY (Inappropriate ioctl for device)
ioctl(1, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, 0x7eb5f0cc) = -1 ENOTTY (Inappropriate ioctl for device)
ioctl(2, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, 0x7eb5f0cc) = -1 ENOTTY (Inappropriate ioctl for device)
open("/usr/share/locale/locale.alias", O_RDONLY|O_CLOEXEC) = 3
fcntl64(3, F_GETFD) = 0x1 (flags FD_CLOEXEC)
fstat64(3, {st_mode=S_IFREG|0644, st_size=2492, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x76f8e000
read(3, "# Locale name alias data base.\n#"..., 4096) = 2492
read(3, "", 4096) = 0
close(3) = 0
munmap(0x76f8e000, 4096) = 0
open("/usr/share/locale/zh_CN.UTF-8/LC_MESSAGES/kbd.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/zh_CN.utf8/LC_MESSAGES/kbd.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/zh_CN/LC_MESSAGES/kbd.mo", O_RDONLY) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=30170, ...}) = 0
mmap2(NULL, 30170, PROT_READ, MAP_PRIVATE, 3, 0) = 0x76f87000
close(3) = 0
open("/usr/lib/arm-linux-gnueabihf/gconv/gconv-modules.cache", O_RDONLY) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=26262, ...}) = 0
mmap2(NULL, 26262, PROT_READ, MAP_SHARED, 3, 0) = 0x76f48000
close(3) = 0
write(2, "\346\227\240\346\263\225\350\216\267\345\217\226\346\214\207\345\220\221\346\216\247\345\210\266\345\217\260\347\232\204\346\226"..., 46无法获取指向控制台的文件描述符
) = 46
exit_group(1) = ?
+++ exited with 1 +++
brk (line 53) 之前的输出都不用看，属于程序初始化的逻辑；值得注意的报错点在 line 70 与 79：
open("/dev/console", O_RDWR) = -1 EACCES (Permission denied)
这两个地方错误相似，都是打开 tty 设备时没有权限，而且是试了三种权限都失败了，分别为：读写 (O_RDWR)、只写 (O_WRONLY)、只读 (O_RDONLY)，看来为了打开这个设备，open 也是尽力了呀～
为什么好端端的会没有权限呢？让我们看下 ls 的输出：
$ ls -lh /dev/console crw------- 1 root root 5, 1 Aug 8 22:37 /dev/console
这个设备是 root 创建、root 拥有、且没有给其它用户开放任何权限，难怪会加载失败。
解决方案
既然根因是权限导致的，那就从权限入手来解决，之前系统总结过 linux 文件权限的规则 (参考 《[apue] linux 文件访问权限那些事儿 》)，要将 root 创建的文件分享给当前用户访问，也不是什么难事。
console group
第一个冒出来的想法，就是创建一个新的用户组 (例如 console)，把 root 和当前用户 pi 都加入到这个组中，然后指定设备的所有组为 console，并开放适当的组权限，这样就可以实现共享啦。有的人可能觉得麻烦，直接把出错的 tty 设备 chown 到当前用户不就行了？对于普通的数据文件，我也经常这样搞，但是对于公共的、系统相关的文件，我劝大家还是不要这么自信，否则可能搞到开不了机 (个人惨痛遭遇就不展开了)。另外这样做还有个额外的好处，就是当新用户遇到了同样问题时，只要把他加入这个组 (console) 就搞定了！下面是对应的脚本：
groups sudo groupadd console usermod -a -G console pi groups su groups usermod -a -G console root groups chgrp console /dev/console chmod g+rw /dev/console exit
夹在 usermod 前后的 groups 命令用来显示当前用户所属的用户组，用以验证添加附加组是否成功 (实际上无效，见后面说明)。注意对 root 帐户的操作需要使用 su 切换，切换后顺便对它拥有的文件 (/dev/console) 进行操作，以避免权限上的错误，最后执行完成后 exit 退回到当前用户。这里有两个点需要注意：
- usermod -a 选项必需和 -G 配合使用，之前我单独使用 -a 时 usemod 会直接打印 Usage，查了下 man 才弄明白；
- 将用户添加到一个组后 groups 不能立即看到结果，必需重新登录才会生效。特别对于 VNC 登录，logout 菜单似乎不起作用，需要重启设备才行。
觉得敲命令麻烦的同学，也可以直接进入 /etc/group 文件进行修改：
$ tail /etc/group
avahi:x:110:
ntp:x:111:
ssh:x:112:
bluetooth:x:113:
spi:x:999:pi
i2c:x:998:pi
gpio:x:997:pi
lightdm:x:114:
lpadmin:x:115:
console:x:1001:pi,root
最后一行就是了。下面是执行成功后新的 ls 的输出：
# ls -lh /dev/console crw-rw---- 1 root console 5, 1 Aug 8 22:37 /dev/console
此时再运行一次 strace open，发现针对 console 的访问成功了！
open("/dev/console", O_RDWR) = 3
虽然因为其它 tty 没有修改仍然出错了，但是至少证明方向是对的。
tty group
上面的方案是可行的，在查看 tty 设备时无意间得到了下面的输出：
$ ls -lh /dev/tty[0-9]* crw------- 1 root root 4, 0 Aug 8 23:51 /dev/tty0 crw------- 1 pi tty 4, 1 Aug 8 23:52 /dev/tty1 crw------- 1 root tty 4, 10 Aug 8 23:51 /dev/tty10 crw------- 1 root tty 4, 11 Aug 8 23:51 /dev/tty11 crw------- 1 root tty 4, 12 Aug 8 23:51 /dev/tty12 crw------- 1 root tty 4, 13 Aug 8 23:51 /dev/tty13 crw------- 1 root tty 4, 14 Aug 8 23:51 /dev/tty14 crw------- 1 root tty 4, 15 Aug 8 23:51 /dev/tty15 crw------- 1 root tty 4, 16 Aug 8 23:51 /dev/tty16 crw------- 1 root tty 4, 17 Aug 8 23:51 /dev/tty17 crw------- 1 root tty 4, 18 Aug 8 23:51 /dev/tty18 crw------- 1 root tty 4, 19 Aug 8 23:51 /dev/tty19 crw------- 1 root root 4, 2 Aug 8 23:51 /dev/tty2 crw------- 1 root tty 4, 20 Aug 8 23:51 /dev/tty20 crw------- 1 root tty 4, 21 Aug 8 23:51 /dev/tty21 crw------- 1 root tty 4, 22 Aug 8 23:51 /dev/tty22 crw------- 1 root tty 4, 23 Aug 8 23:51 /dev/tty23 crw------- 1 root tty 4, 24 Aug 8 23:51 /dev/tty24 crw------- 1 root tty 4, 25 Aug 8 23:51 /dev/tty25 crw------- 1 root tty 4, 26 Aug 8 23:51 /dev/tty26 crw------- 1 root tty 4, 27 Aug 8 23:51 /dev/tty27 crw------- 1 root tty 4, 28 Aug 8 23:51 /dev/tty28 crw------- 1 root tty 4, 29 Aug 8 23:51 /dev/tty29 crw------- 1 root root 4, 3 Aug 8 23:51 /dev/tty3 crw------- 1 root tty 4, 30 Aug 8 23:51 /dev/tty30 crw------- 1 root tty 4, 31 Aug 8 23:51 /dev/tty31 crw------- 1 root tty 4, 32 Aug 8 23:51 /dev/tty32 crw------- 1 root tty 4, 33 Aug 8 23:51 /dev/tty33 crw------- 1 root tty 4, 34 Aug 8 23:51 /dev/tty34 crw------- 1 root tty 4, 35 Aug 8 23:51 /dev/tty35 crw------- 1 root tty 4, 36 Aug 8 23:51 /dev/tty36 crw------- 1 root tty 4, 37 Aug 8 23:51 /dev/tty37 crw------- 1 root tty 4, 38 Aug 8 23:51 /dev/tty38 crw------- 1 root tty 4, 39 Aug 8 23:51 /dev/tty39 crw------- 1 root tty 4, 4 Aug 8 23:51 /dev/tty4 crw------- 1 root tty 4, 40 Aug 8 23:51 /dev/tty40 crw------- 1 root tty 4, 41 Aug 8 23:51 /dev/tty41 crw------- 1 root tty 4, 42 Aug 8 23:51 /dev/tty42 crw------- 1 root tty 4, 43 Aug 8 23:51 /dev/tty43 crw------- 1 root tty 4, 44 Aug 8 23:51 /dev/tty44 crw------- 1 root tty 4, 45 Aug 8 23:51 /dev/tty45 crw------- 1 root tty 4, 46 Aug 8 23:51 /dev/tty46 crw------- 1 root tty 4, 47 Aug 8 23:51 /dev/tty47 crw------- 1 root tty 4, 48 Aug 8 23:51 /dev/tty48 crw------- 1 root tty 4, 49 Aug 8 23:51 /dev/tty49 crw------- 1 root tty 4, 5 Aug 8 23:51 /dev/tty5 crw------- 1 root tty 4, 50 Aug 8 23:51 /dev/tty50 crw------- 1 root tty 4, 51 Aug 8 23:51 /dev/tty51 crw------- 1 root tty 4, 52 Aug 8 23:51 /dev/tty52 crw------- 1 root tty 4, 53 Aug 8 23:51 /dev/tty53 crw------- 1 root tty 4, 54 Aug 8 23:51 /dev/tty54 crw------- 1 root tty 4, 55 Aug 8 23:51 /dev/tty55 crw------- 1 root tty 4, 56 Aug 8 23:51 /dev/tty56 crw------- 1 root tty 4, 57 Aug 8 23:51 /dev/tty57 crw------- 1 root tty 4, 58 Aug 8 23:51 /dev/tty58 crw------- 1 root tty 4, 59 Aug 8 23:51 /dev/tty59 crw------- 1 root tty 4, 6 Aug 8 23:51 /dev/tty6 crw------- 1 root tty 4, 60 Aug 8 23:51 /dev/tty60 crw------- 1 root tty 4, 61 Aug 8 23:51 /dev/tty61 crw------- 1 root tty 4, 62 Aug 8 23:51 /dev/tty62 crw------- 1 root tty 4, 63 Aug 8 23:51 /dev/tty63 crw------- 1 root tty 4, 7 Aug 8 23:51 /dev/tty7 crw------- 1 root tty 4, 8 Aug 8 23:51 /dev/tty8 crw------- 1 root tty 4, 9 Aug 8 23:51 /dev/tty9
除了 tty0 / tty2 / tty3 的用户组属于 root 外，其它 tty 设备都属于 tty 用户组，原来系统早为它们准备了现成的组，我在上面创建 console 组就显得画蛇添足了，而且观察用户 pi 所属的附加组：
$ groups
pi adm tty dialout cdrom sudo audio video plugdev games users input netdev gpio i2c spi console
其中一个就是 tty，所以我从善如流的决定使用使用 tty 了。因此上节的脚本可以简化为：
su chgrp tty /dev/console /dev/tty[0-9]* chmod g+rw /dev/console /dev/tty[0-9]* exit
什么创建用户组、为用户添加附加组统统可以不要了，只需要修正 tty 设备文件的用户组就可以啦。
group permission
重启设备让修改生效后，再运行一次 strace open，发现还是有问题：
$ strace open ./fit.png
execve("/bin/open", ["open", "./fit.png"], [/* 36 vars */]) = 0
brk(0) = 0xe6b000
uname({sys="Linux", node="raspberrypi", ...}) = 0
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
mmap2(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x76f95000
access("/etc/ld.so.preload", R_OK) = 0
open("/etc/ld.so.preload", O_RDONLY|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=42, ...}) = 0
mmap2(NULL, 42, PROT_READ|PROT_WRITE, MAP_PRIVATE, 3, 0) = 0x76f94000
close(3) = 0
open("/usr/lib/arm-linux-gnueabihf/libarmmem.so", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0(\0\1\0\0\0h\5\0\0004\0\0\0"..., 512) = 512
lseek(3, 17960, SEEK_SET) = 17960
read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 960) = 960
lseek(3, 17696, SEEK_SET) = 17696
read(3, "A.\0\0\0aeabi\0\1$\0\0\0\0056\0\6\6\10\1\t\1\n\3\f\1\22\4\24"..., 47) = 47
fstat64(3, {st_mode=S_IFREG|0644, st_size=18920, ...}) = 0
mmap2(NULL, 83236, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x76f53000
mprotect(0x76f58000, 61440, PROT_NONE) = 0
mmap2(0x76f67000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x4000) = 0x76f67000
mprotect(0x7e817000, 4096, PROT_READ|PROT_WRITE|PROT_EXEC|PROT_GROWSDOWN) = 0
close(3) = 0
munmap(0x76f94000, 42) = 0
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=76981, ...}) = 0
mmap2(NULL, 76981, PROT_READ, MAP_PRIVATE, 3, 0) = 0x76f40000
close(3) = 0
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
open("/lib/arm-linux-gnueabihf/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0(\0\1\0\0\0L\204\1\0004\0\0\0"..., 512) = 512
lseek(3, 1239936, SEEK_SET) = 1239936
read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 2840) = 2840
lseek(3, 1236500, SEEK_SET) = 1236500
read(3, "A.\0\0\0aeabi\0\1$\0\0\0\0056\0\6\6\10\1\t\1\n\2\22\4\23\1\24"..., 47) = 47
fstat64(3, {st_mode=S_IFREG|0755, st_size=1242776, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x76f94000
mmap2(NULL, 1312152, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x76dff000
mprotect(0x76f2a000, 65536, PROT_NONE) = 0
mmap2(0x76f3a000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x12b000) = 0x76f3a000
mmap2(0x76f3d000, 9624, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x76f3d000
close(3) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x76f93000
set_tls(0x76f934c0, 0x76f93ba8, 0x76f98050, 0x76f934c0, 0x76f98050) = 0
mprotect(0x76f3a000, 8192, PROT_READ) = 0
mprotect(0x76f53000, 20480, PROT_READ|PROT_WRITE) = 0
mprotect(0x76f53000, 20480, PROT_READ|PROT_EXEC) = 0
cacheflush(0x76f53000, 0x76f58000, 0, 0x15, 0x7e8170b0) = 0
mprotect(0x22000, 4096, PROT_READ) = 0
mprotect(0x76f97000, 4096, PROT_READ) = 0
munmap(0x76f40000, 76981) = 0
brk(0) = 0xe6b000
brk(0xe8c000) = 0xe8c000
open("/usr/lib/locale/locale-archive", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=4395328, ...}) = 0
mmap2(NULL, 2097152, PROT_READ, MAP_PRIVATE, 3, 0) = 0x76bff000
mmap2(NULL, 1720320, PROT_READ, MAP_PRIVATE, 3, 0x28e000) = 0x76a5b000
close(3) = 0
fstat64(0, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 1), ...}) = 0
fstat64(1, {st_mode=S_IFREG|0644, st_size=3755, ...}) = 0
fstat64(2, {st_mode=S_IFREG|0644, st_size=3813, ...}) = 0
open("/proc/self/fd/0", O_RDWR) = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(3, KDGKBTYPE, 0x7e81714b) = -1 ENOTTY (Inappropriate ioctl for device)
close(3) = 0
open("/dev/tty", O_RDWR) = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(3, KDGKBTYPE, 0x7e81714b) = -1 ENOTTY (Inappropriate ioctl for device)
close(3) = 0
open("/dev/tty0", O_RDWR) = -1 EACCES (Permission denied)
open("/dev/tty0", O_WRONLY) = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 -opost -isig -icanon -echo ...}) = 0
ioctl(3, KDGKBTYPE, 0x7e81714b) = 0
ioctl(3, VT_GETSTATE, 0x7e8171a8) = 0
ioctl(3, VIDIOC_QUERYCAP or VT_OPENQRY, 0x7e8171a4) = 0
clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x76f93068) = 1882
exit_group(0) = ?
+++ exited with 0 +++
open: Unable to open /dev/tty2: 权限不够
line 70 有一次失败，但是马上 line 71 对同一个设备操作成功，末尾出现的权限不够失败对应的 /dev/tty2 文件根本在前面没有出现过，amazing～
用 ls 大法看一看：
$ ls -lh /dev/tty[0-9]* crw--w---- 1 root tty 4, 0 Aug 9 00:13 /dev/tty0 crw------- 1 pi tty 4, 1 Aug 9 00:14 /dev/tty1 crw--w---- 1 root tty 4, 10 Aug 9 00:13 /dev/tty10 crw--w---- 1 root tty 4, 11 Aug 9 00:13 /dev/tty11 crw--w---- 1 root tty 4, 12 Aug 9 00:13 /dev/tty12 crw--w---- 1 root tty 4, 13 Aug 9 00:13 /dev/tty13 crw--w---- 1 root tty 4, 14 Aug 9 00:13 /dev/tty14 crw--w---- 1 root tty 4, 15 Aug 9 00:13 /dev/tty15 crw--w---- 1 root tty 4, 16 Aug 9 00:13 /dev/tty16 crw--w---- 1 root tty 4, 17 Aug 9 00:13 /dev/tty17 crw--w---- 1 root tty 4, 18 Aug 9 00:13 /dev/tty18 crw--w---- 1 root tty 4, 19 Aug 9 00:13 /dev/tty19 crw--w---- 1 root tty 4, 2 Aug 9 00:13 /dev/tty2 crw--w---- 1 root tty 4, 20 Aug 9 00:13 /dev/tty20 crw--w---- 1 root tty 4, 21 Aug 9 00:13 /dev/tty21 crw--w---- 1 root tty 4, 22 Aug 9 00:13 /dev/tty22 crw--w---- 1 root tty 4, 23 Aug 9 00:13 /dev/tty23 crw--w---- 1 root tty 4, 24 Aug 9 00:13 /dev/tty24 crw--w---- 1 root tty 4, 25 Aug 9 00:13 /dev/tty25 crw--w---- 1 root tty 4, 26 Aug 9 00:13 /dev/tty26 crw--w---- 1 root tty 4, 27 Aug 9 00:13 /dev/tty27 crw--w---- 1 root tty 4, 28 Aug 9 00:13 /dev/tty28 crw--w---- 1 root tty 4, 29 Aug 9 00:13 /dev/tty29 crw--w---- 1 root tty 4, 3 Aug 9 00:13 /dev/tty3 crw--w---- 1 root tty 4, 30 Aug 9 00:13 /dev/tty30 crw--w---- 1 root tty 4, 31 Aug 9 00:13 /dev/tty31 crw--w---- 1 root tty 4, 32 Aug 9 00:13 /dev/tty32 crw--w---- 1 root tty 4, 33 Aug 9 00:13 /dev/tty33 crw--w---- 1 root tty 4, 34 Aug 9 00:13 /dev/tty34 crw--w---- 1 root tty 4, 35 Aug 9 00:13 /dev/tty35 crw--w---- 1 root tty 4, 36 Aug 9 00:13 /dev/tty36 crw--w---- 1 root tty 4, 37 Aug 9 00:13 /dev/tty37 crw--w---- 1 root tty 4, 38 Aug 9 00:13 /dev/tty38 crw--w---- 1 root tty 4, 39 Aug 9 00:13 /dev/tty39 crw--w---- 1 root tty 4, 4 Aug 9 00:13 /dev/tty4 crw--w---- 1 root tty 4, 40 Aug 9 00:13 /dev/tty40 crw--w---- 1 root tty 4, 41 Aug 9 00:13 /dev/tty41 crw--w---- 1 root tty 4, 42 Aug 9 00:13 /dev/tty42 crw--w---- 1 root tty 4, 43 Aug 9 00:13 /dev/tty43 crw--w---- 1 root tty 4, 44 Aug 9 00:13 /dev/tty44 crw--w---- 1 root tty 4, 45 Aug 9 00:13 /dev/tty45 crw--w---- 1 root tty 4, 46 Aug 9 00:13 /dev/tty46 crw--w---- 1 root tty 4, 47 Aug 9 00:13 /dev/tty47 crw--w---- 1 root tty 4, 48 Aug 9 00:13 /dev/tty48 crw--w---- 1 root tty 4, 49 Aug 9 00:13 /dev/tty49 crw--w---- 1 root tty 4, 5 Aug 9 00:13 /dev/tty5 crw--w---- 1 root tty 4, 50 Aug 9 00:13 /dev/tty50 crw--w---- 1 root tty 4, 51 Aug 9 00:13 /dev/tty51 crw--w---- 1 root tty 4, 52 Aug 9 00:13 /dev/tty52 crw--w---- 1 root tty 4, 53 Aug 9 00:13 /dev/tty53 crw--w---- 1 root tty 4, 54 Aug 9 00:13 /dev/tty54 crw--w---- 1 root tty 4, 55 Aug 9 00:13 /dev/tty55 crw--w---- 1 root tty 4, 56 Aug 9 00:13 /dev/tty56 crw--w---- 1 root tty 4, 57 Aug 9 00:13 /dev/tty57 crw--w---- 1 root tty 4, 58 Aug 9 00:13 /dev/tty58 crw--w---- 1 root tty 4, 59 Aug 9 00:13 /dev/tty59 crw--w---- 1 root tty 4, 6 Aug 9 00:13 /dev/tty6 crw--w---- 1 root tty 4, 60 Aug 9 00:13 /dev/tty60 crw--w---- 1 root tty 4, 61 Aug 9 00:13 /dev/tty61 crw--w---- 1 root tty 4, 62 Aug 9 00:13 /dev/tty62 crw--w---- 1 root tty 4, 63 Aug 9 00:13 /dev/tty63 crw--w---- 1 root tty 4, 7 Aug 9 00:13 /dev/tty7 crw--w---- 1 root tty 4, 8 Aug 9 00:13 /dev/tty8 crw--w---- 1 root tty 4, 9 Aug 9 00:13 /dev/tty9
所有 tty 设置的组权限变成了只写？难道是上一步中重启设备前我忘了设置各个文件的权限? 用 root 帐号 chmod 一下，输出这次正常了：
$ strace open ./fit.png
execve("/bin/open", ["open", "./fit.png"], [/* 36 vars */]) = 0
brk(0) = 0x288000
uname({sys="Linux", node="raspberrypi", ...}) = 0
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
mmap2(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x76efd000
access("/etc/ld.so.preload", R_OK) = 0
open("/etc/ld.so.preload", O_RDONLY|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=42, ...}) = 0
mmap2(NULL, 42, PROT_READ|PROT_WRITE, MAP_PRIVATE, 3, 0) = 0x76efc000
close(3) = 0
open("/usr/lib/arm-linux-gnueabihf/libarmmem.so", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0(\0\1\0\0\0h\5\0\0004\0\0\0"..., 512) = 512
lseek(3, 17960, SEEK_SET) = 17960
read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 960) = 960
lseek(3, 17696, SEEK_SET) = 17696
read(3, "A.\0\0\0aeabi\0\1$\0\0\0\0056\0\6\6\10\1\t\1\n\3\f\1\22\4\24"..., 47) = 47
fstat64(3, {st_mode=S_IFREG|0644, st_size=18920, ...}) = 0
mmap2(NULL, 83236, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x76ebb000
mprotect(0x76ec0000, 61440, PROT_NONE) = 0
mmap2(0x76ecf000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x4000) = 0x76ecf000
mprotect(0x7ea80000, 4096, PROT_READ|PROT_WRITE|PROT_EXEC|PROT_GROWSDOWN) = 0
close(3) = 0
munmap(0x76efc000, 42) = 0
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=76981, ...}) = 0
mmap2(NULL, 76981, PROT_READ, MAP_PRIVATE, 3, 0) = 0x76ea8000
close(3) = 0
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
open("/lib/arm-linux-gnueabihf/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0(\0\1\0\0\0L\204\1\0004\0\0\0"..., 512) = 512
lseek(3, 1239936, SEEK_SET) = 1239936
read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 2840) = 2840
lseek(3, 1236500, SEEK_SET) = 1236500
read(3, "A.\0\0\0aeabi\0\1$\0\0\0\0056\0\6\6\10\1\t\1\n\2\22\4\23\1\24"..., 47) = 47
fstat64(3, {st_mode=S_IFREG|0755, st_size=1242776, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x76efc000
mmap2(NULL, 1312152, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x76d67000
mprotect(0x76e92000, 65536, PROT_NONE) = 0
mmap2(0x76ea2000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x12b000) = 0x76ea2000
mmap2(0x76ea5000, 9624, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x76ea5000
close(3) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x76efb000
set_tls(0x76efb4c0, 0x76efbba8, 0x76f00050, 0x76efb4c0, 0x76f00050) = 0
mprotect(0x76ea2000, 8192, PROT_READ) = 0
mprotect(0x76ebb000, 20480, PROT_READ|PROT_WRITE) = 0
mprotect(0x76ebb000, 20480, PROT_READ|PROT_EXEC) = 0
cacheflush(0x76ebb000, 0x76ec0000, 0, 0x15, 0x7ea800b0) = 0
mprotect(0x22000, 4096, PROT_READ) = 0
mprotect(0x76eff000, 4096, PROT_READ) = 0
munmap(0x76ea8000, 76981) = 0
brk(0) = 0x288000
brk(0x2a9000) = 0x2a9000
open("/usr/lib/locale/locale-archive", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=4395328, ...}) = 0
mmap2(NULL, 2097152, PROT_READ, MAP_PRIVATE, 3, 0) = 0x76b67000
mmap2(NULL, 1720320, PROT_READ, MAP_PRIVATE, 3, 0x28e000) = 0x769c3000
close(3) = 0
fstat64(0, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 1), ...}) = 0
fstat64(1, {st_mode=S_IFREG|0644, st_size=3755, ...}) = 0
fstat64(2, {st_mode=S_IFREG|0644, st_size=3813, ...}) = 0
open("/proc/self/fd/0", O_RDWR) = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(3, KDGKBTYPE, 0x7ea8014b) = -1 ENOTTY (Inappropriate ioctl for device)
close(3) = 0
open("/dev/tty", O_RDWR) = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(3, KDGKBTYPE, 0x7ea8014b) = -1 ENOTTY (Inappropriate ioctl for device)
close(3) = 0
open("/dev/tty0", O_RDWR) = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 -opost -isig -icanon -echo ...}) = 0
ioctl(3, KDGKBTYPE, 0x7ea8014b) = 0
ioctl(3, VT_GETSTATE, 0x7ea801a8) = 0
ioctl(3, VIDIOC_QUERYCAP or VT_OPENQRY, 0x7ea801a4) = 0
clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x76efb068) = 2013
exit_group(0) = ?
+++ exited with 0 +++
不过再次重启后，这个文件权限仍然会被重置，之前使用 console group 时，也发现重启设备后 /dev/console 文件的用户组会自动重置为 root，组权限重置为空。好在这里只是将权限重置为只写，难道每次登录都要设置一遍 tty 文件权限？
xdg-open
答案是 no，倒不是因为找到了解决方案，而是即使在权限正确的情况下、open 不再报错了，图片还是没有自动打开 (汗)。于是我决定试试其它的命令，经过一番百度，找到了一个在 Ubuntu 下很常用的 xdg-open 命令，参数是要打开的图片路径，调用方式和 open 一样。话说回来，open 是我在 mac 上使用的命令，不适用 raspberrypi 是情理之中的，至于这个上的 open 是干啥的，我没找到对应的 man 记录 (还是个三无命令)。另外一开始如果使用 xdg-open，是不是就不用设置设备文件的用户组和权限了？没有试，不得而知。
结语
本文探索了诡异的 tty 设备文件权限问题，结合 linux 文件权限相关的知识进行了一番大胆的尝试，最后却另辟蹊径解决了问题。如果你在现实中遇到了类似的问题，不妨可以试试上面的方案，不管成功与否，都欢迎在文章下面留言反馈。
后记
和问题相关的都说的差不多了，下面来聊两句题外话。这次收拾尘封的树莓派，我深切的体会到了设置密码要简单的重要性，ssh 登录过程中试遍了我常用的所有密码，都不行，最后还是 1-6 拯救了我，而且 root 密码也是这个呦～
最后看着破破烂烂的亚克力塑料外壳，实在不能忍了，果断淘宝了一套新的外壳换上，换壳前后对比明显：
最后上一张树莓派 VNC 的截图：
参考
[1]. Ubuntu下通过命令打开图片
