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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)

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：
- fedora - 错误：无法获得指向控制台的文件描述符
- Error in linux console : Couldn't get a file descriptor referring to the console
loadkeys 命令：
- loadkeys says “Couldn't get a file descriptor referring to the console” in chroot on ChromeOS

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, "177ELF1113(1h5004"..., 512) = 512
lseek(3, 17960, SEEK_SET)               = 17960
read(3, ""..., 960) = 960
lseek(3, 17696, SEEK_SET)               = 17696
read(3, "A.aeabi1$05666101	1
3f122424"..., 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, "177ELF1113(1L2041004"..., 512) = 512
lseek(3, 1239936, SEEK_SET)             = 1239936
read(3, ""..., 2840) = 2840
lseek(3, 1236500, SEEK_SET)             = 1236500
read(3, "A.aeabi1$05666101	1
222423124"..., 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.
#"..., 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, "346227240346263225350216267345217226346214207345220221346216247345210266345217260347232204346226"..., 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, "177ELF1113(1h5004"..., 512) = 512
lseek(3, 17960, SEEK_SET)               = 17960
read(3, ""..., 960) = 960
lseek(3, 17696, SEEK_SET)               = 17696
read(3, "A.aeabi1$05666101	1
3f122424"..., 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, "177ELF1113(1L2041004"..., 512) = 512
lseek(3, 1239936, SEEK_SET)             = 1239936
read(3, ""..., 2840) = 2840
lseek(3, 1236500, SEEK_SET)             = 1236500
read(3, "A.aeabi1$05666101	1
222423124"..., 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, "177ELF1113(1h5004"..., 512) = 512
lseek(3, 17960, SEEK_SET)               = 17960
read(3, ""..., 960) = 960
lseek(3, 17696, SEEK_SET)               = 17696
read(3, "A.aeabi1$05666101	1
3f122424"..., 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, "177ELF1113(1L2041004"..., 512) = 512
lseek(3, 1239936, SEEK_SET)             = 1239936
read(3, ""..., 2840) = 2840
lseek(3, 1236500, SEEK_SET)             = 1236500
read(3, "A.aeabi1$05666101	1
222423124"..., 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

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原文地址：https://www.cnblogs.com/goodcitizen/p/problem_couldnt_get_a_file_descriptor_referring_to_the_console.html