标题: Linux Magic System Request Key Hacks

https://scz.617.cn/unix/201611291822.txt

参看内核源码中的"Documentation/sysrq.txt"

Q:

什么是"magic SysRq key"

A:

是一系列魔法按键，按下它们时，不管内核正在干什么都会立即响应，进行指定操作，
除非内核已经彻底失去响应。

echo "number" > /proc/sys/kernel/sysrq

--------------------------------------------------------------------------
0   disable sysrq completely
1   enable all functions of sysrq (default)
>1  bitmask of allowed sysrq functions
    2       0x2     enable control of console logging level
    4       0x4     enable control of keyboard (SAK, unraw)
    8       0x8     enable debugging dumps of processes etc.
    16      0x10    enable sync command
    32      0x20    enable remount read-only
    64      0x40    enable signalling of processes (term, kill, oom-kill)
    128     0x80    allow reboot/poweroff
    256     0x100   allow nicing of all RT tasks
--------------------------------------------------------------------------

"/proc/sys/kernel/sysrq"只影响通过键盘引发"Magic System Request"。无论如何
总是允许通过"/proc/sysrq-trigger"引发"Magic System Request"。

Q:

如何按下魔法按键

A:

x86

    Alt-SysRq-<command key>
    Alt-PrScrn-<command key>

serial console

    You send a BREAK, then within 5 seconds a command key.

Q:

都有哪些"Magic System Request"

A:

echo "..." > /proc/sysrq-trigger

--------------------------------------------------------------------------
b   immediately reboot the system without syncing or unmounting your disks
c   perform a system crash by a NULL pointer dereference. A crashdump will
    be taken if configured
d   shows all locks that are held
e   send a SIGTERM to all processes, except for init
f   call oom_kill to kill a memory hog process
g   used by kgdb (kernel debugger)
h   display help (actually any other key than those listed here will display help)
i   send a SIGKILL to all processes, except for init
j   forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl
k   Secure Access Key (SAK) Kills all programs on the current virtual console
l   shows a stack backtrace for all active CPUs
m   dump current memory info to your console
n   used to make RT tasks nice-able
o   shut your system off (if configured and supported)
p   dump the current registers and flags to your console
q   dump per CPU lists of all armed hrtimers (but NOT regular timer_list
    timers) and detailed information about all clockevent devices
r   turns off keyboard raw mode and sets it to XLATE
s   attempt to sync all mounted filesystems
t   dump a list of current tasks and their information to your console
u   attempt to remount all mounted filesystems read-only
v   forcefully restores framebuffer console

    causes ETM buffer dump [ARM-specific]

w   dumps tasks that are in uninterruptable (blocked) state
x   used by xmon interface on ppc/powerpc platforms

    show global PMU Registers on sparc64

y   show global CPU Registers [SPARC-64 specific]
z   dump the ftrace buffer
0-9 sets the console_loglevel. for example 0 would make it so that only
    emergency messages like PANICs or OOPSes would make it to your console.
--------------------------------------------------------------------------

# echo "h" > /proc/sysrq-trigger
# dmesg | tail -1
[100383.935040] SysRq : HELP : ...

原来是单行输出，整理一下:

loglevel(0-9)
reboot(b)
crash(c)
terminate-all-tasks(e)
memory-full-oom-kill(f)
kill-all-tasks(i)
thaw-filesystems(j)
sak(k)
show-backtrace-all-active-cpus(l)
show-memory-usage(m)
nice-all-RT-tasks(n)
poweroff(o)
show-registers(p)
show-all-timers(q)
unraw(r)
sync(s)
show-task-states(t)
unmount(u)
show-blocked-tasks(w)
dump-ftrace-buffer(z)

在我这个测试环境中，不支持"d"。