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Security researcher and MateSSL founder, Andrew Ayer has uncovered a bug which will either crash or make systemd unstable (depending on who you talk to) on pretty much every linux distro. David Strauss posted a highly critical response to Ayer. In true pedantic nerd-fight fashion there is a bit of back and forth between them over the "true" severity of the issue and what not.
Nerd fights aside, how you feel about this bug, will probably largely depend on how you feel about systemd in general.
The following command, when run as any user, will crash systemd:
NOTIFY_SOCKET=/run/systemd/notify systemd-notify ""
After running this command, PID 1 is hung in the
pausesystem call. You can no longer start and stop daemons. inetd-style services no longer accept connections. You cannot cleanly reboot the system. The system feels generally unstable (e.g. ssh and su hang for 30 seconds since systemd is now integrated with the login system). All of this can be caused by a command that's short enough to fit in a Tweet.Edit (2016-09-28 21:34): Some people can only reproduce if they wrap the command in a
while trueloop. Yay non-determinism!
(Score: 3, Informative) by butthurt on Wednesday October 05 2016, @02:03AM
> You know NT is a microkernel, right?
That was abandoned with Windows NT 4.0:
The major architectural change in this version of Windows NT is the move of the Window Manager, Graphics Device Interface (GDI), and higher-level device drivers from the Win32 environment subsystem into the Windows NT Executive as an executive service.
[...]
The most significant aspect of this change is that graphics services, which used to run in user mode like applications, now run in kernel mode like most of the operating system. The idea behind kernel mode and its alternative, user mode, is to separate applications from the operating system. Applications run in user mode; operating systems run in kernel mode.
Kernel mode is a highly privileged mode of operation where the code has direct access to all memory, including the address spaces of all user-mode processes and applications, and to hardware.
-- https://www.microsoft.com/resources/documentation/windowsnt/4/workstation/reskit/en-us/archi.mspx [microsoft.com]