[systemd-devel] why log_set_prohibit_ipc() is set in journald

[Lennart Poettering]

On Fr, 27.08.21 10:01, Mantas Mikulėnas (grawity at gmail.com) wrote:

> On Fri, Aug 27, 2021, 08:52 Nishant Nayan <nayan.nishant2000 at gmail.com>
> wrote:
>
> > I have just started to learn journald and in its main function (in
> > journald.c) I encountered a function call "log_set_prohibit_ipc(true);"
> > In systemd source, I can see the declaration in src/basic/log.h:/*
> >
> > If turned on, then we'll never use IPC-based logging, * i.e. never log to
> > syslog or the journal. We'll only * log to stderr, the console or kmsg
> > */void log_set_prohibit_ipc(bool b);
> >
> > I did not get this because Journald not writing to journal itself by
> > default is strange, isn't it?
> > What is the reason behind it?
> >
>
> My understanding is that the point isn't to prevent logging to journal, but
> to prevent logging *through IPC* specifically, i.e. make sure journald
> doesn't try to create loopback connections to its own sockets. The journald
> daemon is single-threaded, so if it tries to connect to itself, it'll
> deadlock.
>
> But also if journald wants to log a critical error (e.g. running out of
> space or something like that), then it can't really *rely* on journal still
> working...
>
> Afaik, messages written to kmsg will be imported back into the journal
> anyway, but that happens asynchronously so it's fine.

The above describes exactly how it is, and why journald turns of
logging via IPC. journald should not be a client to itself.

Lennart

--
Lennart Poettering, Berlin