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Subject[PATCH] init: Allow distribution configuration of default init
Some init systems (eg. systemd) have init at their own paths, for
example, /usr/lib/systemd/systemd. A compatibility symlink to one of the
hardcoded init paths is provided by another package, usually named
something like systemd-sysvcompat or similar.

Currently distro maintainers who are hands-off on the bootloader are
more or less required to include those compatibility links as part of
their base distribution, because it's hard to migrate away from them
since there's a risk some users will not get the message to set init= on
the kernel command line appropriately.

Moreover, for distributions where the init system is something the
distribution itself is opinionated about (eg. Arch, which has systemd in
the required `base` package), we could usually reasonably configure this
ahead of time when building the distribution kernel. However, we
currently simply don't have any way to configure the kernel to do this.
Here's an example discussion where removing sysvcompat was discussed by
distro maintainers[0].

This patch adds a new Kconfig tunable, CONFIG_DEFAULT_INIT, which if set
is tried before the hardcoded fallback list. So the order of precedence
is now thus:

1. init= on command line (on failure: panic)
2. CONFIG_DEFAULT_INIT (on failure: try #3)
3. Hardcoded fallback list (on failure: panic)

This new config parameter will allow distribution maintainers to move
away from these compatibility links safely, without having to worry that
their users might not have the right init=.

There are also two other benefits of this over having the distribution
maintain a symlink:

1. One of the value propositions over simply having distributions
maintain a /sbin/init symlink via a package is that it also frees
distributions which have a preferred default, but not mandatory, init
system from having their package manager fight with their users for
control of /{s,}bin/init. Instead, the distribution simply makes
their preference known in CONFIG_DEFAULT_INIT, and if the user
installs another init system and uninstalls the default one they can
still make use of /{s,}bin/init and friends for their own uses. This
makes more cases Just Work(tm) without the user having to perform
extra configuration via init=.
2. Since before this we don't know which path the distribution actually
_intends_ to serve init from, we don't pr_err if it is simply
missing, and usually will just silently put the user in a /bin/sh
shell. Now that the distribution can make a declaration of intent, we
can be more vocal when this init system fails to launch for any
reason, even if it's simply because no file exists at that location,
speeding up the palaver of init/mount dependency/etc debugging a bit.

0: https://lists.archlinux.org/pipermail/arch-dev-public/2019-January/029435.html

Signed-off-by: Chris Down <chris@chrisdown.name>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
---
init/Kconfig | 10 ++++++++++
init/main.c | 10 ++++++++++
2 files changed, 20 insertions(+)

diff --git a/init/Kconfig b/init/Kconfig
index bff51e542353..a8136131c108 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -284,6 +284,16 @@ config KERNEL_UNCOMPRESSED

endchoice

+config DEFAULT_INIT
+ string "Default init path"
+ default ""
+ help
+ This option determines the default init for the system if no init=
+ option is passed on the kernel command line. If the requested path is
+ not present, we will still then move on to attempting further
+ locations (e.g. /sbin/init, etc). If this is empty, we will just use
+ the fallback list when init= is not passed.
+
config DEFAULT_HOSTNAME
string "Default hostname"
default "(none)"
diff --git a/init/main.c b/init/main.c
index eb7e0d16e958..0ead83e86b5a 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1437,6 +1437,16 @@ static int __ref kernel_init(void *unused)
panic("Requested init %s failed (error %d).",
execute_command, ret);
}
+
+ if (CONFIG_DEFAULT_INIT[0] != '\0') {
+ ret = run_init_process(CONFIG_DEFAULT_INIT);
+ if (ret)
+ pr_err("Default init %s failed (error %d)\n",
+ CONFIG_DEFAULT_INIT, ret);
+ else
+ return 0;
+ }
+
if (!try_to_run_init_process("/sbin/init") ||
!try_to_run_init_process("/etc/init") ||
!try_to_run_init_process("/bin/init") ||
--
2.26.2
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