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authorChris Down <chris@chrisdown.name>2020-06-04 16:50:53 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2020-06-04 19:06:25 -0700
commitada4ab7af1a67db7eb8596709071a97afb2bf7de (patch)
treee1f089e040d15f88d70cf96c00f973509a8dcbf9 /init
parent51da9dfb7f20911ae4e79e9b412a9c2d4c373d4b (diff)
downloadlwn-ada4ab7af1a67db7eb8596709071a97afb2bf7de.tar.gz
lwn-ada4ab7af1a67db7eb8596709071a97afb2bf7de.zip
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> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Masami Hiramatsu <mhiramat@kernel.org> Link: http://lkml.kernel.org/r/20200522160234.GA1487022@chrisdown.name Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'init')
-rw-r--r--init/Kconfig10
-rw-r--r--init/main.c10
2 files changed, 20 insertions, 0 deletions
diff --git a/init/Kconfig b/init/Kconfig
index fdb4f52609c6..2d12d38cdd88 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -260,6 +260,16 @@ 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 df32f67214d2..76df62fc3e2c 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1433,6 +1433,16 @@ 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") ||