linux-kernel - [PATCH] Wire up CONFIG_DEVTMPFS

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Message-ID: <d53f041a-83f7-57f7-28ae-eb7b23034f83@landley.net>
Date:   Sun, 16 Jan 2022 12:36:49 -0600
From:   Rob Landley <rob@...dley.net>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: [PATCH] Wire up CONFIG_DEVTMPFS_MOUNT to initramfs.

From: Rob Landley <rob@...dley.net>

The kernel has had CONFIG_DEVTMPFS_MOUNT for years, but it only applied to
fallback ROOT= not initramfs/initmpfs. As long as the config option exists, it
might as well work.

I use this for board bringup: populating a chdir and calling cpio as a normal
user often leaves /dev empty (because mknod requires root access), meaning no
/dev/console for init/main.c to open, meaning init runs without
stdin/stdout/stderr and has to mount devtmpfs and redirect the filehandles blind
with no error output if something goes wrong.

Signed-off-by: Rob Landley <rob@...dley.net>
---
Previously: https://lkml.org/lkml/2017/9/13/651

 init/main.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/init/main.c b/init/main.c
index bb984ed79de0..288c73db57f4 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1612,7 +1612,6 @@ static noinline void __init kernel_init_freeable(void)
 	kunit_run_all_tests();

 	wait_for_initramfs();
-	console_on_rootfs();

 	/*
 	 * check if there is an early userspace init.  If yes, let it do all
@@ -1621,7 +1620,11 @@ static noinline void __init kernel_init_freeable(void)
 	if (init_eaccess(ramdisk_execute_command) != 0) {
 		ramdisk_execute_command = NULL;
 		prepare_namespace();
+	} else if (IS_ENABLED(CONFIG_DEVTMPFS_MOUNT)) {
+		sys_mkdir("/dev", 0755);
+		devtmpfs_mount();
 	}
+	console_on_rootfs();

 	/*
 	 * Ok, we have completed the initial bootup, and