FAILED: patch "[PATCH] rootfs: Fix support for rootfstype= when root= is given" failed to apply to 4.19-stable tree

[<gregkh@xxxxxxxxxxxxxxxxxxx>]

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@xxxxxxxxxxxxxxx>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-4.19.y
git checkout FETCH_HEAD
git cherry-pick -x 21528c69a0d8483f7c6345b1a0bc8d8975e9a172
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@xxxxxxxxxxxxxxx>' --in-reply-to '2024012238-coke-debrief-5c9b@gregkh' --subject-prefix 'PATCH 4.19.y' HEAD^..

Possible dependencies:

21528c69a0d8 ("rootfs: Fix support for rootfstype= when root= is given")
037f11b4752f ("mnt_init(): call shmem_init() unconditionally")
fd3e007f6c6a ("don't bother with registering rootfs")
14a253ce4210 ("init_rootfs(): don't bother with init_ramfs_fs()")
9bc61ab18b1d ("vfs: Introduce fs_context, switch vfs_kern_mount() to it.")
505b050fdf42 ("Merge branch 'mount.part1' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs")

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

>From 21528c69a0d8483f7c6345b1a0bc8d8975e9a172 Mon Sep 17 00:00:00 2001
From: Stefan Berger <stefanb@xxxxxxxxxxxxx>
Date: Sun, 19 Nov 2023 20:12:48 -0500
Subject: [PATCH] rootfs: Fix support for rootfstype= when root= is given

Documentation/filesystems/ramfs-rootfs-initramfs.rst states:

  If CONFIG_TMPFS is enabled, rootfs will use tmpfs instead of ramfs by
  default.  To force ramfs, add "rootfstype=ramfs" to the kernel command
  line.

This currently does not work when root= is provided since then
saved_root_name contains a string and rootfstype= is ignored. Therefore,
ramfs is currently always chosen when root= is provided.

The current behavior for rootfs's filesystem is:

   root=       | rootfstype= | chosen rootfs filesystem
   ------------+-------------+--------------------------
   unspecified | unspecified | tmpfs
   unspecified | tmpfs       | tmpfs
   unspecified | ramfs       | ramfs
    provided   | ignored     | ramfs

rootfstype= should be respected regardless whether root= is given,
as shown below:

   root=       | rootfstype= | chosen rootfs filesystem
   ------------+-------------+--------------------------
   unspecified | unspecified | tmpfs  (as before)
   unspecified | tmpfs       | tmpfs  (as before)
   unspecified | ramfs       | ramfs  (as before)
    provided   | unspecified | ramfs  (compatibility with before)
    provided   | tmpfs       | tmpfs  (new)
    provided   | ramfs       | ramfs  (new)

This table represents the new behavior.

Fixes: 6e19eded3684 ("initmpfs: use initramfs if rootfstype= or root= specified")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Rob Landley <rob@xxxxxxxxxxx>
Link: https://lore.kernel.org/lkml/8244c75f-445e-b15b-9dbf-266e7ca666e2@xxxxxxxxxxx/
Reviewed-and-Tested-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20231120011248.396012-1-stefanb@xxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

diff --git a/init/do_mounts.c b/init/do_mounts.c
index 5fdef94f0864..279ad28bf4fb 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -510,7 +510,10 @@ struct file_system_type rootfs_fs_type = {
 
 void __init init_rootfs(void)
 {
-	if (IS_ENABLED(CONFIG_TMPFS) && !saved_root_name[0] &&
-		(!root_fs_names || strstr(root_fs_names, "tmpfs")))
-		is_tmpfs = true;
+	if (IS_ENABLED(CONFIG_TMPFS)) {
+		if (!saved_root_name[0] && !root_fs_names)
+			is_tmpfs = true;
+		else if (root_fs_names && !!strstr(root_fs_names, "tmpfs"))
+			is_tmpfs = true;
+	}
 }