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From:   Leon Romanovsky <leon@kernel.org>
To:     Al Viro <viro@zeniv.linux.org.uk>, Christoph Hellwig <hch@lst.de>
Cc:     Leon Romanovsky <leonro@nvidia.com>, linux-kernel@vger.kernel.org,
        Jason Gunthorpe <jgg@nvidia.com>
Subject: [PATCH -rc] init: don't panic if mount_nodev_root failed
Date:   Tue, 14 Sep 2021 12:12:10 +0300
Message-Id: <b83c61f6ab34c8f17672d88e0853a9d324c3d48a.1631610487.git.leonro@nvidia.com>
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From: Leon Romanovsky <leonro@nvidia.com>

Attempt to mount 9p file system as root gives the following kernel panic:

 9pnet_virtio: no channels available for device root
 Kernel panic - not syncing: VFS: Unable to mount root "root" (9p), err=-2
 CPU: 2 PID: 1 Comm: swapper/0 Not tainted 5.15.0-rc1+ #127
 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
 Call Trace:
  dump_stack_lvl+0x45/0x59
  panic+0x1e2/0x44b
  ? __warn_printk+0xf3/0xf3
  ? free_unref_page+0x2d4/0x4a0
  ? trace_hardirqs_on+0x32/0x120
  ? free_unref_page+0x2d4/0x4a0
  mount_root+0x189/0x1e0
  prepare_namespace+0x136/0x165
  kernel_init_freeable+0x3b8/0x3cb
  ? rest_init+0x2e0/0x2e0
  kernel_init+0x19/0x130
  ret_from_fork+0x1f/0x30
 Kernel Offset: disabled
 ---[ end Kernel panic - not syncing: VFS: Unable to mount root "root" (9p), err=-2 ]---

QEMU command line:
 "qemu-system-x86_64 -append root=/dev/root rw rootfstype=9p rootflags=trans=virtio ..."

This error is because root_device_name is truncated in prepare_namespace() from
being "/dev/root" to be "root" prior to call to mount_nodev_root().

As a solution, don't treat errors in mount_nodev_root() as errors that
require panics and allow failback to the mount flow that existed before
patch citied in Fixes tag.

Fixes: f9259be6a9e7 ("init: allow mounting arbitrary non-blockdevice filesystems as root")
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
---
I'm not sure if this is the right thing to do, but it works for me.
---
 init/do_mounts.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/init/do_mounts.c b/init/do_mounts.c
index 2ed30ff6c906..b423ea7dcc6b 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -553,9 +553,6 @@ static int __init mount_nodev_root(void)
 				    root_mount_data);
 		if (!err)
 			break;
-		if (err != -EACCES && err != -EINVAL)
-			panic("VFS: Unable to mount root \"%s\" (%s), err=%d\n",
-			      root_device_name, fstype, err);
 	}
 
 	free_page((unsigned long)fs_names);
-- 
2.31.1