linux-kernel - Re: [PATCH -rc] init: don't panic if mount_nodev

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Date:   Sun, 19 Sep 2021 08:46:39 +0300
From:   Leon Romanovsky <leonro@...dia.com>
To:     Al Viro <viro@...iv.linux.org.uk>, Christoph Hellwig <hch@....de>
CC:     <linux-kernel@...r.kernel.org>, Jason Gunthorpe <jgg@...dia.com>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>
Subject: Re: [PATCH -rc] init: don't panic if mount_nodev_root failed

On Tue, Sep 14, 2021 at 12:12:10PM +0300, Leon Romanovsky wrote:
> From: Leon Romanovsky <leonro@...dia.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@...dia.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(-)

Hi,

I assume that after Christoph gave his ROB [1], the patch is ok.
Should I do anything else in order to progress with merging it?

Thanks

[1] https://lore.kernel.org/all/20210914150709.GA32708@lst.de/