[PATCH] erofs: set SB_NODEV sb_flags when mounting with fsid

[Christian Brauner]

On Mon, Apr 15, 2024 at 08:17:46PM +0800, Baokun Li wrote:
> When erofs_kill_sb() is called in block dev based mode, s_bdev may not have
> been initialised yet, and if CONFIG_EROFS_FS_ONDEMAND is enabled, it will
> be mistaken for fscache mode, and then attempt to free an anon_dev that has
> never been allocated, triggering the following warning:
> 
> ============================================
> ida_free called for id=0 which is not allocated.
> WARNING: CPU: 14 PID: 926 at lib/idr.c:525 ida_free+0x134/0x140
> Modules linked in:
> CPU: 14 PID: 926 Comm: mount Not tainted 6.9.0-rc3-dirty #630
> RIP: 0010:ida_free+0x134/0x140
> Call Trace:
>  <TASK>
>  erofs_kill_sb+0x81/0x90
>  deactivate_locked_super+0x35/0x80
>  get_tree_bdev+0x136/0x1e0
>  vfs_get_tree+0x2c/0xf0
>  do_new_mount+0x190/0x2f0
>  [...]
> ============================================
> 
> To avoid this problem, add SB_NODEV to fc->sb_flags after successfully
> parsing the fsid, and then the superblock inherits this flag when it is
> allocated, so that the sb_flags can be used to distinguish whether it is
> in block dev based mode when calling erofs_kill_sb().
> 
> Signed-off-by: Baokun Li <libaokun1 at huawei.com>
> ---
>  fs/erofs/super.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/erofs/super.c b/fs/erofs/super.c
> index b21bd8f78dc1..7539ce7d64bc 100644
> --- a/fs/erofs/super.c
> +++ b/fs/erofs/super.c
> @@ -520,6 +520,7 @@ static int erofs_fc_parse_param(struct fs_context *fc,
>  		ctx->fsid = kstrdup(param->string, GFP_KERNEL);
>  		if (!ctx->fsid)
>  			return -ENOMEM;
> +		fc->sb_flags |= SB_NODEV;

Hm, I wouldn't do it this way. That's an abuse of that flag imho.
Record the information in the erofs_fs_context if you need to.