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

[Baokun Li <libaokun1@huawei.com>]

On 2024/4/16 8:57, Gao Xiang wrote:
> Hi Christian, Baokun,
>
> On Mon, Apr 15, 2024 at 11:23:58PM +0800, Baokun Li wrote:
>> On 2024/4/15 21:38, Christian Brauner wrote:
>>> 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@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.
>> The stack diagram that triggers the problem is as follows, the call to
>> erofs_kill_sb() fails before fill_super() has been executed, and we can
>> only use super_block to determine whether it is currently in nodev
>> fscahe mode or block device based mode. So if it is recorded in
>> erofs_fs_context (aka fc->fs_private), we can't access the recorded data
>> unless we pass fc into erofs_kill_sb() as well.
>>
> If I understand correctly, from the discussion above, I think
> there exists a gap between alloc_super() and sb->s_bdev is set.
> But .kill_sb() can be called between them and fc is not passed
> into .kill_sb().
>
> I'm not sure how to resolve it in EROFS itself, anyway...
>
> Thanks,
> Gao Xiang
Yes, exactly!

There is no fill_super between alloc_super() and kill_sb(), so erofs has
no way to set a flag for the superblock directly. The only way I can
think of now is to modify fc->sb_flags in erofs_fc_parse_param()
to indirectly set a flag for the superblock.

Cheers,
Baokun