linux-kernel - Re: [PATCH] erofs: set SB_NODEV sb

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Date: Mon, 15 Apr 2024 23:23:58 +0800
From: Baokun Li <libaokun1@...wei.com>
To: Christian Brauner <brauner@...nel.org>
CC: <linux-erofs@...ts.ozlabs.org>, <xiang@...nel.org>, <chao@...nel.org>,
	<huyue2@...lpad.com>, <jefflexu@...ux.alibaba.com>,
	<viro@...iv.linux.org.uk>, <linux-kernel@...r.kernel.org>,
	<yangerkun@...wei.com>, <houtao1@...wei.com>, Baokun Li
	<libaokun1@...wei.com>
Subject: Re: [PATCH] erofs: set SB_NODEV sb_flags when mounting with fsid

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@...wei.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.

do_new_mount
  fs_context_for_mount
   alloc_fs_context
    erofs_init_fs_context
     // Mount options have not been parsed yet,
     // it is not clear if there is an fsid
  parse_monolithic_mount_data
   generic_parse_monolithic
    erofs_fc_parse_param
     case Opt_fsid:
       // fc->sb_flags |= SB_NODEV;
  vfs_get_tree
   erofs_fc_get_tree
    get_tree_bdev
     s = sget_dev(fc, dev)
       s = alloc_super()
         s->s_flags = fc->sb_flags;
     setup_bdev_super
       // return error
       sb->s_bdev = bdev // s_bdev is NULL
     deactivate_locked_super
       erofs_kill_sb
         // Block device based mode is considered fscahe mode for nodev
         erofs_is_fscache_mode(sb)
           IS_ENABLED(CONFIG_EROFS_FS_ONDEMAND) && !sb->s_bdev
          kill_anon_super(sb)
            free_anon_bdev(dev)
              ida_free(&unnamed_dev_ida, MINOR(dev))
              ((int)id < 0)
              WARN(1, "ida_free called for id=%d which is not 
allocated.\n", id);


Thanks!
-- 
With Best Regards,
Baokun Li