linux-kernel - Re: [PATCH] debugfs: fix potential infinite loop in debugfs_remove

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Date:   Thu, 14 Nov 2019 10:01:23 +0800
From:   "yukuai (C)" <yukuai3@...wei.com>
To:     Steven Rostedt <rostedt@...dmis.org>
CC:     <gregkh@...uxfoundation.org>, <rafael@...nel.org>,
        <oleg@...hat.com>, <jack@...e.cz>, <linux-kernel@...r.kernel.org>,
        <zhengbin13@...wei.com>, <yi.zhang@...wei.com>,
        <chenxiang66@...ilicon.com>, <xiexiuqi@...wei.com>
Subject: Re: [PATCH] debugfs: fix potential infinite loop in
 debugfs_remove_recursive

Thanks for your explanation

On 2019/11/14 4:17, Steven Rostedt wrote:
> On Thu, 31 Oct 2019 21:34:44 +0800
> yu kuai <yukuai3@...wei.com> wrote:
> 
>> debugfs_remove_recursive uses list_empty to judge weather a dentry has
>> any subdentry or not. This can lead to infinite loop when any subdir is in
>> use.
>>
>> The problem was discoverd by the following steps in the console.
>> 1. use debugfs_create_dir to create a dir and multiple subdirs(insmod);
>> 2. cd to the subdir with depth not less than 2;
>> 3. call debugfs_remove_recursive(rmmod).
>>
>> After removing the subdir, the infinite loop is triggered bucause
> 
>    s/bucause/because/
> 
>> debugfs_remove_recursive uses list_empty to judge if the current dir
>> doesn't have any subdentry, if so, remove the current dir and which
>> will never happen.
>>
>> Fix the problem by using simple_empty instead of list_empty.
>>
>> Fixes: 776164c1faac ('debugfs: debugfs_remove_recursive() must not rely on list_empty(d_subdirs)')
>> Reported-by: chenxiang66@...ilicon.com
>> Signed-off-by: yu kuai <yukuai3@...wei.com>
>> ---
>>   fs/debugfs/inode.c | 5 +++--
>>   1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
>> index 7b975db..42b28acc 100644
>> --- a/fs/debugfs/inode.c
>> +++ b/fs/debugfs/inode.c
>> @@ -773,8 +773,10 @@ void debugfs_remove_recursive(struct dentry *dentry)
>>   		if (!simple_positive(child))
>>   			continue;
>>   
>> -		/* perhaps simple_empty(child) makes more sense */
>> -		if (!list_empty(&child->d_subdirs)) {
>> +		/* use simple_empty to prevent infinite loop when any
>> +		 * subdentry of child is in use
>> +		 */
> 
> Nit, multi-line comments should be of the form:
> 
> 	/*
> 	 * comment line 1
> 	 * comment line 2
> 	 */
> 
> Not
> 
> 	/* comment line 1
> 	 * comment line 2
> 	 */
> 
> It's known that the networking folks like that method, but it's not
> acceptable anywhere outside of networking.
> 
Do you agree with that list_empty(&chile->d_subdirs) here is not 
appropriate? Since it can't skip the subdirs that is not 
simple_positive(simple_positive() will return false), which is the 
reason of infinite loop.
>> +		if (!simple_empty(child)) {
> 
> Have you tried this with lockdep enabled? I'm thinking that you might
> get a splat with holding parent->d_lock and simple_empty(child) taking
> the child->d_lock.
The locks are taken and released in the right order:
take parent->d_lock
	take child->d_lock
		list_for_each_entry(c, &child->d_sundirs, d_child)
			take c->d_lock
			release c->d_lock
	release child->d_lock
release parent->d_lock
I don't see anything wrong, am I missing something?

Thanks
Yu Kuai
> 
> -- Steve
> 
> 
>>   			spin_unlock(&parent->d_lock);
>>   			inode_unlock(d_inode(parent));
>>   			parent = child;
> 
> 
> .
>