linux-kernel - Re: [PATCH] ocfs2: fix check if list iterator did find an element

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Date:   Tue, 22 Mar 2022 10:15:34 +0800
From:   Joseph Qi <joseph.qi@...ux.alibaba.com>
To:     Jakob Koschel <jakobkoschel@...il.com>
Cc:     Mark Fasheh <mark@...heh.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ocfs2-devel@....oracle.com, Joel Becker <jlbec@...lplan.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Miguel Ojeda <ojeda@...nel.org>,
        Mike Rapoport <rppt@...nel.org>,
        Brian Johannesmeyer <bjohannesmeyer@...il.com>,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        "Bos, H.J." <h.j.bos@...nl>,
        Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: [PATCH] ocfs2: fix check if list iterator did find an element



On 3/21/22 9:34 PM, Jakob Koschel wrote:
> 
>> On 21. Mar 2022, at 02:50, Joseph Qi <joseph.qi@...ux.alibaba.com> wrote:
>>
>>
>>
>> On 3/20/22 4:31 AM, Jakob Koschel wrote:
>>> Instead of setting 'res' to NULL, it should only be set if
>>> the suitable element was found.
>>>
>>> In the original code 'res' would have been set to an incorrect pointer
>>> if the list is empty.
>>>
>> The logic before iteration can make sure track_list won't be empty.
>> Please refer the discussion via:
>> https://lore.kernel.org/ocfs2-devel/bd0ec87e-b490-83dc-2363-5e5342c59fa4@linux.alibaba.com/T/#m96d4397930201d83d68677c33a9721ae8dbd8f15
> 
> ah yes, I just read up on the discussion there, sorry for having duplicated it
> here.
> 
> Was any conclusion reached there which fixes can/should be merged?
> 
> This code obviously can always be safe if the list cannot be empty.
> That's also not necessarily the reason I'm fixing this. The reason is that
> we want to get rid of any use of the list iterator variable after the loop
> ('res' in this case). This will allow moving the list iterator variable
> into the scope of the list iterator macro to forbid any invalid use of it
> at compile time. Like this you don't have to rely on assumptions that are
> hard to validate (e.g. that a certain list is never empty).
> 
> The patch here is the minimal change to simply do that but looking at
> Dan Carpenter patch there might be more things in this code that can
> be simplified.
> 
Agree, so I'm fine with this change.
So could you please update the description and send v2?

Thanks,
Joseph

> [CC'd Dan Carpenter]
> 
> See [1] for changes that have already been merged:
> 
> [1] https://lore.kernel.org/linux-kernel/20220308171818.384491-3-jakobkoschel@gmail.com/
> 
>>
>> Thanks,
>> Joseph
>>
>>> In preparation to limit the scope of the list iterator to the list
>>> traversal loop, use a dedicated pointer pointing to the found element [1].
>>>
>>> Link: https://lore.kernel.org/all/YhdfEIwI4EdtHdym@kroah.com/
>>> Signed-off-by: Jakob Koschel <jakobkoschel@...il.com>
>>> ---
>>> fs/ocfs2/dlm/dlmdebug.c | 12 ++++++------
>>> 1 file changed, 6 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/fs/ocfs2/dlm/dlmdebug.c b/fs/ocfs2/dlm/dlmdebug.c
>>> index d442cf5dda8a..be5e9ed7da8d 100644
>>> --- a/fs/ocfs2/dlm/dlmdebug.c
>>> +++ b/fs/ocfs2/dlm/dlmdebug.c
>>> @@ -541,7 +541,7 @@ static void *lockres_seq_start(struct seq_file *m, loff_t *pos)
>>> 	struct debug_lockres *dl = m->private;
>>> 	struct dlm_ctxt *dlm = dl->dl_ctxt;
>>> 	struct dlm_lock_resource *oldres = dl->dl_res;
>>> -	struct dlm_lock_resource *res = NULL;
>>> +	struct dlm_lock_resource *res = NULL, *iter;
>>> 	struct list_head *track_list;
>>>
>>> 	spin_lock(&dlm->track_lock);
>>> @@ -556,11 +556,11 @@ static void *lockres_seq_start(struct seq_file *m, loff_t *pos)
>>> 		}
>>> 	}
>>>
>>> -	list_for_each_entry(res, track_list, tracking) {
>>> -		if (&res->tracking == &dlm->tracking_list)
>>> -			res = NULL;
>>> -		else
>>> -			dlm_lockres_get(res);
>>> +	list_for_each_entry(iter, track_list, tracking) {
>>> +		if (&iter->tracking != &dlm->tracking_list) {
>>> +			dlm_lockres_get(iter);
>>> +			res = iter;
>>> +		}
>>> 		break;
>>> 	}
>>> 	spin_unlock(&dlm->track_lock);
>>>
>>> base-commit: 34e047aa16c0123bbae8e2f6df33e5ecc1f56601
>>> --
>>> 2.25.1
> 
> 	Jakob