linux-kernel - Re: [PATCH] block/rnbd-clt: improve find_or_create

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Date:   Mon, 11 Jan 2021 06:49:39 -0800
From:   Tom Rix <trix@...hat.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     danil.kipnis@...ud.ionos.com, jinpu.wang@...ud.ionos.com,
        axboe@...nel.dk, ndesaulniers@...gle.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] block/rnbd-clt: improve find_or_create_sess() return
 check


On 1/10/21 9:14 PM, Nathan Chancellor wrote:
> On Sun, Jan 10, 2021 at 01:57:26PM -0800, trix@...hat.com wrote:
>> From: Tom Rix <trix@...hat.com>
>>
>> clang static analysis reports this problem
>>
>> rnbd-clt.c:1212:11: warning: Branch condition evaluates to a
>>   garbage value
>>         else if (!first)
>>                  ^~~~~~
> Ah, is it complaining that the 'if (IS_ERR(sess)) {' section in
> find_or_create_sess() does not initialize first even though that will be
> caught by the 'if (sess == ERR_PTR(-ENOMEM))' in
> find_and_get_or_create_sess() because alloc_sess() only returns an
> -ENOMEM error pointer?

Reviewing the code, failure looks like it returns only -ENOMEM.

So the check is correct but brittle.

>
>> This is triggered in the find_and_get_or_create_sess() call
>> because the variable first is not initialized and the
>> earlier check is specifically for
>>
>> 	if (sess == ERR_PTR(-ENOMEM))
>>
>> This is false positive.
>>
>> But the if-check can be reduced by initializing first to
>> false and then returning if the call to find_or_creat_sess()
>> does not set it to true.  When it remains false, either
>> sess will be valid or not.  The not case is caught by
>> find_and_get_or_create_sess()'s caller rnbd_clt_map_device()
>>
>> 	sess = find_and_get_or_create_sess(...);
>> 	if (IS_ERR(sess))
>> 		return ERR_CAST(sess);
>>
>> Since find_and_get_or_create_sess() initializes first to false
>> setting it in find_or_create_sess() is not needed.
>>
>> Signed-off-by: Tom Rix <trix@...hat.com>
> Every maintainer has their preference for where and how stuff gets
> initialized but this makes sense to me. I am not sure the commit above
> find_or_create_sess() is needed but again, personal preference.

Mostly this removes two unneeded branches at the cost of initializing a variable.

Secondary, the static analysis complaint is resolved.

Tom

>
> Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
>
>> ---
>>  drivers/block/rnbd/rnbd-clt.c | 10 ++++------
>>  1 file changed, 4 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/block/rnbd/rnbd-clt.c b/drivers/block/rnbd/rnbd-clt.c
>> index 96e3f9fe8241..251f747cf10d 100644
>> --- a/drivers/block/rnbd/rnbd-clt.c
>> +++ b/drivers/block/rnbd/rnbd-clt.c
>> @@ -919,6 +919,7 @@ static struct rnbd_clt_session *__find_and_get_sess(const char *sessname)
>>  	return NULL;
>>  }
>>  
>> +/* caller is responsible for initializing 'first' to false */
>>  static struct
>>  rnbd_clt_session *find_or_create_sess(const char *sessname, bool *first)
>>  {
>> @@ -934,8 +935,7 @@ rnbd_clt_session *find_or_create_sess(const char *sessname, bool *first)
>>  		}
>>  		list_add(&sess->list, &sess_list);
>>  		*first = true;
>> -	} else
>> -		*first = false;
>> +	}
>>  	mutex_unlock(&sess_lock);
>>  
>>  	return sess;
>> @@ -1203,13 +1203,11 @@ find_and_get_or_create_sess(const char *sessname,
>>  	struct rnbd_clt_session *sess;
>>  	struct rtrs_attrs attrs;
>>  	int err;
>> -	bool first;
>> +	bool first = false;
>>  	struct rtrs_clt_ops rtrs_ops;
>>  
>>  	sess = find_or_create_sess(sessname, &first);
>> -	if (sess == ERR_PTR(-ENOMEM))
>> -		return ERR_PTR(-ENOMEM);
>> -	else if (!first)
>> +	if (!first)
>>  		return sess;
>>  
>>  	if (!path_cnt) {
>> -- 
>> 2.27.0
>>