linux-kernel - Re: Explanation on Uninitialized Variable bio in blk_rq_prep

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Message-ID: <759193c0-c831-46bc-8194-9b3525eeeae5@gmail.com>
Date: Sun, 6 Oct 2024 12:41:34 +0530
From: Suraj Sonawane <surajsonawane0215@...il.com>
To: John Garry <john.g.garry@...cle.com>, hch@...radead.org
Cc: axboe@...nel.dk, linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Explanation on Uninitialized Variable bio in blk_rq_prep_clone

On 06/10/24 12:28, Suraj Sonawane wrote:
> On 04/10/24 20:03, John Garry wrote:
>> On 04/10/2024 15:10, SurajSonawane2415 wrote:
>>> Explaination of how bio could be used uninitialized in this function:
>>>
>>> In the function blk_rq_prep_clone, the variable bio is declared but 
>>> can remain uninitialized
>>> if the allocation with bio_alloc_clone fails. This can lead to 
>>> undefined behavior when the
>>> function attempts to free bio in the error handling section using 
>>> bio_put(bio).
>>> By initializing bio to NULL at declaration, we ensure that the 
>>> cleanup code will only
>>> interact with bio if it has been successfully allocated.
>>>
>>>
>>
>> What about if rq_src->bio is NULL for blk_rq_prep_clone() -> 
>> __rq_for_each_bio(,rq_src):
>>
>> #define __rq_for_each_bio(_bio, rq)    \
>>      if ((rq->bio))            \
>>          for (_bio = (rq)->bio; _bio; _bio = _bio->bi_next)
>>
>> Then I don't think bio it get init'ed. Whether this is possible 
>> (rq_src->bio is NULL) is another question.
> 
> Hi Keith,

I realized I mistakenly addressed my reply to you as "Keith" in this 
message. Apologies for the confusion. Thank you again for your input!

> 
> You're right to bring this up. If rq_src->bio is NULL, the 
> __rq_for_each_bio macro will skip the loop, meaning the bio variable 
> won't be used at all. So, even if bio isn’t initialized, it won't cause 
> any issues in that case.
> 
> Thanks for pointing that out.
> 
> Best regards,
> Suraj

Best regards,
Suraj