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

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Message-ID: <Zv_-SRguWC4sxWBJ@kbusch-mbp>
Date: Fri, 4 Oct 2024 08:40:09 -0600
From: Keith Busch <kbusch@...nel.org>
To: John Garry <john.g.garry@...cle.com>
Cc: SurajSonawane2415 <surajsonawane0215@...il.com>, hch@...radead.org,
	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 Fri, Oct 04, 2024 at 03:33:00PM +0100, 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.

If the source request doesn't have a bio, then the onstack 'bio' is
never referenced, so should be okay if it's not initialized in that
case.