netdev - Re: [PATCH net-next] sctp: avoid list_del_init if it's freeing the memory right away

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Message-ID: <20170207194508.GD3414@localhost.localdomain>
Date:   Tue, 7 Feb 2017 17:45:09 -0200
From:   Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, linux-sctp@...r.kernel.org,
        vyasevich@...il.com, nhorman@...driver.com
Subject: Re: [PATCH net-next] sctp: avoid list_del_init if it's freeing the
 memory right away

On Tue, Feb 07, 2017 at 02:21:21PM -0500, David Miller wrote:
> From: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
> Date: Tue,  7 Feb 2017 17:03:21 -0200
> 
> > There is no reason to use list_del_init() in these places as we are
> > going to free/destroy the memory in a few lines below.
> > 
> > Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
> > ---
> >  net/sctp/associola.c     | 14 ++++----------
> >  net/sctp/auth.c          |  8 ++------
> >  net/sctp/chunk.c         |  4 ++--
> >  net/sctp/outqueue.c      | 14 +++++++-------
> >  net/sctp/sm_make_chunk.c |  3 +--
> >  5 files changed, 16 insertions(+), 27 deletions(-)
> > 
> > diff --git a/net/sctp/associola.c b/net/sctp/associola.c
> > index e50dc6d7543fd6acfa7442f3a9ee575203c7718d..7eb9dacfa53a438b20a34319cf01c6c9a591f0c3 100644
> > --- a/net/sctp/associola.c
> > +++ b/net/sctp/associola.c
> > @@ -1638,25 +1638,19 @@ int sctp_assoc_set_id(struct sctp_association *asoc, gfp_t gfp)
> >  static void sctp_assoc_free_asconf_queue(struct sctp_association *asoc)
> >  {
> >  	struct sctp_chunk *asconf;
> > -	struct sctp_chunk *tmp;
> >  
> > -	list_for_each_entry_safe(asconf, tmp, &asoc->addip_chunk_list, list) {
> > -		list_del_init(&asconf->list);
> > +	list_for_each_entry(asconf, &asoc->addip_chunk_list, list)
> >  		sctp_chunk_free(asconf);
> > -	}
> >  }
> 
> This leave freed memory on the asoc->addip_chunk_list, in fact why aren't you seeing

This should be alright, because here we are purging the entire list and
the asoc will also be free right after.

> the BUG_ON() in sctp_chunk_destroy() get triggered?  If you elide the list_del() here
> then the "list_empty(&chunk->list)" check there will not be true.
> 

Good question. I have to double check this, but you're probably right.

> I don't think this transformation here is legal at all.

I removed the BUG_ON check on sctp_auth_shkey_free() for the same
reason, not sure why not for sctp_chunk_destroy().

  Marcelo