linux-kernel - RE: [PATCH 3/7] fs, xfs: convert xfs_buf_log_item.bli_refcount from atomic_t to refcount

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Date:   Tue, 21 Feb 2017 16:06:30 +0000
From:   "Reshetova, Elena" <elena.reshetova@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "darrick.wong@...cle.com" <darrick.wong@...cle.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        Kees Cook <keescook@...omium.org>,
        David Windsor <dwindsor@...il.com>
Subject: RE: [PATCH 3/7] fs, xfs: convert xfs_buf_log_item.bli_refcount from
 atomic_t to refcount_t

> On Tue, Feb 21, 2017 at 05:49:03PM +0200, Elena Reshetova wrote:
> > refcount_t type and corresponding API should be
> > used instead of atomic_t when the variable is used as
> > a reference counter. This allows to avoid accidental
> > refcounter overflows that might lead to use-after-free
> > situations.
> 
> Changelog forgets to mention if this was runtime tested..

It was boot-tested in the whole refcount_t changes pile, which is not very useful for fs anyway. 
What's why we are sending this through maintainers to get through their tests. 
I am sure that testing would be better than what we can do. 

> 
> 
> > @@ -371,7 +371,7 @@ xfs_trans_brelse(xfs_trans_t	*tp,
> >  	ASSERT(bip->bli_item.li_type == XFS_LI_BUF);
> >  	ASSERT(!(bip->bli_flags & XFS_BLI_STALE));
> >  	ASSERT(!(bip->__bli_format.blf_flags & XFS_BLF_CANCEL));
> > -	ASSERT(atomic_read(&bip->bli_refcount) > 0);
> > +	ASSERT(refcount_read(&bip->bli_refcount) > 0);
> >
> >  	trace_xfs_trans_brelse(bip);
> >
> > @@ -419,7 +419,7 @@ xfs_trans_brelse(xfs_trans_t	*tp,
> >  	/*
> >  	 * Drop our reference to the buf log item.
> >  	 */
> > -	atomic_dec(&bip->bli_refcount);
> > +	refcount_dec(&bip->bli_refcount);
> >
> >  	/*
> >  	 * If the buf item is not tracking data in the log, then
> > @@ -432,7 +432,7 @@ xfs_trans_brelse(xfs_trans_t	*tp,
> >  /***
> >  		ASSERT(bp->b_pincount == 0);
> >  ***/
> > -		ASSERT(atomic_read(&bip->bli_refcount) == 0);
> > +		ASSERT(refcount_read(&bip->bli_refcount) == 0);
> >  		ASSERT(!(bip->bli_item.li_flags & XFS_LI_IN_AIL));
> >  		ASSERT(!(bip->bli_flags &
> XFS_BLI_INODE_ALLOC_BUF));
> >  		xfs_buf_item_relse(bp);
> 
> 
> This for example looks dodgy.
> 
> That seems to suggest the atomic_dec() there can actually hit 0, which
> _will_ generate a WARN.

True, but in some of this cases WARN might be ok, I think? As soon as functionality is not changed and object is not reused (by doing refcount_inc on it) anywhere later on.