Re: [PATCH 2/3] xfs: fix off-by-one error when the last rt extent is in use

From: Chandan Babu R <chandanrlinux@gmail.com>
To: "Darrick J. Wong" <djwong@kernel.org>
Cc: linux-xfs@vger.kernel.org
Subject: Re: [PATCH 2/3] xfs: fix off-by-one error when the last rt extent is in use
Date: Fri, 13 Aug 2021 16:20:34 +0530	[thread overview]
Message-ID: <87fsvdlijp.fsf@garuda> (raw)
In-Reply-To: <162872992772.1220643.10308054638747493338.stgit@magnolia>

On 11 Aug 2021 at 17:58, "Darrick J. Wong" <djwong@kernel.org> wrote:
> From: Darrick J. Wong <djwong@kernel.org>
>
> The fsmap implementation for realtime devices uses the gap between
> info->next_daddr and a free rtextent reported by xfs_rtalloc_query_range
> to feed userspace fsmap records with an "unknown" owner.  We use this
> trick to report to userspace when the last rtextent in the filesystem is
> in use by synthesizing a null rmap record starting at the next block
> after the query range.
>
> Unfortunately, there's a minor accounting bug in the way that we
> construct the null rmap record.  Originally, ahigh.ar_startext contains
> the last rtextent for which the user wants records.  It's entirely
> possible that number is beyond the end of the rt volume, so the location
> synthesized rmap record /must/ be constrained to the minimum of the high
> key and the number of extents in the rt volume.
>

When the number of blocks on the realtime device is not an integral multiple
of xfs_sb->sb_rextsize, ahigh.ar_startext can contain a value which is one
more than the highest valid rtextent. Hence, without this patch, the last
record reported to the userpace might contain an invalid upper bound. Assuming
that my understanding is indeed correct,

Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>

> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> ---
>  fs/xfs/xfs_fsmap.c |   22 +++++++++++++++++-----
>  1 file changed, 17 insertions(+), 5 deletions(-)
>
>
> diff --git a/fs/xfs/xfs_fsmap.c b/fs/xfs/xfs_fsmap.c
> index 7d0b09c1366e..a0e8ab58124b 100644
> --- a/fs/xfs/xfs_fsmap.c
> +++ b/fs/xfs/xfs_fsmap.c
> @@ -523,27 +523,39 @@ xfs_getfsmap_rtdev_rtbitmap_query(
>  {
>  	struct xfs_rtalloc_rec		alow = { 0 };
>  	struct xfs_rtalloc_rec		ahigh = { 0 };
> +	struct xfs_mount		*mp = tp->t_mountp;
>  	int				error;
>  
> -	xfs_ilock(tp->t_mountp->m_rbmip, XFS_ILOCK_SHARED);
> +	xfs_ilock(mp->m_rbmip, XFS_ILOCK_SHARED);
>  
> +	/*
> +	 * Set up query parameters to return free extents covering the range we
> +	 * want.
> +	 */
>  	alow.ar_startext = info->low.rm_startblock;
> +	do_div(alow.ar_startext, mp->m_sb.sb_rextsize);
> +
>  	ahigh.ar_startext = info->high.rm_startblock;
> -	do_div(alow.ar_startext, tp->t_mountp->m_sb.sb_rextsize);
> -	if (do_div(ahigh.ar_startext, tp->t_mountp->m_sb.sb_rextsize))
> +	if (do_div(ahigh.ar_startext, mp->m_sb.sb_rextsize))
>  		ahigh.ar_startext++;
> +
>  	error = xfs_rtalloc_query_range(tp, &alow, &ahigh,
>  			xfs_getfsmap_rtdev_rtbitmap_helper, info);
>  	if (error)
>  		goto err;
>  
> -	/* Report any gaps at the end of the rtbitmap */
> +	/*
> +	 * Report any gaps at the end of the rtbitmap by simulating a null
> +	 * rmap starting at the block after the end of the query range.
> +	 */
>  	info->last = true;
> +	ahigh.ar_startext = min(mp->m_sb.sb_rextents, ahigh.ar_startext);
> +
>  	error = xfs_getfsmap_rtdev_rtbitmap_helper(tp, &ahigh, info);
>  	if (error)
>  		goto err;
>  err:
> -	xfs_iunlock(tp->t_mountp->m_rbmip, XFS_ILOCK_SHARED);
> +	xfs_iunlock(mp->m_rbmip, XFS_ILOCK_SHARED);
>  	return error;
>  }
>  

-- 
chandan