Re: [PATCH 3/3] xfs: check _alloc_read_agf buffer pointer before using

[Brian Foster <bfoster@redhat.com>]

On Tue, Jul 18, 2017 at 11:24:28AM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> In some circumstances, _alloc_read_agf can return an error code of zero
> but also a null AGF buffer pointer.  Check for this and jump out.
> 

It looks like this is only possible in trylock cases. Otherwise (and
unless I'm missing something), it should always return a buffer or
error.

This is circuitous regardless and so seems fine if it shuts up a
coverity warning:

Reviewed-by: Brian Foster <bfoster@redhat.com>

> Fixes-coverity-id: 1415250
> Fixes-coverity-id: 1415320
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
>  fs/xfs/libxfs/xfs_refcount.c |    4 ++++
>  fs/xfs/xfs_reflink.c         |    2 ++
>  2 files changed, 6 insertions(+)
> 
> 
> diff --git a/fs/xfs/libxfs/xfs_refcount.c b/fs/xfs/libxfs/xfs_refcount.c
> index 900ea23..45b1c3b 100644
> --- a/fs/xfs/libxfs/xfs_refcount.c
> +++ b/fs/xfs/libxfs/xfs_refcount.c
> @@ -1638,6 +1638,10 @@ xfs_refcount_recover_cow_leftovers(
>  	error = xfs_alloc_read_agf(mp, tp, agno, 0, &agbp);
>  	if (error)
>  		goto out_trans;
> +	if (!agbp) {
> +		error = -ENOMEM;
> +		goto out_trans;
> +	}
>  	cur = xfs_refcountbt_init_cursor(mp, tp, agbp, agno, NULL);
>  
>  	/* Find all the leftover CoW staging extents. */
> diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> index d9b3d57..f45fbf0 100644
> --- a/fs/xfs/xfs_reflink.c
> +++ b/fs/xfs/xfs_reflink.c
> @@ -170,6 +170,8 @@ xfs_reflink_find_shared(
>  	error = xfs_alloc_read_agf(mp, tp, agno, 0, &agbp);
>  	if (error)
>  		return error;
> +	if (!agbp)
> +		return -ENOMEM;
>  
>  	cur = xfs_refcountbt_init_cursor(mp, tp, agbp, agno, NULL);
>  
> 
> --
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