linux-kernel - Re: [PATCH 4/5] fs, xfs: convert xfs_cui_log_item.cui_refcount from atomic_t to refcount

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Date:   Mon, 6 Mar 2017 11:14:46 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Elena Reshetova <elena.reshetova@...el.com>
Cc:     linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        peterz@...radead.org, gregkh@...uxfoundation.org,
        david@...morbit.com, Hans Liljestrand <ishkamiel@...il.com>,
        Kees Cook <keescook@...omium.org>,
        David Windsor <dwindsor@...il.com>
Subject: Re: [PATCH 4/5] fs, xfs: convert xfs_cui_log_item.cui_refcount from
 atomic_t to refcount_t

On Wed, Mar 01, 2017 at 10:47:39AM +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.
> 
> Signed-off-by: Elena Reshetova <elena.reshetova@...el.com>
> Signed-off-by: Hans Liljestrand <ishkamiel@...il.com>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> Signed-off-by: David Windsor <dwindsor@...il.com>
> ---
>  fs/xfs/xfs_refcount_item.c | 4 ++--
>  fs/xfs/xfs_refcount_item.h | 4 +++-
>  2 files changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/xfs/xfs_refcount_item.c b/fs/xfs/xfs_refcount_item.c
> index 6e4c744..61bc570 100644
> --- a/fs/xfs/xfs_refcount_item.c
> +++ b/fs/xfs/xfs_refcount_item.c
> @@ -205,7 +205,7 @@ xfs_cui_init(
>  	cuip->cui_format.cui_nextents = nextents;
>  	cuip->cui_format.cui_id = (uintptr_t)(void *)cuip;
>  	atomic_set(&cuip->cui_next_extent, 0);
> -	atomic_set(&cuip->cui_refcount, 2);
> +	refcount_set(&cuip->cui_refcount, 2);

I'm assuming the refcount design is ok with the log item refcounts
starting at 2 and marching down to zero?  The code seems to be
fine with it; I just want to make sure everyone's ok with supporting
this use case.

>  	return cuip;
>  }
> @@ -221,7 +221,7 @@ void
>  xfs_cui_release(
>  	struct xfs_cui_log_item	*cuip)
>  {
> -	if (atomic_dec_and_test(&cuip->cui_refcount)) {
> +	if (refcount_dec_and_test(&cuip->cui_refcount)) {

I suppose it's useful to have refcount_dec_and_test complain loudly if
we ever have a dangling pointer... Friday I was chasing a possible
use-after-free of the EFI items in generic/388.

>  		xfs_trans_ail_remove(&cuip->cui_item, SHUTDOWN_LOG_IO_ERROR);
>  		xfs_cui_item_free(cuip);
>  	}
> diff --git a/fs/xfs/xfs_refcount_item.h b/fs/xfs/xfs_refcount_item.h
> index 5b74ddd..7f23ff8 100644
> --- a/fs/xfs/xfs_refcount_item.h
> +++ b/fs/xfs/xfs_refcount_item.h
> @@ -20,6 +20,8 @@
>  #ifndef	__XFS_REFCOUNT_ITEM_H__
>  #define	__XFS_REFCOUNT_ITEM_H__
>  
> +#include <linux/refcount.h>

I still think this include should go in xfs_linux.h, the same as most of
the other linux/*.h includes in XFS.

--D

> +
>  /*
>   * There are (currently) two pairs of refcount btree redo item types:
>   * increase and decrease.  The log items for these are CUI (refcount
> @@ -63,7 +65,7 @@ struct kmem_zone;
>   */
>  struct xfs_cui_log_item {
>  	struct xfs_log_item		cui_item;
> -	atomic_t			cui_refcount;
> +	refcount_t			cui_refcount;
>  	atomic_t			cui_next_extent;
>  	unsigned long			cui_flags;	/* misc flags */
>  	struct xfs_cui_log_format	cui_format;
> -- 
> 2.7.4
> 
> --
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