Re: [PATCH] maple_tree: Fix mas_prev() state separation code

[Kees Cook]

On Thu, Dec 07, 2023 at 02:33:19PM -0500, Liam R. Howlett wrote:
> mas_prev() was setting the ma_underflow condition when the limit was
> reached and not when the limit was attempting to go lower.  This
> resulted in the incorrect behaviour on subsequent actions.
> 
> This commit fixes the status setting to only set ma_underflow when the
> lower limit is attempted to be decremented, and modifies the testing to
> ensure that's the case.
> 
> Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>

Is this is related to the report[1] I forwarded? If so, please add these tags
too:

 Reported-by: Linux Kernel Functional Testing <lkft@xxxxxxxxxx>
 Closes: https://lore.kernel.org/all/CA+G9fYs-j2FYZSFSVZj48mgoM9gQd4-7M2mu2Ez3D1DqDdW2bQ@xxxxxxxxxxxxxx/ [1]

Thanks either way!

-Kees

> ---
> 
> Andrew,
> 
> This should be clean to squash into 7f79fdb1d94d7 ("maple_tree: separate
> ma_state node from status.") which is currently in your mm-unstable
> branch.
> 
> Thanks,
> Liam
> 
> 
>  lib/maple_tree.c      | 16 ++++++++++++----
>  lib/test_maple_tree.c |  9 +++++++--
>  2 files changed, 19 insertions(+), 6 deletions(-)
> 
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index 89f8d21602774..47f2a7a973852 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -4432,6 +4432,9 @@ static void *mas_prev_slot(struct ma_state *mas, unsigned long min, bool empty)
>  		mas->last = mas->index - 1;
>  		mas->index = mas_safe_min(mas, pivots, mas->offset);
>  	} else  {
> +		if (mas->index <= min)
> +			goto underflow;
> +
>  		if (mas_prev_node(mas, min)) {
>  			mas_rewalk(mas, save_point);
>  			goto retry;
> @@ -4452,15 +4455,15 @@ static void *mas_prev_slot(struct ma_state *mas, unsigned long min, bool empty)
>  	if (unlikely(mas_rewalk_if_dead(mas, node, save_point)))
>  		goto retry;
>  
> -	if (mas->index <= min)
> -		mas->status = ma_underflow;
>  
>  	if (likely(entry))
>  		return entry;
>  
>  	if (!empty) {
> -		if (mas_is_underflow(mas))
> +		if (mas->index <= min) {
> +			mas->status = ma_underflow;
>  			return NULL;
> +		}
>  
>  		goto again;
>  	}
> @@ -4596,7 +4599,7 @@ static void *mas_next_slot(struct ma_state *mas, unsigned long max, bool empty)
>  		if (unlikely(mas_rewalk_if_dead(mas, node, save_point)))
>  			goto retry;
>  
> -		if (pivot >= max) {
> +		if (pivot >= max) { /* Was at the limit, next will extend beyond */
>  			mas->status = ma_overflow;
>  			return NULL;
>  		}
> @@ -4611,6 +4614,11 @@ static void *mas_next_slot(struct ma_state *mas, unsigned long max, bool empty)
>  		else
>  			mas->last = mas->max;
>  	} else  {
> +		if (mas->last >= max) {
> +			mas->status = ma_overflow;
> +			return NULL;
> +		}
> +
>  		if (mas_next_node(mas, node, max)) {
>  			mas_rewalk(mas, save_point);
>  			goto retry;
> diff --git a/lib/test_maple_tree.c b/lib/test_maple_tree.c
> index 15fbeb788f3ac..29185ac5c727f 100644
> --- a/lib/test_maple_tree.c
> +++ b/lib/test_maple_tree.c
> @@ -3294,8 +3294,8 @@ static noinline void __init check_state_handling(struct maple_tree *mt)
>  	MT_BUG_ON(mt, mas.last != 0x1500);
>  	MT_BUG_ON(mt, !mas_is_active(&mas));
>  
> -	/* next:active ->active */
> -	entry = mas_next(&mas, ULONG_MAX);
> +	/* next:active ->active (spanning limit) */
> +	entry = mas_next(&mas, 0x2100);
>  	MT_BUG_ON(mt, entry != ptr2);
>  	MT_BUG_ON(mt, mas.index != 0x2000);
>  	MT_BUG_ON(mt, mas.last != 0x2500);
> @@ -3360,6 +3360,11 @@ static noinline void __init check_state_handling(struct maple_tree *mt)
>  	MT_BUG_ON(mt, entry != ptr);
>  	MT_BUG_ON(mt, mas.index != 0x1000);
>  	MT_BUG_ON(mt, mas.last != 0x1500);
> +	MT_BUG_ON(mt, !mas_is_active(&mas)); /* spanning limit */
> +	entry = mas_prev(&mas, 0x1200); /* underflow */
> +	MT_BUG_ON(mt, entry != NULL);
> +	MT_BUG_ON(mt, mas.index != 0x1000);
> +	MT_BUG_ON(mt, mas.last != 0x1500);
>  	MT_BUG_ON(mt, !mas_is_underflow(&mas));
>  
>  	/* prev:underflow -> underflow (lower limit) spanning end range */
> -- 
> 2.40.1
> 

-- 
Kees Cook