Re: [PATCH v5 5/7] futex: Remove the limit of elements for sys_set_robust_list2 lists

[Thomas Gleixner]

On Thu, Jun 26 2025 at 14:11, André Almeida wrote:
> Remove the limit of ROBUST_LIST_LIMIT elements that a robust list can
> have, for the ones created with the new interface. This is done by

With which new interface?

> overwritten the list as it's proceeded in a way that we avoid circular

overwriting each processed list entry to point at ...., which eliminates
a potential circular list.


> lists.
>
> For the old interface, we keep the limited behavior to avoid changing

s/we//

> the API.

Which API would be violated?

Overwriting the dying tasks robust list entries is not violating any
ABI. The task's memory is on the way to be destroyed.

> Signed-off-by: André Almeida <andrealmeid@xxxxxxxxxx>
> ---
>  kernel/futex/core.c | 37 +++++++++++++++++++++++++++++--------
>  1 file changed, 29 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/futex/core.c b/kernel/futex/core.c
> index 1049f8ef3ce3c611b3be0ca12df34a98f710121d..942b66facdea16cd7be2235d95c2bbbae8d7cc63 100644
> --- a/kernel/futex/core.c
> +++ b/kernel/futex/core.c
> @@ -1152,7 +1152,8 @@ static inline int fetch_robust_entry(struct robust_list __user **entry,
>   * We silently return on any sign of list-walking problem.
>   */
>  static void exit_robust_list64(struct task_struct *curr,
> -			       struct robust_list_head __user *head)
> +			       struct robust_list_head __user *head,
> +			       bool destroyable)
>  {
>  	struct robust_list __user *entry, *next_entry, *pending;
>  	unsigned int limit = ROBUST_LIST_LIMIT, pi, pip;
> @@ -1196,13 +1197,17 @@ static void exit_robust_list64(struct task_struct *curr,
>  		}
>  		if (rc)
>  			return;
> -		entry = next_entry;
> -		pi = next_pi;
> +
>  		/*
>  		 * Avoid excessively long or circular lists:
>  		 */
> -		if (!--limit)
> +		if (!destroyable && !--limit)
>  			break;
> +		else
> +			put_user(&head->list, &entry->next);

Unchecked put_user() with zero explanation what it actually does.

> +
> +		entry = next_entry;
> +		pi = next_pi;
>  
>  		cond_resched();
>  	}
> @@ -1214,7 +1219,8 @@ static void exit_robust_list64(struct task_struct *curr,
>  }
>  #else
>  static void exit_robust_list64(struct task_struct *curr,
> -			       struct robust_list_head __user *head)
> +			       struct robust_list_head __user *head,
> +			       bool destroyable)
>  {
>  	pr_warn("32bit kernel should not allow ROBUST_LIST_64BIT");
>  }
> @@ -1252,7 +1258,8 @@ fetch_robust_entry32(u32 *uentry, struct robust_list __user **entry,
>   * We silently return on any sign of list-walking problem.
>   */
>  static void exit_robust_list32(struct task_struct *curr,
> -			       struct robust_list_head32 __user *head)
> +			       struct robust_list_head32 __user *head,
> +			       bool destroyable)
>  {
>  	struct robust_list __user *entry, *next_entry, *pending;
>  	unsigned int limit = ROBUST_LIST_LIMIT, pi, pip;

So this get's a destroyable argument as well, but no implementation?

> @@ -1474,10 +1481,19 @@ static void exit_pi_state_list(struct task_struct *curr)
>  static inline void exit_pi_state_list(struct task_struct *curr) { }
>  #endif
>  
> +/*
> + * futex_cleanup - After the task exists, process the robust lists
> + *
> + * Walk through the linked list, parsing robust lists and freeing the
> + * allocated lists. Lists created with the set_robust_list2 don't have a limit
> + * for sizing and can be destroyed while we walk on it to avoid circular list.
> + */
>  static void futex_cleanup(struct task_struct *tsk)
>  {
>  	struct robust_list2_entry *curr, *n;
>  	struct list_head *list2 = &tsk->robust_list2;
> +	bool destroyable = true;
> +	int i = 0;
>  
>  	/*
>  	 * Walk through the linked list, parsing robust lists and freeing the
> @@ -1485,15 +1501,20 @@ static void futex_cleanup(struct task_struct *tsk)
>  	 */
>  	if (unlikely(!list_empty(list2))) {
>  		list_for_each_entry_safe(curr, n, list2, list) {
> +			destroyable = true;
> +			if (tsk->robust_list_index == i)
> +				destroyable = false;

Oh well.....