Re: [PATCH v2] uprobes: Use flexible array for xol_area bitmap

[Peter Zijlstra]

On Tue, Nov 10, 2026 at 06:26:05PM -0800, Rosen Penev wrote:
> The XOL slot bitmap has the same lifetime as struct xol_area, but it
> is currently allocated separately.  That adds another allocation
> failure path and a matching cleanup branch without buying any extra
> flexibility.
> 
> Store the bitmap as a flexible array member and allocate it together
> with the xol_area using kzalloc_flex().  The bitmap remains
> zero-initialized, while the allocation and error handling become
> simpler.
> 
> Signed-off-by: Rosen Penev <rosenp@xxxxxxxxx>
> Acked-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> ---
>  v2: add Acked-by. remove Assisted-by
>  kernel/events/uprobes.c | 14 +++-----------
>  1 file changed, 3 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 853fc952331c..876c3cfe8031 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -108,7 +108,6 @@ static LIST_HEAD(delayed_uprobe_list);
>   */
>  struct xol_area {
>  	wait_queue_head_t		wq;		/* if all slots are busy */
> -	unsigned long			*bitmap;	/* 0 = free slot */
>  
>  	struct page			*page;
>  	/*
> @@ -117,6 +116,7 @@ struct xol_area {
>  	 * the vma go away, and we must handle that reasonably gracefully.
>  	 */
>  	unsigned long			vaddr;		/* Page(s) of instruction slots */
> +	unsigned long			bitmap[];	/* 0 = free slot */
>  };
>  
>  static void uprobe_warn(struct task_struct *t, const char *msg)
> @@ -1760,18 +1760,13 @@ static struct xol_area *__create_xol_area(unsigned long vaddr)
>  	struct xol_area *area;
>  	void *insns;
>  
> -	area = kzalloc_obj(*area);
> +	area = kzalloc_flex(*area, bitmap, BITS_TO_LONGS(UINSNS_PER_PAGE));

I am confused; this is a compile time constant. Why are we using flex
arrays?