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

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

[Rosen Penev]

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@gmail.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 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));
 	if (unlikely(!area))
 		goto out;
 
-	area->bitmap = kcalloc(BITS_TO_LONGS(UINSNS_PER_PAGE), sizeof(long),
-			       GFP_KERNEL);
-	if (!area->bitmap)
-		goto free_area;
-
 	area->page = alloc_page(GFP_HIGHUSER | __GFP_ZERO);
 	if (!area->page)
-		goto free_bitmap;
+		goto free_area;
 
 	area->vaddr = vaddr;
 	init_waitqueue_head(&area->wq);
@@ -1784,8 +1779,6 @@ static struct xol_area *__create_xol_area(unsigned long vaddr)
 		return area;
 
 	__free_page(area->page);
- free_bitmap:
-	kfree(area->bitmap);
  free_area:
 	kfree(area);
  out:
@@ -1826,7 +1819,6 @@ void uprobe_clear_state(struct mm_struct *mm)
 		return;
 
 	put_page(area->page);
-	kfree(area->bitmap);
 	kfree(area);
 }
 
-- 
2.56.0

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@gmail.com>
> Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> ---
>  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?

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

[Oleg Nesterov]

On 11/10, Rosen Penev wrote:
>
> --- 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 */

I agree with Peter, you can just use
	
	unsigned long			bitmap[BITS_TO_LONGS(UINSNS_PER_PAGE)];

Oleg.