[PATCH] uprobes: Use flexible array for xol_area bitmap

[PATCH] 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.

Assisted-by: Codex:GPT-5.5
Signed-off-by: Rosen Penev <rosenp@gmail.com>
---
 kernel/events/uprobes.c | 13 +++----------
 1 file changed, 3 insertions(+), 10 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 4084e926e284..9ef74c2ad390 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)
@@ -1755,18 +1755,13 @@ 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);
@@ -1779,8 +1774,6 @@ 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:
-- 
2.54.0

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

[Masami Hiramatsu]

On Sun, 10 May 2026 14:41:18 -0700
Rosen Penev <rosenp@gmail.com> 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.
> 

You also have to update uprobe_clear_state(), because area->bitmap
is no longer allocated separately.

Thank you,


> Assisted-by: Codex:GPT-5.5
> Signed-off-by: Rosen Penev <rosenp@gmail.com>
> ---
>  kernel/events/uprobes.c | 13 +++----------
>  1 file changed, 3 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 4084e926e284..9ef74c2ad390 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)
> @@ -1755,18 +1755,13 @@ 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);
> @@ -1779,8 +1774,6 @@ 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:
> -- 
> 2.54.0
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>