Re: [PATCH v1 1/2] tracing/remotes: Account for ring buffer page header in size calculation

[Steven Rostedt <rostedt@goodmis.org>]

On Mon,  7 Sep 2026 20:26:42 +0100
Vincent Donnefort <vdonnefort@google.com> wrote:

> trace_buffer_desc_size() and trace_remote_alloc_buffer undercount the
> required pages because every ring buffer page contains a header
> (BUF_PAGE_HDR_SIZE). Account for that header to ensure allocated remote
> ring buffers aren't smaller than requested by the user.
> 
> While at it, ensure those functions catch nr_pages overflow.

Fixes patches should never have "while at it". That means it belongs as a
separate patch. The only "while at it" that is acceptable is white space
fixes or other formatting changes along with new code that is not a fix.

> 
> Fixes: 2e67fabd8b77 ("ring-buffer: Introduce ring-buffer remotes")
> Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
> 
> diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
> index afc7daa6ee7d..7a1a92f87650 100644
> --- a/include/linux/ring_buffer.h
> +++ b/include/linux/ring_buffer.h
> @@ -8,6 +8,8 @@
>  
>  #include <uapi/linux/trace_mmap.h>
>  
> +#include <linux/ring_buffer_types.h>

Hmm, why is this after the uapi header? It should be part of the linux/
headers.

> +
>  struct trace_buffer;
>  struct ring_buffer_iter;
>  
> @@ -281,9 +283,14 @@ static inline struct ring_buffer_desc *__first_ring_buffer_desc(struct trace_buf
>  
>  static inline size_t trace_buffer_desc_size(size_t buffer_size, unsigned int nr_cpus)
>  {
> -	unsigned int nr_pages = max(DIV_ROUND_UP(buffer_size, PAGE_SIZE), 2UL) + 1;
> +	unsigned long nr_pages =
> +		max(DIV_ROUND_UP(buffer_size, PAGE_SIZE - BUF_PAGE_HDR_SIZE), 2UL) + 1;

This is getting a little unwieldy. Let's break it up:

	unsigned long pages_req = DIV_ROUND_UP(buffer_size, PAGE_SIZE - BUF_PAGE_HDR_SIZE);
	unsigned long nr_pages = max(pages_req, 2UL) + 1;

Or better yet, let's add a new helper function:

static inline unsigned long calculate_nr_pages(size_t buffer_size)
{
	unsigned long req_pages = DIV_ROUND_UP(buffer_size, PAGE_SIZE - BUF_PAGE_HDR_SIZE);
	return max(pages_req, 2UL) + 1;
}

Then this could be simply:

	unsigned long nr_pages = calculate_nr_pages(buffer_size);

>  	struct ring_buffer_desc *rbdesc;
>  
> +	/* Capped by ring_buffer_desc::nr_page_va */
> +	if (nr_pages > UINT_MAX)
> +		return SIZE_MAX;

Separate patch.

> +
>  	return size_add(offsetof(struct trace_buffer_desc, __data),
>  			size_mul(nr_cpus, struct_size(rbdesc, page_va, nr_pages)));
>  }
> diff --git a/kernel/trace/trace_remote.c b/kernel/trace/trace_remote.c
> index 75fa1ffc4c96..2e0fdbb730b7 100644
> --- a/kernel/trace/trace_remote.c
> +++ b/kernel/trace/trace_remote.c
> @@ -980,9 +980,12 @@ int trace_remote_alloc_buffer(struct trace_buffer_desc *desc, size_t desc_size,
>  			      const struct cpumask *cpumask)
>  {
>  	size_t min_desc_size = trace_buffer_desc_size(buffer_size, cpumask_weight(cpumask));
> -	unsigned int nr_pages = max(DIV_ROUND_UP(buffer_size, PAGE_SIZE), 2UL) + 1;
>  	struct ring_buffer_desc *rb_desc;
>  	int cpu, ret = -ENOMEM;
> +	unsigned int nr_pages;
> +


> +	if (min_desc_size == SIZE_MAX)
> +		return -E2BIG;

Looks like this should be a separate patch too.

>  
>  	if (desc_size < min_desc_size)
>  		return -EINVAL;
> @@ -991,6 +994,7 @@ int trace_remote_alloc_buffer(struct trace_buffer_desc *desc, size_t desc_size,
>  	desc->struct_len = min_desc_size;
>  
>  	rb_desc = __first_ring_buffer_desc(desc);
> +	nr_pages = max(DIV_ROUND_UP(buffer_size, PAGE_SIZE - BUF_PAGE_HDR_SIZE), 2UL) + 1;

And here we can have;

	nr_pages = calculate_nr_pages(buffer_size);

-- Steve

>  
>  	for_each_cpu(cpu, cpumask) {
>  		unsigned int id;