On Mon, 7 Sep 2026 20:26:42 +0100
Vincent Donnefort <[email protected]> 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 <[email protected]>
>
> 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;
