On Wed, Sep 09, 2026 at 06:13:16PM -0400, Steven Rostedt wrote:
> 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.
ack.
>
> >
> > 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;
> }
I'll introduce the helper from this patch as suggested.
>
> 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;
>
--
Vincent
