Re: [PATCH v9 3/4] ring-buffer: Cap static ring buffer nr_pages

On Thu, Sep 03, 2026 at 12:56:21PM -0400, Steven Rostedt wrote:

On Tue,  1 Sep 2026 16:54:44 +0100
Vincent Donnefort [off-list ref] wrote:

quoted

Static ring buffers (i.e. persistent, user-mapped and remote) rely on
the bpage::id field. The number of pages for those ring buffers must fit
into that variable. Enforce this limit on ring buffer creation or
user-mapping.

While at it, make buffer_page::id 31 bits. This does not change the
struct buffer_page size.

Let's not add that change to this patch. Especially since this has a fixes
tag to it. That change has nothing to do with the fix.

The reason I had it as 30 to begin with was to reserve a bit in case I
found another reason for it. If 1<<30 is too small for the number of boot
buffer pages, we can always up in another order in the future.

quoted

Fixes: be68d63a139b ("ring-buffer: Add ring_buffer_alloc_range()")
Signed-off-by: Vincent Donnefort <redacted>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 28dd76edfecf..c4260d6ecdfc 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c

@@ -350,7 +350,7 @@ struct buffer_page {
 	local_t		 entries;	/* entries on this page */
 	unsigned long	 real_end;	/* real end of data */
 	unsigned	 order;		/* order of the page */
-	u32		 id:30;		/* ID for external mapping */
+	u32		 id:31;		/* ID for external mapping */
 	u32		 range:1;	/* Mapped via a range */
 	struct buffer_data_page *page;	/* Actual data page */
 };

@@ -657,6 +657,15 @@ static bool rb_is_static(struct ring_buffer_per_cpu *cpu_buffer)
 	return cpu_buffer->user_mapped || cpu_buffer->remote || cpu_buffer->ring_meta;
 }
 
+static unsigned long rb_static_max_pages(void)
+{
+	/*
+	 * Static ring buffers are using bpage::id and must account for the
+	 * reader page.
+	 */
+	return (1UL << 31) - 1;
+}
+
 struct ring_buffer_iter {
 	struct ring_buffer_per_cpu	*cpu_buffer;
 	unsigned long			head;

@@ -2842,6 +2851,10 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
 		 */
 		nr_pages = (size - sizeof(struct ring_buffer_cpu_meta)) /
 			(subbuf_size + sizeof(int));
+
+		if (nr_pages > rb_static_max_pages())
+			goto fail_free_buffers;
+

If you want to add something, we could add to the beginning of this
function:

	/* Prevent ridiculously small sizes */
	if (size < PAGE_SIZE)
		return NULL;

to shut up Sashiko about overflows :-p

-- Steve

tracer_alloc_buffers() uses size of 1 for non-expanded buffers.

I'll test size just before 

  nr_pages = (size - sizeof(struct ring_buffer_cpu_meta)) /
  	(subbuf_size + sizeof(int));

quoted

 		/* Need at least two pages plus the reader page */
 		if (nr_pages < 3)
 			goto fail_free_buffers;

@@ -2874,6 +2887,10 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
 		/* The writer is remote. This ring-buffer is read-only */
 		atomic_inc(&buffer->record_disabled);
 		nr_pages = desc->nr_page_va - 1;
+
+		if (nr_pages > rb_static_max_pages())
+			goto fail_free_buffers;
+
 		if (nr_pages < 2)
 			goto fail_free_buffers;
 	} else {

@@ -7836,6 +7853,9 @@ int ring_buffer_map(struct trace_buffer *buffer, int cpu,
 	/* prevent another thread from changing buffer/sub-buffer sizes */
 	guard(mutex)(&buffer->mutex);
 
+	if (cpu_buffer->nr_pages > rb_static_max_pages())
+		return -E2BIG;
+
 	err = rb_alloc_meta_page(cpu_buffer);
 	if (err)
 		return err;