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

[Vincent Donnefort <vdonnefort@google.com>]

On Thu, Sep 03, 2026 at 12:56:21PM -0400, Steven Rostedt wrote:
> On Tue,  1 Sep 2026 16:54:44 +0100
> Vincent Donnefort <vdonnefort@google.com> wrote:
> 
> > 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.
> 
> > 
> > Fixes: be68d63a139b ("ring-buffer: Add ring_buffer_alloc_range()")
> > Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
> > 
> > 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));

> 
> 
> 
> >  		/* 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;
>