Re: [PATCH net-next 2/3] bpf: test_run: Use system page pool for XDP live frame mode

[Paolo Abeni <pabeni@redhat.com>]

On Tue, 2024-02-20 at 10:06 +0100, Daniel Borkmann wrote:
> On 2/15/24 2:26 PM, Toke Høiland-Jørgensen wrote:
> > The BPF_TEST_RUN code in XDP live frame mode creates a new page pool
> > each time it is called and uses that to allocate the frames used for the
> > XDP run. This works well if the syscall is used with a high repetitions
> > number, as it allows for efficient page recycling. However, if used with
> > a small number of repetitions, the overhead of creating and tearing down
> > the page pool is significant, and can even lead to system stalls if the
> > syscall is called in a tight loop.
> > 
> > Now that we have a persistent system page pool instance, it becomes
> > pretty straight forward to change the test_run code to use it. The only
> > wrinkle is that we can no longer rely on a custom page init callback
> > from page_pool itself; instead, we change the test_run code to write a
> > random cookie value to the beginning of the page as an indicator that
> > the page has been initialised and can be re-used without copying the
> > initial data again.
> > 
> > Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
> 
> [...]
> > -
> >   	/* We create a 'fake' RXQ referencing the original dev, but with an
> >   	 * xdp_mem_info pointing to our page_pool
> >   	 */
> >   	xdp_rxq_info_reg(&xdp->rxq, orig_ctx->rxq->dev, 0, 0);
> > -	xdp->rxq.mem.type = MEM_TYPE_PAGE_POOL;
> > -	xdp->rxq.mem.id = pp->xdp_mem_id;
> > +	xdp->rxq.mem.type = MEM_TYPE_PAGE_POOL; /* mem id is set per-frame below */
> >   	xdp->dev = orig_ctx->rxq->dev;
> >   	xdp->orig_ctx = orig_ctx;
> >   
> > +	/* We need a random cookie for each run as pages can stick around
> > +	 * between runs in the system page pool
> > +	 */
> > +	get_random_bytes(&xdp->cookie, sizeof(xdp->cookie));
> > +
> 
> So the assumption is that there is only a tiny chance of collisions with
> users outside of xdp test_run. If they do collide however, you'd leak data.

Good point. @Toke: what is the worst-case thing that could happen in
case a page is recycled from another pool's user?

could we possibly end-up matching the cookie for a page containing
'random' orig_ctx/ctx, so that bpf program later tries to access
equally random ptrs?

Thanks!

Paolo