Re: [PATCH v14 bpf-next 00/18] mvneta: introduce XDP multi-buffer support

[Jakub Kicinski <kuba@kernel.org>]

On Sat, 18 Sep 2021 13:53:35 +0200 Toke Høiland-Jørgensen wrote:
> I'm OK with a bpf_header_pointer()-type helper - I quite like the
> in-kernel version of this for SKBs, so replicating it as a BPF helper
> would be great. But I'm a little worried about taking a performance hit.
> 
> I.e., if you do:
> 
> ptr = bpf_header_pointer(pkt, offset, len, stack_ptr)
> *ptr = xxx;
> 
> then, if the helper ended up copying the data into the stack pointer,
> you didn't actually change anything in the packet, so you need to do a
> writeback.
> 
> Jakub suggested up-thread that this should be done with some kind of
> flush() helper. But you don't know whether the header_pointer()-helper
> copied the data, so you always need to call the flush() helper, which
> will incur overhead. If the verifier can in-line the helpers that will
> lower it, but will it be enough to make it negligible?

Depends on the assumptions the program otherwise makes, right?

For reading I'd expect a *layout-independent* TC program would 
replace approximately:

ptr = <some_ptr>;
if (ptr + CONST >= md->ptr_end)
	if (bpf_pull_data(md, off + CONST))
		return DROP;
	ptr = <some_ptr>;
	if (ptr + CONST >= md->ptr_end)
		return DROP; /* da hell? */
}

With this (pre-inlining):

ptr = bpf_header_pointer(md, offset, len, stack);
if (!ptr)
	return DROP;

Post-inlining (assuming static validation of args to prevent wraps):

if (md->ptr + args->off + args->len < md->ptr_end)
	ptr = md->ptr + args->off;
else
	ptr = __bpf_header_pointer(md, offset, len, stack);
if (!ptr)
	return DROP;

But that's based on guesswork so perhaps I'm off base.


Regarding the flush() I was expecting that most progs will not modify
the packet (or at least won't modify most headers they load) so no
point paying the price of tracking changes auto-magically.

In fact I don't think there is anything infra can do better for
flushing than the prog itself:

	bool mod = false;

	ptr = bpf_header_pointer(...);
	...
	if (some_cond(...)) {
		change_packet(...);
		mod = true;
	}
	...
	if (mod)
		bpf_header_pointer_flush();


is simple enough.. to me.