Jakub Kicinski [off-list ref] writes:

On Sat, 18 Sep 2021 13:53:35 +0200 Toke Høiland-Jørgensen wrote:

quoted

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.

Yeah, that's more or less what I was thinking...

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.

... but I guess my mental model assumed that packet writes would be just
as frequent as reads, in which case it would be a win if you could amend
your example like:

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)

to have an additional check like:

if (mod && ptr == stack)

(or something to that effect). No?

-Toke

		bpf_header_pointer_flush();


is simple enough.. to me.