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.
