Re: [PATCH bpf-next v2 1/8] xdp: restructure redirect actions

Björn Töpel [off-list ref] writes:

On 2021-01-20 13:44, Toke Høiland-Jørgensen wrote:

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Björn Töpel [off-list ref] writes:

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From: Björn Töpel <redacted>

The XDP_REDIRECT implementations for maps and non-maps are fairly
similar, but obviously need to take different code paths depending on
if the target is using a map or not. Today, the redirect targets for
XDP either uses a map, or is based on ifindex.

Future commits will introduce yet another redirect target via the a
new helper, bpf_redirect_xsk(). To pave the way for that, we introduce
an explicit redirect type to bpf_redirect_info. This makes the code
easier to follow, and makes it easier to add new redirect targets.

Further, using an explicit type in bpf_redirect_info has a slight
positive performance impact by avoiding a pointer indirection for the
map type lookup, and instead use the hot cacheline for
bpf_redirect_info.

The bpf_redirect_info flags member is not used by XDP, and not
read/written any more. The map member is only written to when
required/used, and not unconditionally.

I like the simplification. However, the handling of map clearing becomes
a bit murky with this change:

You're not changing anything in bpf_clear_redirect_map(), and you're
removing most of the reads and writes of ri->map. Instead,
bpf_xdp_redirect_map() will store the bpf_dtab_netdev pointer in
ri->tgt_value, which xdp_do_redirect() will just read and use without
checking. But if the map element (or the entire map) has been freed in
the meantime that will be a dangling pointer. I *think* the RCU callback
in dev_map_delete_elem() and the rcu_barrier() in dev_map_free()
protects against this, but that is by no means obvious. So confirming
this, and explaining it in a comment would be good.

Yes, *most* of the READ_ONCE(ri->map) are removed, it's pretty much only 
the bpf_redirect_map(), and as you write, the tracepoints.

The content/element of the map is RCU protected, and actually even the
map will be around until the XDP processing is complete. Note the
synchronize_rcu() followed after all bpf_clear_redirect_map() calls.

I'll try to make it clearer in the commit message! Thanks for pointing 
that out!

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Also, as far as I can tell after this, ri->map is only used for the
tracepoint. So how about just storing the map ID and getting rid of the
READ/WRITE_ONCE() entirely?

...and the bpf_redirect_map() helper. Don't you think the current
READ_ONCE(ri->map) scheme is more obvious/clear?

Yeah, after your patch we WRITE_ONCE() the pointer in
bpf_redirect_map(), but the only place it is actually *read* is in the
tracepoint. So the only purpose of bpf_clear_redirect_map() is to ensure
that an invalid pointer is not read in the tracepoint function. Which
seems a bit excessive when we could just store the map ID for direct use
in the tracepoint and get rid of bpf_clear_redirect_map() entirely, no?

Besides, from a UX point of view, having the tracepoint display the map
ID even if that map ID is no longer valid seems to me like it makes more
sense than just displaying a map ID of 0 and leaving it up to the user
to figure out that this is because the map was cleared. I mean, at the
time the redirect was made, that *was* the map ID that was used...

Oh, and as you say due to the synchronize_rcu() call in dev_map_free() I
think this whole discussion is superfluous anyway, since it can't
actually happen that the map gets freed between the setting and reading
of ri->map, no?

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(Oh, and related to this I think this patch set will conflict with
Hangbin's multi-redirect series, so maybe you two ought to coordinate? :))

Yeah, good idea! I would guess Hangbin's would go in before this, so I
would need to adapt.


Thanks for taking of look at the series, Toke! Much appreciated!

You're welcome :)

-Toke