Date:   Wed, 20 Jan 2021 16:49:22 +0100
From:   Björn Töpel <bjorn.topel@...el.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Björn Töpel <bjorn.topel@...il.com>,
        ast@...nel.org, daniel@...earbox.net, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     magnus.karlsson@...el.com, maciej.fijalkowski@...el.com,
        kuba@...nel.org, jonathan.lemon@...il.com, maximmi@...dia.com,
        davem@...emloft.net, hawk@...nel.org, john.fastabend@...il.com,
        ciara.loftus@...el.com, weqaar.a.janjua@...el.com
Subject: Re: [PATCH bpf-next v2 1/8] xdp: restructure redirect actions

On 2021-01-20 15:52, Toke Høiland-Jørgensen wrote:
> Björn Töpel <bjorn.topel@...el.com> writes:
> 
>> On 2021-01-20 13:44, Toke Høiland-Jørgensen wrote:
>>> Björn Töpel <bjorn.topel@...il.com> writes:
>>>
>>>> From: Björn Töpel <bjorn.topel@...el.com>
>>>>
>>>> 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!
>>
>>> 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...
>

Convinced! Getting rid of bpf_clear_redirect_map() would be good! I'll
take a stab at this for v3!


> 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?
>

It can't be free'd but, ri->map can be cleared via
bpf_clear_redirect_map(). So, between the helper (setting) and the
tracepoint in xdp_do_redirect() it can be cleared (say if the XDP
program is swapped out, prior running xdp_do_redirect()).

Moving to the scheme you suggested, does make the discussion
superfluous. :-)


Thanks for the input!
Björn

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