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SubjectRe: [PATCH v3 net-next] bpf/verifier: track liveness for pruning
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On 18/08/17 04:21, Alexei Starovoitov wrote:
> On 8/15/17 12:34 PM, Edward Cree wrote:
>> State of a register doesn't matter if it wasn't read in reaching an exit;
>> a write screens off all reads downstream of it from all explored_states
>> upstream of it.
>> This allows us to prune many more branches; here are some processed insn
>> counts for some Cilium programs:
>> Program before after
>> bpf_lb_opt_-DLB_L3.o 6515 3361
>> bpf_lb_opt_-DLB_L4.o 8976 5176
>> bpf_lb_opt_-DUNKNOWN.o 2960 1137
>> bpf_lxc_opt_-DDROP_ALL.o 95412 48537
>> bpf_lxc_opt_-DUNKNOWN.o 141706 78718
>> bpf_netdev.o 24251 17995
>> bpf_overlay.o 10999 9385
>>
>> The runtime is also improved; here are 'time' results in ms:
>> Program before after
>> bpf_lb_opt_-DLB_L3.o 24 6
>> bpf_lb_opt_-DLB_L4.o 26 11
>> bpf_lb_opt_-DUNKNOWN.o 11 2
>> bpf_lxc_opt_-DDROP_ALL.o 1288 139
>> bpf_lxc_opt_-DUNKNOWN.o 1768 234
>> bpf_netdev.o 62 31
>> bpf_overlay.o 15 13
>>
>> Signed-off-by: Edward Cree <ecree@solarflare.com>
>
> this is one ingenious hack. Love it!
> I took me whole day to understand most of it, but I still have
> few questions:
>
>> +
>> +static void propagate_liveness(const struct bpf_verifier_state *state,
>> + struct bpf_verifier_state *parent)
>
> here the name 'parent' is very confusing, since for the first
> iteration of the loop below it transfers lives from 'neighbor'
> state to the current state and only then traverses the link
> of parents in the current.
> Would be good to document it, since I was struggling the most
> with this name until I realized that the way you build parent link list
> in is_state_visited() is actual sequence of roughly basic blocks and
> the name 'parent' applies there, but not for the first iteration
> of this function.
In the way I think about it, it really is a parent, by which I mean "a
block whose output registers are our inputs". When we call
propagate_liveness(), we're saying "here's another block whose outputs
could be our inputs". So while the 'state->parent' datastructure is a
linked list, the 'parentage' relationship is really a DAG.
While 'state->parent' always has to point at an explored_state (which are
roughly immutable), the 'parent' we pass into propagate_liveness is just
env->cur_state which is mutable. The weird "it's not actually
state->parent" comes from (a) there's only room for one state->parent and
(b) we didn't create a new_sl for it because we're pruning.
I agree this is not at all explained in the code or comments, except for
the glib "Registers read by the continuation are read by us". I will try
to write some comments and/or documentation explaining how and why the
liveness tracking works, because it _is_ subtle and a week from now _I_
probably won't understand it either.

>> @@ -3407,6 +3501,14 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
>> memcpy(&new_sl->state, &env->cur_state, sizeof(env->cur_state));
>> new_sl->next = env->explored_states[insn_idx];
>> env->explored_states[insn_idx] = new_sl;
>> + /* connect new state to parentage chain */
>> + env->cur_state.parent = &new_sl->state;
>> + /* clear liveness marks in current state */
>> + for (i = 0; i < BPF_REG_FP; i++)
>> + env->cur_state.regs[i].live = REG_LIVE_NONE;
>> + for (i = 0; i < MAX_BPF_STACK / BPF_REG_SIZE; i++)
>> + if (env->cur_state.stack_slot_type[i * BPF_REG_SIZE] == STACK_SPILL)
>> + env->cur_state.spilled_regs[i].live = REG_LIVE_NONE;
>
> and this part I don't get at all.
The idea behind the liveness marks is that 'writes go down and reads go up
and up'. That is, each state 'tells' its parent state 'I read from you'
(which can then end up recursing), but it remembers for itself that it
wrote to a register and therefore should swallow subsequent reads rather
than forwarding them to its parent.
While a block is being walked, its liveness marks _only_ record writes, and
then only writes _made in that block_. The read marks go to the parent,
which is "some block that has been walked", but whose continuations haven't
all been walked yet so (by looplessness) won't show up as a pruning
opportunity. An explored_state's liveness marks record the writes _done in
reaching that state_ from its parent, but the reads _done by the state's
children_. A cur_state's liveness marks do the same, but it doesn't have
any children yet so it never gets read-marks (until it gets turned into an
explored_state, of course).
We clear our liveness marks because the writes our parent block did are not
writes we did, so they don't screen off our reads from our parent; they
only screen off our (or our parent's) reads from our grandparent.
> It seems you're trying to sort-of do per-fake-basic block liveness
> analysis, but our state_list_marks are not correct if we go with
> canonical basic block definition, since we mark the jump insn and
> not insn after the branch and not every basic block boundary is
> properly detected.
I think the reason this works is that jump insns can't do writes.
Whenever we pop a branch and restore its register state, we _also_ restore
its parentage state. Then if we decide to prune, we're saying that
whatever it takes to get from sl->state to an exit, we must also do.
A read mark on sl->state says that in the process of getting to an exit, the
register was written before it was read. A write mark on sl->state says
that the straight-line code resulting in sl->state wrote to the register,
so reads shouldn't propagate to its ->parent. But by the time we're using
sl->state in is_state_visited(), its continuations have all been probed, so
it can never gain more reads, so its write marks are irrelevant; they
mustn't stop the state's reads propagating to new 'pruning parents' because
those didn't arrive at the state through the straight-line code.
So maybe the first iteration through propagate_liveness() really _is_
special, and that if it were done differently (ignoring write marks) then
it really wouldn't matter at all where our state_list_marks were in
relation to the basic blocks. But I _think_ that, because we mark jump
insns as well as their destinations (and a popped branch is always a
destination, rather than the 'insn after the branch'), the sl->state will
never have any write marks and it'll all just work.
But I should really test that!
> So if algorithm should only work for basic blocks (for sequences of
> instructions without control flow changes) then it's broken.
> If it should work with control flow insns then it should also work
> for the whole chain of insns from the first one till bpf_exit...
> So I tried removing two above clearing loops and results are much
> better:
> before after
> bpf_lb-DLB_L3.o 2604 1120
> bpf_lb-DLB_L4.o 11159 1371
> bpf_lb-DUNKNOWN.o 1116 485
> bpf_lxc-DDROP_ALL.o 34566 12758
> bpf_lxc-DUNKNOWN.o 53267 18337
> bpf_netdev.o 17843 10564
> bpf_overlay.o 8672 5513
>
> but it feels too good to be true and probably not correct.
> So either way we need to fix something it seems.
Without that clearing, write marks will never be cleared, meaning that once
a register has been written to it will _never again_ forward a read. Since
every register (except ctx and fp) must be written before it can be read,
no register (except ctx) will ever be marked as read, and all the branches
will be pruned away.

Reads are a property of a state - roughly, "what do we read in getting from
this state to a BPF_EXIT?" - whereas writes are a property of a block,
"what do we write in getting from the parent to here?"
Thus, reads may (indeed must) propagate (upwards), but writes must _never_
spread beyond the 'end-state' of their block.

I hope this answers your questions, because I'm not entirely sure _I_
understand it either. Or rather, when I think about it for an hour, it
makes sense for about fifteen seconds, and then it's gone again and all I
can remember is "write down, read up".

-Ed

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