On Fri, 2023-06-02 at 13:17 -0700, Andrii Nakryiko wrote: > On Fri, Jun 2, 2023 at 12:13 PM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote: > > > > On Fri, 2023-06-02 at 11:50 -0700, Andrii Nakryiko wrote: > > [...] > > > > > The thread is long. Could you please describe it again in pseudo code? > > > > > > > > - Add a function mark_precise_scalar_ids(struct bpf_verifier_env *env, > > > > struct bpf_verifier_state *st) > > > > such that it: > > > > - collect PRECISE_IDS: a set of IDs of all registers marked in env->bt > > > > - visit all registers with ids from PRECISE_IDS and make sure > > > > that these registers are marked in env->bt > > > > - Call mark_precise_scalar_ids() from __mark_chain_precision() > > > > for each state 'st' visited by states chain processing loop, > > > > so that: > > > > - mark_precise_scalar_ids() is called for current state when > > > > __mark_chain_precision() is entered, reusing id assignments in > > > > current state; > > > > - mark_precise_scalar_ids() is called for each parent state, reusing > > > > id assignments valid at 'last_idx' instruction of that state. > > > > > > > > The idea is that in situations like below: > > > > > > > > 4: if (r6 > r7) goto +1 > > > > 5: r7 = r6 > > > > --- checkpoint #1 --- > > > > 6: <something> > > > > 7: if (r7 > X) goto ... > > > > 8: r7 = 0 > > > > 9: r9 += r6 > > > > > > > > The mark_precise_scalar_ids() would be called at: > > > > - (9) and current id assignments would be used. > > > > - (6) and id assignments saved in checkpoint #1 would be used. > > > > > > > > If <something> is the code that modifies r6/r7 the link would be > > > > broken and we would overestimate the set of precise registers. > > > > > > > > > > To avoid this we need to recalculate these IDs on each new parent > > > state, based on requested precision marks. If we keep a simple and > > > small array of IDs and do a quick linear search over them for each > > > SCALAR register, I suspect it should be very fast. I don't think in > > > practice we'll have more than 1-2 IDs in that array, right? > > > > I'm not sure I understand, could you please describe how it should > > work for e.g.?: > > > > 3: r6 &= 0xf // assume safe bound > > 4: if (r6 > r7) goto +1 > > 5: r7 = r6 > > --- checkpoint #1 --- > > 6: r7 = 0 > > 7: if (r7 > 10) goto exit; > > 8: r7 = 0 > > 9: r9 += r6 > > > > __mark_chain_precision() would get to checkpoint #1 with only r6 as > > precise, what should happen next? > > it should mark all SCALARs that have r6's ID in env->bt, and then > proceed with precision propagation until next parent state? This is > where you'll mark r7, because in parent state (checkpoint #1) r6.id == > r7.id. That's what I do now. Sorry, I thought you had a suggestion on how to avoid the precise set overestimation (e.g. how to detect that "6: r7 = 0" breaks the link). > It might be easier to just discuss latest code you have, there are > lots of intricacies, and code wins over words :) Here is what I have now: https://github.com/kernel-patches/bpf/compare/bpf-next_base...eddyz87:bpf:verify-ids-for-scalars-in-regsafe-v3 The interesting part is mark_precise_scalar_ids(). But a few tests are not passing because expected messages have to be adjusted. And a lot of tests have to be added. We can delay discussion until I submit v3 (worst case tomorrow). > > As a side note: I added several optimizations: > > - avoid allocation of scalar ids for constants; > > - remove sole scalar ids from cached states; > > so that's what I was proposing earlier, Yes, it turned out beneficial when I inspected logs for bpf_xdp.o. > but not just from cached > states, but from any state. As soon as we get SCALAR with some ID that > is not shared by any other SCALAR, we should try to drop that ID. The > question is only in how to implement this efficiently. No, we don't want to do it for non-cached state, not until we generate scalar ids on stack spills and fills. Otherwise we would break find_equal_scalars() for the following case: r1 = r2 // r1 gains ID fp[-8] = r1 // r2 = 0 // r1 = 0 // fp[-8] has a unique ID now --- checkpoint --- r1 = fp[-8] r2 = fp[-8] if r1 > 10 goto exit; // range propagates to r2 now, // but won't propagate if fp[-8] ID // is cleared at checkpoint (A bit contrived, but conveys the idea) And we don't really need to bother about unique IDs in non-cached state when rold->id check discussed in a sibling thread is used: if (rold->precise && rold->id && !check_ids(rold->id, rcur->id, idmap)) return false; Here, if rcur->id is unique there are two cases: - rold->id == 0: then rcur->id is just ignored - rold->id != 0: then rold->id/rcur->id pair would be added to idmap, there is some other precise old register with the same id as rold->id, so eventually check_ids() would make regsafe() return false. > > - do a check as follows: > > if (rold->precise && rold->id && !check_ids(idmap, rold, rcur)) > > return false; > > Hm.. do we need extra special case here? With precision changes we are > discussion, and this removing singular SCALAR IDs you are proposing, > just extending existing logic to: > > if (regs_exact(rold, rcur, idmap)) > return true; > if (env->explore_alu_limits) > return false; > if (!rold->precise) > return true; > /* new val must satisfy old val knowledge */ > return range_within(rold, rcur) && > check_ids(rold->id, rcur->id, idmap) && > check_ids(rold->ref_obj_id, rcur->ref_obj_id, idmap) && > tnum_in(rold->var_off, rcur->var_off); > > wouldn't be enough? Yes, it could be shortened as below: return range_within(rold, rcur) && (rold->id == 0 || check_ids(rold->id, rcur->id, idmap)) && check_ids(rold->ref_obj_id, rcur->ref_obj_id, idmap) && tnum_in(rold->var_off, rcur->var_off); but I wanted a separate place to put a long comment at. > > > > And I'm seeing almost zero performance overhead now. > > So, maybe what we figured so far is good enough. > > Need to add more tests, though.
