On Wed, 4 Dec 2024 at 22:37, Eduard Zingerman <eddyz87@xxxxxxxxx> wrote: > > On Wed, 2024-12-04 at 13:13 -0800, Alexei Starovoitov wrote: > > On Wed, Dec 4, 2024 at 1:08 PM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote: > > > > > > On Wed, 2024-12-04 at 21:48 +0100, Kumar Kartikeya Dwivedi wrote: > > > > > > [...[ > > > > > > (A) ----. > > > | > > > v > > > > > > What this will do in both cases:: > > > > > > First, avoid walking states when off != 0, and reset id. > > > > > > If off == 0, go inside mark_ptr_or_null_reg and walk all regs, and > > > > > > remove marks for those with off != 0. > > > > > > (B) ----. > > > | > > > v > > > > > That's getting intrusive. > > > > > How about we reset id=0 in adjust_ptr_min_max_vals() > > > > > right after we suppressed "null-check it first" message for raw_tp-s. > > > > > > > > > > That will address the issue as well, right? > > > > > > > > Yes (minor detail, it needs to be reset to a new id, otherwise we have > > > > warn on maybe_null set but !reg->id, but the idea is the same). > > > > Let's see what Eduard thinks and then I can give it a go. > > > > > > Sorry for delay. > > > > > > I like what Kumar is proposing in (A) because it could be generalized: > > > there is no real harm in doing 'r2 = r1; r2 += 8; r1 != 0; ...' > > > and what Kumar suggests could be used to lift the "null-check it first ..." > > > restriction. > > > > I don't see how it can be generalized. > > Also 'avoid walking states when off != 0' is far from simple. > > We call into mark_ptr_or_null_regs() with id == 0 already > > and with reg->off != 0 for RCU and alloc_obj. > > I did not try to implement this, so there might be a devil in the details. > The naive approach looks as below. > > Suppose we want to allow 'rX += K' when rX is PTR_MAYBE_NULL. > Such operations generate a set of pointers with different .off values > but same .id . > For a regular (non raw_tp) case: > - dereferencing PTR_MAYBE_NULL is disallowed; > - if there is a check 'if rY != 0' and rY.off == 0, > the non-null status could be propagated to each > register in a set (and PTR_MAYBE_NULL mark removed); > - if there is a check 'if rY != 0' and rY.off != 0, > nothing happens, no marks are changed. Yes, also I realized after some thinking that when rY with off != 0 is checked, it just needs to be a no-op (in context of this solution), we don't need to remove it from the set. Later if rX with off == 0 sharing the same id is checked rY should be marked not null. > > For a raw_tp case: > - dereferencing PTR_MAYBE_NULL is allowed (as it is already); > - the mechanics for 'if rY != 0' and rY.off ==/!= 0 can remain the same, > nothing is wrong with removing PTR_MAYBE_NULL marks from such pointers. Yes, it can be generalized, this solution to generalize to all types does not require the state forking approach, which is different. It is getting late for me here so I will continue looking at this tomorrow, but I can do this for raw_tp in this patch as a fix for the warning. Then, I can send a follow up doing it for all pointer types against bpf-next where can continue discussion based on concrete code. Alexei said he was giving the state forking a go in parallel (which is much more involved and impact on veristat needs to be analyzed). Anyhow, I will continue tomorrow. Let me know what you think. > > > 'avoid walking with off != 0' doesn't look trivial. > > It would need to be special cased to raw_tp and some other > > conditions. > > I could be missing something. > > > > Let's see how patches look. > > > > > However, as far as I understand, the plan is to fix this by generating > > > two entry tracepoint states: one with parameter as null, another with > > > parameter not-null (all combinations for every parameter). > > > If that is the plan, what Alexei suggests in (B) is simpler. >
