On Tue, Jan 03, 2023 at 02:04:44PM -0800, Andrii Nakryiko wrote: > > It sounds logical, but it can get tricky with ranges and branch taken logic. > > Consider something like: > > R1=(min=2,max=8), R2=(min=1, max=10) > > if (R1 within R2) // bpf prog is doing its own 'within' > > a bit confused what is "R1 within R2" here and what you mean "bpf prog > is doing its own 'within'"? Any sort of `R1 < R2` checks (and any > other op: <=, >=, etc) can't really kick in branch elimination because > R2_min=1 < R1_max=8, so arithmetically speaking we can't conclude that > "R1 is always smaller than R2", so both branches would have to be > examined. Something like that. Details didn't matter to me. It was hypothetical 'within' operation just to illustrate the point. > But I probably misunderstood your example, sorry. > > > // branch taken kicks in > > else > > // issues that were never checked > > > > Now new state has: > > R1=(min=4,max=6), R2=(min=5, max=5) > > > > Both R1 and R2 of new state individually range_within of old safe state, > > but together the prog may go to the unverified path. > > Not sure whether it's practical today. > > You asked for hypothetical, so here it goes :) > > No problem with "hypothetical-ness". But my confusion and argument is > similarly "in principle"-like. Because if such an example above can be > constructed then this would be an issue for SCALAR as well, right? And > if you can bypass verifier's safety with SCALAR, you (hypothetically) > could use that SCALAR to do out-of-bounds memory access by adding this > SCALAR to some mem-like register. Correct. The issue would apply to regular scalar if such 'within' operation was available. > So that's my point and my source of confusion: if we don't trust > var_off+range_within() logic to handle *all* situations correctly, > then we should be worried about SCALARs just as much as anything else > (unless, as usual, I missed something). Yes. I personally don't believe that doing range_within for all regtypes by default is a safer way forward. The example wasn't real. It was trying to demonstrate a possible issue. You insist to see a real example with range_within. I don't have it. It's a gut feel that it could be there because I could construct it with fake 'within'. > > More gut feel than real issue. > > > > > > > > > > > > > SCALARS and PTR_TO_BTF_ID will likely dominate future bpf progs. > > > > Keeping default as regs_exact (that does ID match) is safer default. > > > > > > It's fine, though the point of this patch set was patch #7, enabling > > > logic similar to PTR_TO_MAP_VALUE for PTR_TO_MEM and PTR_TO_BUF. I can > > > send specific fixes for that, no problem. But as I said above, I'm > > > really curious to understand what kind of situations will lead to > > > unsafety if we do var_off+range_within checks. > > > > PTR_TO_MEM and PTR_TO_BUF explicitly are likely ok despite my convoluted > > example above. > > I'm less sure about PTR_TO_BTF_ID. It could be ok. > > Just feels safer to opt-in each type explicitly. > > Sure, I can just do a simple opt-in, no problem. As I said, mostly > trying to understand the issue overall. > > For PTR_TO_BTF_ID specifically, I can see how we can enable > var_off+range_within for cases when we access some array, right? But > then I think we'll be enforcing that we are staying within the > boundaries of a single array field, never crossing into another field. Likely yes, but why? You're trying hard to collapse the switch statement in regsafe() while claiming it's a safer way. I don't see it this way. For example the upcoming active_lock_id would need its own check_ids() call. It will be necessary for PTR_TO_BTF_ID only. Why collapse the switch into 'default:' just to bring some back? The default without checking active_lock_id through check_ids would be wrong, so collapsed switch doesn't make things safer. > But just to take a step back, from my perspective var_off and > range_within are complementary and solve slightly different uses, but > should be both satisfied: > - var_off is not precise with range boundaries (due to some bits too > coarsely marked as unknown), but it's useful to enforce having a value > being a multiple of some power-of-2 (e.g., knowing for sure that > lowest 2 bits are zero means that value is multiple of 4; I haven't > checked, but I assume we check with for various pointer accesses to > ensure we don't have misaligned reads). They can be only approximately > used for actual possible range of values. Right. var_off is used for alignment checking too. grep tnum_is_aligned. We have bare minimum of testing for that though. Only few tests in the test_verifier use BPF_F_STRICT_ALIGNMENT > - range_within() can and should be used for *precise* range of value > tracking, but it can't express that alignment restriction. Right. > So while I previously thought that we can do away without var_off, I > now think there are cases when it's necessary. But if we are sure that > we handle any SCALAR case correctly for any possible var_off + > range_within situation, it should be fine to do that for any mem-like > pointer just as much, as var_off+range_within is basically a MEM + > SCALAR combined case. Right. Likely true. > Anyways, I'm not blocked on this, but I think we'll benefit from > taking this discussion to its logical conclusion. Not sure what conclusion you're looking for.
