Verify that equivalent RawVals and ScVals compare the same

[brson]

Per 225cf4f#r1144119247 it is crucial that for all equivalent pairs of RawVals and ScVals, the pair has the same comparison results.

It looks to me that RawVal uses a custom Compare trait - RawVals don't appear to implement Ord; ScVal does implement Ord. So presumably the result of Compare on RawVals needs to be equivalent to the results of Ord::cmp (and PartialOrd::partial_cmp).

This should be testable with the fuzzing infrastructure in stellar/rs-soroban-sdk#878 so I intend to try that. Fuzzing won't guarantee all possible values compare the same, but better than nothing.

cc @graydon @dmkozh

[brson]

I have started fuzzing as described and have found differences in how RawVal and ScVal implement comparison.

ScVal's derive Ord, and RawVal implements Compare by hand, so to remediate issues I am currently focusing on making the RawVal implementations match the derived ScVal implementations.

It may be prudent though to write the Ord impls for XDR types by hand if it is important that they not change. As an example, the ScVal enum's Ord impl depends on the lexical order of ScVal variants, something that is prone to changing as the XDR definitions change.

[brson]

If it is crucial for these comparison functions to be the same then perhaps it is possible to just remove the Ord impl from ScVal and always do comparisons as RawVals. It seems very high-risk to have these two impls that might diverge over time, even if we fix them once with some confidence.

Or maybe there is some way to unify the impls, though it seems difficult.

[dmkozh]

Thanks for looking into this!

The issue is indeed concerning, though maybe it's not as bad as it seems.

For the particular case described in the PR only equivalence of Eq comparisons is important - the internal order divergence doesn't really matter as we're interested in the key retrieval interface only.

However, I agree that Ord doesn't seem useful/correct for ScVal. Besides the mentioned issue of RawVal divergence, since ScVal is defined in XDR, different XDR libraries might define different comparison algorithms (as it's not a part of XDR standard). So even if I wanted to do some client-side sorting of ScVal, there is no way to guarantee that it would match RawVal comparison logic. I don't think we should have the cases where comparing ScVals is useful on the host-side, though I might be wrong.

Looking for comments from @graydon as well.

[brson]

Thanks for the clarifications @dmkozh.

I've continued this fuzzing under the assumption that the only comparisons that matter are those where one of the RawVal or ScVal comparisons returns Ordering::Equal.

Is it important that impl Compare<ScVal> for Budget has the same logic as impl Compare<RawVal> for Host and/or PartialOrd for ScVal?

I have found one bug in Compare<ScVec> for Budget where differing vector lengths are not accounted for. Likewise for Compare<ScMap> for Budget.

[graydon]

Unfortunately it's not just a matter of equal comparisons, we also need identical behaviour wrt. the total order of the values. This is because we treat maps as ordered collections, and their order is canonical so as not to admit logically-equal maps that have physically-different (encoded) representations. This is better than the alternative, in which we say we don't canonicalize the order of values and then a signature on a message {"a": 10, "b": 11} doesn't apply to a message {"b":11, "a": 10}.

Put another way: users think of maps as sets-of-pairs, and sets-of-pairs have no logical order, but hashing relies on a physical order, so we impose a canonical order to the not-logically-ordered artifact to eliminate the ability to make logically-equal but physically-different maps.

This is all 100% intentional and part of the design.

[graydon]

(Also practically speaking the "key-retrieval interface" relies on binary search in sorted maps, which relies on proper ordering, so key retrieval will fail if the ordering isn't correct.)

[dmkozh]

Also practically speaking the "key-retrieval interface" relies on binary search in sorted maps, which relies on proper ordering, so key retrieval will fail if the ordering isn't correct.

Actually, what happens if we provide an incorrectly sorted map as a user input? Will the host fail or will we just instantiate a 'broken' map?

I think it would be nice to avoid having two different comparison implementations. The comparisons should normally happen only on RawVals. ScVal is just a serialization format, so as long as it doesn't shuffle the data during conversions, we should be good w.r.t. hashing. Basically the host implementation of RawVal comparison should be the source of truth and we shouldn't really need to sort ScVals on the host side, should we? The client implementations of course need to follow the host implementation, but it's much less dangerous than host itself having divergent implementations.

[graydon]

If you build a map out of a mis-sorted vector of pairs, the call to initialize the map fails. If you try to deserialize a mis-sorted ScMap into a host map, the deserialization fails.

The host never sorts ScVal by Ord but an external user / guest / other client can. Even if we turned off Ord on the client, nothing stops someone else from turning it on. If there are divergences between a naive structural sort as generated by #[derive(Ord)] and what we're doing in Compare<RawVal> I actually really want to know and understand what's causing them.

[graydon]

(if divergence is arising from something deeply subtle and impossible to guard against, I can imagine being persuaded to turn off #[derive(Ord)] on the XDR, but it's generally a useful feature in XDR and it seems much more likely to me that a divergence arises from a bug in Compare<RawVal>. I want to find those!)

[dmkozh]

The host never sorts ScVal by Ord

But why do we implement Compare<ScVal> for Budget then if it's not intended to be called by host? Ord on XDR is fine because it's not a part of the host implementation and divergence should (hopefully) only affect the client.

I agree that there might be bugs in RawVal comparisons as well and we should make sure it works as expected.

[graydon]

The host does call Compare<ScVal> for Budget in the storage map. It doesn't call ScVal::Ord because that's un-metered.

[dmkozh]

Right, that's what we do now. Would it be too silly to use RawVals for the storage comparisons as well? I realize that we have to compute the LedgerKey anyway to verify its presence in the footprint, but that would be a bit safer and the overhead IIUC would come only from storing an additional RawVal per map entry. Maybe it's not worth the effort, just something to consider.

[brson]

There are many discrepancies between impl Ord for ScVal and the impls of Compare,
so many that it would help to know for sure before continuing that it is definitely important that they agree.
Furthermore, I have found that some cases of Compare<ScVal> for Budget do defer to impls of Ord for ScVal,
and that does seem to result in potentially incorrect behavior that deviates from Compare<RawVal> for Env.

I'm a bit overwhelmed, not sure what should be changed and what should be ignored, but here are my concrete findings so far.

Patches

These are patches I've made to change the behavior of comparison functions,
with explanations.

 impl Ord for Status {
     #[inline(always)]
     fn cmp(&self, other: &Self) -> Ordering {
-        let self_tup = (self.as_raw().get_major(), self.as_raw().get_minor());
-        let other_tup = (other.as_raw().get_major(), other.as_raw().get_minor());
+        let self_tup = (self.as_raw().get_minor(), self.as_raw().get_major());
+        let other_tup = (other.as_raw().get_minor(), other.as_raw().get_major());
         self_tup.cmp(&other_tup)
     }
 }

This makes Compare<RawVal> for Env agree with Ord for ScStatus -
the minor part is the ScStatus variant, the major part the contained value.

---

+// Note that these must have the same order as the impl
+// of Ord for ScVal, re https://github.com/stellar/rs-soroban-env/issues/743
 fn host_obj_discriminant(ho: &HostObject) -> usize {
     match ho {
-        HostObject::Vec(_) => 0,
-        HostObject::Map(_) => 1,
-        HostObject::U64(_) => 2,
-        HostObject::I64(_) => 3,
-        HostObject::TimePoint(_) => 4,
-        HostObject::Duration(_) => 5,
-        HostObject::U128(_) => 6,
-        HostObject::I128(_) => 7,
-        HostObject::U256(_) => 8,
-        HostObject::I256(_) => 9,
-        HostObject::Bytes(_) => 10,
-        HostObject::String(_) => 11,
-        HostObject::Symbol(_) => 12,
-        HostObject::Address(_) => 13,
-        HostObject::ContractExecutable(_) => 14,
+        HostObject::U64(_) => 0,
+        HostObject::I64(_) => 1,
+        HostObject::TimePoint(_) => 2,
+        HostObject::Duration(_) => 3,
+        HostObject::U128(_) => 4,
+        HostObject::I128(_) => 5,
+        HostObject::U256(_) => 6,
+        HostObject::I256(_) => 7,
+        HostObject::Bytes(_) => 8,
+        HostObject::String(_) => 9,
+        HostObject::Symbol(_) => 10,
+        HostObject::Vec(_) => 11,
+        HostObject::Map(_) => 12,
+        HostObject::ContractExecutable(_) => 13,
+        HostObject::Address(_) => 14,
         HostObject::NonceKey(_) => 15,
     }

This makes the discriminants agree with Ord for ScVal, so that when objects of different types are compared they compare the same as Ord for ScVal.

---

@@ -211,13 +214,9 @@ impl Compare<ScVec> for Budget {
     type Error = HostError;

     fn compare(&self, a: &ScVec, b: &ScVec) -> Result<Ordering, Self::Error> {
-        for (a, b) in a.iter().zip(b.iter()) {
-            match self.compare(a, b)? {
-                Ordering::Equal => (),
-                unequal => return Ok(unequal),
-            }
-        }
-        Ok(Ordering::Equal)
+        let a: &Vec<ScVal> = &*a;
+        let b: &Vec<ScVal> = &*b;
+        self.compare(a, b)
     }
 }

This handles the case where the vecs have different lengths. Seems to just be a plain bug.

---

@@ -225,13 +224,22 @@ impl Compare<ScMap> for Budget {
     type Error = HostError;

     fn compare(&self, a: &ScMap, b: &ScMap) -> Result<Ordering, Self::Error> {
-        for (a, b) in a.iter().zip(b.iter()) {
-            match self.compare(&(&a.key, &a.val), &(&b.key, &b.val))? {
-                Ordering::Equal => (),
-                unequal => return Ok(unequal),
+        let a: &Vec<ScMapEntry> = &*a;
+        let b: &Vec<ScMapEntry> = &*b;
+        self.compare(a, b)
+    }
+}
+
+impl Compare<ScMapEntry> for Budget {
+    type Error = HostError;
+
+    fn compare(&self, a: &ScMapEntry, b: &ScMapEntry) -> Result<Ordering, Self::Error> {
+        match self.compare(&a.key, &b.key)? {
+            Ordering::Equal => {
+                self.compare(&a.val, &b.val)
             }
+            cmp => Ok(cmp),
         }
-        Ok(Ordering::Equal)
     }

This handles the case where the maps have different lengths. Seems to just be a plain bug.

---

Unresolved findings

I haven't made patches for these because they would take
a bunch of effort that might not be necessary depending on whether
it matters how Ord for ScVal is implemented.

i128/u128 comparisons

When both values are i128 or u128, Compare does a 128-bit numeric comparison;
whereas the Ord XDR implementations are doing comparisons on the layout
of two u64 struct fields, ordered lo, hi.

pub struct Int128Parts {
    pub lo: u64,
    pub hi: u64,
}

Compare<ScVal> for Budget falls back to Ord for ScVal

Related to the above.

impl Compare<ScVal> for Budget {
    type Error = HostError;

    fn compare(&self, a: &ScVal, b: &ScVal) -> Result<Ordering, Self::Error> {
        use ScVal::*;
        match (a, b) {
            (Vec(Some(a)), Vec(Some(b))) => self.compare(a, b),
            (Map(Some(a)), Map(Some(b))) => self.compare(a, b),

            (Vec(None), _) | (_, Vec(None)) | (Map(None), _) | (_, Map(None)) => {
                Err(ScHostValErrorCode::MissingObject.into())
            }

            (Bytes(a), Bytes(b)) => {
                <Self as Compare<&[u8]>>::compare(self, &a.as_slice(), &b.as_slice())
            }

            (String(a), String(b)) => {
                <Self as Compare<&[u8]>>::compare(self, &a.as_slice(), &b.as_slice())
            }

            (Symbol(a), Symbol(b)) => {
                <Self as Compare<&[u8]>>::compare(self, &a.as_slice(), &b.as_slice())
            }

            (Bool(_), _)
            | (Void, _)
            | (Status(_), _)
            | (U32(_), _)
            | (I32(_), _)
            | (U64(_), _)
            | (I64(_), _)
            | (Timepoint(_), _)
            | (Duration(_), _)
            | (U128(_), _)
            | (I128(_), _)
            | (U256(_), _)
            | (I256(_), _)
            | (Bytes(_), _)
            | (String(_), _)
            | (Symbol(_), _)
            | (Vec(_), _)
            | (Map(_), _)
            | (ContractExecutable(_), _)
            | (Address(_), _)
            | (LedgerKeyContractExecutable, _)
            | (LedgerKeyNonce(_), _) => Ok(a.cmp(b)),
        }
    }
}

The fallthrough case defers to Ord for ScVal.
Mentioned above, the i128 case at least disagrees with Compare<RawVal> for Env
because XDR i128s are not 128-bit ints but are two 64-bit uints.

Tag values disagree with ScVal variants

This shows up when e.g. a comparison between a U64 object and a small I128:
the Compare<RawVal> falls back to compare the Tag discriminant;
and a U64 object compares greater than a I128 smallval,
whereas in XDR all U64s are less than all I128s.

#[repr(u8)]
#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
pub enum Tag {

    /// Tag for a [RawVal] that contains a [u64] small enough to fit in 56 bits.
    U64Small = 6,

    /// Tag for a [RawVal] that contains an [i64] small enough to fit in 56 bits.
    I64Small = 7,

    ...

    /// Tag for a [RawVal] that contains a [u128] small enough to fit in 56 bits.
    U128Small = 10,

    /// Tag for a [RawVal] that contains a [i128] small enough to fit in 56 bits.
    I128Small = 11,

    ...

    /// Tag for a [RawVal] that refers to a host-side [u64] number.
    U64Object = 64,

    /// Tag for a [RawVal] that refers to a host-side [i64] number.
    I64Object = 65,
}

[graydon]

These are fantastic findings @brson thank you! Much to think about and discuss here. The i128 case probably has a similar companion issue around signed 256-bit values (since they're stored as 32-byte octet arrays in XDR, which won't sort correctly for negative values).

Concerning the vexing cases you identified but haven't patched yet, I think there are fixes to be had (besides "abandon the whole exercise and revisit the question of even having Ord on XDR", which I don't feel we're quite at the point of yet):

• I think we can fix the i128-bit and 256-bit issues by changing the XDR:

  • Switching the representation of 128-bit numbers to a big-endian byte array, as with i256
  • Adding an explicit bool positive field before, so negatives sort before positives, and the body is always unsigned
  • Extending the ScVal validation code to exclude too-small, too-big, and negative-zero signed-number cases, for both i128 and i256
    (though we could also treat this whole exercise as a signal to remove signed 128 and 256 options, but that'd be much more invasive)

• I'm not 100% sure about the Compare<RawVal> case you're describing. In the code I see, if either input to the comparison is an object type, we delegate to the environment's obj_cmp function (which has special cases for all obj-vs-small and small-vs-obj mixtures) and we only do a tag-vs-tag comparison in Compare<RawVal> in the residual cases which is small-vs-small. Though .. I see now that in the environment itself, in Host::obj_cmp, in the final residual branch (host.rs line 1714) when we were given a mix of object and small and they're not of the same underlying logical-ScValType, we do a tag-based comparison, which .. yeah, I suppose that case is wrong! Is that the one you meant? If so, great catch! I think it's fairly readily solvable by making a helper method on Tag that maps from Tag to its underlying (coarser) ScValType, and then comparing those. We only ever wind up in this branch if we were given values of unequal logical-ScValType, so making that inequality the basis for comparison should work and make things like U64-object-less-than-i128-small compare the same as they do in XDR.