overflow-safety

OVERFLOW_SAFETY Skill (Soroban)

Trigger Pattern: Always required for Soroban audits Inject Into: Breadth agents, depth-edge-case Finding prefix: [OF-N] Rules referenced: R10, R14

Soroban contracts are compiled Rust. Rust's overflow behavior depends on the build profile: in debug builds, overflows panic; in release builds (used for deployment), overflows silently wrap by default unless overflow-checks = true is set. Silent wrapping in financial arithmetic is a critical vulnerability.

1. Profile Check

Before tracing any arithmetic, inspect Cargo.toml for the release profile overflow setting:

Interpretation:

• overflow-checks = true → all integer arithmetic panics on overflow in release builds. The codebase is safe from silent wrapping.
• overflow-checks = false or missing → overflows silently wrap in release. Proceed to Section 2.
• If the file does not exist or does not contain [profile.release]: treat as false (Rust default for release).

Finding threshold: If overflow-checks is not true, the entire overflow safety of the contract depends on manual use of checked/saturating arithmetic. This is a configuration-level finding regardless of whether Section 2 finds specific overflow sites.

2. Arithmetic Trace (if overflow-checks is false/missing)

If the profile check from Section 1 found that overflow protection is NOT enabled, trace ALL arithmetic operations in financial paths:

Financial paths to prioritize:

• Token balance calculations (balance + amount, total_supply + mint_amount)
• Share/ratio calculations (shares * price / precision)
• Fee calculations (amount * fee_bps / 10000)
• Interest accrual (principal * rate * time)
• Reward distributions (rewards_per_token * user_balance)

Wrapping arithmetic consequences:

• u128 overflows near 2^128 ≈ 3.4 × 10^38 — practically unreachable for balances
• i128 overflows near 2^127 ≈ 1.7 × 10^38 — practically unreachable for balances
• u64 overflows near 1.8 × 10^19 — reachable with large token amounts in 6-decimal tokens
• u32 overflows near 4.3 × 10^9 — reachable in ledger numbers, timestamps, counts

3. Checked Arithmetic Patterns

Identify all financial arithmetic and classify whether safe arithmetic methods are used:

Safe methods:

• checked_add(b) → returns Option<T>, panics or propagates None on overflow
• checked_mul(b) → returns Option<T>
• saturating_add(b) → clamps at MAX (safe for balances where MAX means "very rich")
• checked_div(b) → also catches division by zero

Unsafe methods:

• +, -, * without overflow-checks = true → silent wrapping in release
• wrapping_add, wrapping_sub, wrapping_mul → explicitly wraps (intentionally unsafe for most contexts)
• / → panics on divide-by-zero regardless of overflow-checks (covered in Section 5)

Flag any unchecked arithmetic where:

• Operands are user-controlled (amounts, durations, counts)
• The result feeds into a balance, share count, or reward calculation

4. i128 Boundary Analysis

Soroban's native token and SEP-41 tokens frequently use i128 for amounts. Check operations near the boundaries:

Specific checks:

• Share calculations: shares = (amount * total_shares) / total_assets — if total_shares is near i128::MAX, multiplication overflows before division
• Cumulative reward trackers: reward_per_token_stored += rewards * PRECISION / total_supply — accumulation can overflow over time
• Negative balance checks: i128 allows negative values; verify contracts reject negative amount parameters via explicit require!(amount > 0)
• Cast safety: u128 as i128 silently truncates if the u128 value exceeds i128::MAX

5. Division Precision

Soroban has no floating-point arithmetic. All division truncates toward zero (integer division). Incorrect division ordering causes precision loss or incorrect results:

Anti-pattern (division before multiplication):

// BAD: (amount / total_supply) loses precision before multiplying by rewards
let user_share = (user_balance / total_supply) * total_rewards;

Correct pattern (multiplication before division):

// GOOD: multiply first to preserve precision
let user_share = (user_balance * total_rewards) / total_supply;

Additional checks:

• Division by zero: verify all divisors are checked for zero before use. require!(total_supply > 0) before x / total_supply
• Rounding direction: does truncation favor the protocol (rounding down on user withdrawals) or systematically favor attackers?
• Precision constants: verify PRECISION / SCALAR constants match the token decimals being used
• Intermediate type precision: if the protocol routes values through a fixed-point intermediate (e.g., FixedI128 with DENOMINATOR=10^N), verify N >= max(token_decimals) for all supported tokens. Double decimal conversion (token A decimals → intermediate → token B decimals) truncates at each step; if the intermediate has fewer precision digits than either token, significant value is silently lost

Finding Template

**ID**: [OF-N]
**Severity**: [Critical if balance wrapping, High if unchecked overflow in financial path, Medium if precision loss, Low if config-only]
**Step Execution**: ✓1,2,3,4,5 | ✗(reasons) | ?(uncertain)
**Rules Applied**: [R10:✓/✗, R14:✓/✗]
**Location**: src/{contract}.rs:LineN (or Cargo.toml for Section 1)
**Title**: {Overflow / Precision loss} in {fn_name} — {impact}
**Description**: [Specific arithmetic expression, operand types, worst-case values, and wrapping consequence]
**Impact**: [Balance wraps to near-zero / share inflation / reward theft / incorrect accounting]

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Step Execution Checklist (MANDATORY)