Division by Zero Remediation

Overview

Division by zero in Solana BPF programs causes a runtime panic that crashes the transaction. Attackers exploit this for denial-of-service by submitting zero divisors. The fix is to validate divisors before every division and modulo operation, or use checked_div/checked_rem which return None on zero.

Recommended Fix

Before (Vulnerable)

pub fn calculate_share(total: u64, num_recipients: u64) -> ProgramResult {
    // VULNERABLE: panics if num_recipients == 0
    let share = total / num_recipients;
    let remainder = total % num_recipients;
    Ok(())
}

After (Fixed)

pub fn calculate_share(total: u64, num_recipients: u64) -> ProgramResult {
    // FIXED: explicit validation + checked arithmetic
    if num_recipients == 0 {
        return Err(ProgramError::InvalidArgument);
    }

    let share = total
        .checked_div(num_recipients)
        .ok_or(ProgramError::ArithmeticOverflow)?;
    let remainder = total
        .checked_rem(num_recipients)
        .ok_or(ProgramError::ArithmeticOverflow)?;
    Ok(())
}

Alternative Mitigations

1. Default value on zero

When zero has a meaningful interpretation (e.g., “no stakers means no distribution”):

pub fn per_staker_reward(total: u64, stakers: u64) -> u64 {
    if stakers == 0 {
        return 0;  // No stakers, no distribution
    }
    total / stakers
}

2. Clamp divisor to minimum

pub fn calculate_rate(amount: u64, period: u64) -> Result<u64, ProgramError> {
    // Ensure period is at least 1
    let safe_period = period.max(1);
    Ok(amount / safe_period)
}

3. Use an account-validation framework constraints for input validation

#[derive(Accounts)]
pub struct Distribute<'info> {
    #[account(
        constraint = config.num_recipients > 0 @ ErrorCode::NoRecipients
    )]
    pub config: Account<'info, DistributionConfig>,
}

Common Mistakes

Mistake 1: Checking only division, not modulo

if divisor == 0 { return Err(...); }
let quotient = total / divisor;
let remainder = total % divisor;  // Also needs the check!

Both division and modulo panic on zero. Validate once, then use for both operations.

Mistake 2: Checking after the division

// WRONG: division already executed (and panicked) before the check
let result = amount / rate;
if rate == 0 {
    return Err(ProgramError::InvalidArgument);
}

Always validate the divisor before performing the operation.

Mistake 3: Trusting derived values without validation

// WRONG: supply could be 0 even if derived from "trusted" source
let supply = token_account.supply;
let price_per_token = total_value / supply;

Even values from on-chain accounts can be zero (newly created token mints, depleted pools). Always validate.

Mistake 4: Using `unwrap()` on checked division

// WRONG: defeats the purpose of checked arithmetic
let result = total.checked_div(divisor).unwrap();

Use .ok_or(error)? to propagate the error gracefully.