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util: introduce TrySub to prevent unsigned underflow

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Introduce `TrySub(T&, U)` which subtracts an unsigned integral `U` from an unsigned integral `T`, returning `false` on underflow.
Use with `Assume(TrySub(...))` at coins cache accounting decrement sites so invariant violations fail immediately rather than silently wrapping.

Co-authored-by: MarcoFalke <*~=`'#}+{/-|&$^_@721217.xyz>
Co-authored-by: Pieter Wuille <pieter@wuille.net>
This commit is contained in:
Lőrinc 2026-02-22 19:05:48 +01:00
parent d9c7364ac5
commit b8fa6f0f70
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GPG Key ID: 669FFF0FFA477A76
3 changed files with 18 additions and 9 deletions
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16
src/coins.cpp
View File

@ -113,8 +113,8 @@ void CCoinsViewCache::AddCoin(const COutPoint &outpoint, Coin&& coin, bool possi
fresh = !it->second.IsDirty();
}
if (!inserted) {
m_dirty_count -= it->second.IsDirty();
cachedCoinsUsage -= it->second.coin.DynamicMemoryUsage();
Assume(TrySub(m_dirty_count, it->second.IsDirty()));
Assume(TrySub(cachedCoinsUsage, it->second.coin.DynamicMemoryUsage()));
}
it->second.coin = std::move(coin);
CCoinsCacheEntry::SetDirty(*it, m_sentinel);
@ -153,8 +153,8 @@ void AddCoins(CCoinsViewCache& cache, const CTransaction &tx, int nHeight, bool
bool CCoinsViewCache::SpendCoin(const COutPoint &outpoint, Coin* moveout) {
CCoinsMap::iterator it = FetchCoin(outpoint);
if (it == cacheCoins.end()) return false;
m_dirty_count -= it->second.IsDirty();
cachedCoinsUsage -= it->second.coin.DynamicMemoryUsage();
Assume(TrySub(m_dirty_count, it->second.IsDirty()));
Assume(TrySub(cachedCoinsUsage, it->second.coin.DynamicMemoryUsage()));
TRACEPOINT(utxocache, spent,
outpoint.hash.data(),
(uint32_t)outpoint.n,
@ -248,12 +248,12 @@ void CCoinsViewCache::BatchWrite(CoinsViewCacheCursor& cursor, const uint256& ha
if (itUs->second.IsFresh() && it->second.coin.IsSpent()) {
// The grandparent cache does not have an entry, and the coin
// has been spent. We can just delete it from the parent cache.
m_dirty_count -= itUs->second.IsDirty();
cachedCoinsUsage -= itUs->second.coin.DynamicMemoryUsage();
Assume(TrySub(m_dirty_count, itUs->second.IsDirty()));
Assume(TrySub(cachedCoinsUsage, itUs->second.coin.DynamicMemoryUsage()));
cacheCoins.erase(itUs);
} else {
// A normal modification.
cachedCoinsUsage -= itUs->second.coin.DynamicMemoryUsage();
Assume(TrySub(cachedCoinsUsage, itUs->second.coin.DynamicMemoryUsage()));
if (cursor.WillErase(*it)) {
// Since this entry will be erased,
// we can move the coin into us instead of copying it
@ -311,7 +311,7 @@ void CCoinsViewCache::Uncache(const COutPoint& hash)
{
CCoinsMap::iterator it = cacheCoins.find(hash);
if (it != cacheCoins.end() && !it->second.IsDirty()) {
cachedCoinsUsage -= it->second.coin.DynamicMemoryUsage();
Assume(TrySub(cachedCoinsUsage, it->second.coin.DynamicMemoryUsage()));
TRACEPOINT(utxocache, uncache,
hash.hash.data(),
(uint32_t)hash.n,

3
src/coins.h
View File

@ -15,6 +15,7 @@
#include <support/allocators/pool.h>
#include <uint256.h>
#include <util/check.h>
#include <util/overflow.h>
#include <util/hasher.h>
#include <cassert>
@ -278,7 +279,7 @@ struct CoinsViewCacheCursor
inline CoinsCachePair* NextAndMaybeErase(CoinsCachePair& current) noexcept
{
const auto next_entry{current.second.Next()};
m_dirty_count -= current.second.IsDirty();
Assume(TrySub(m_dirty_count, current.second.IsDirty()));
// If we are not going to erase the cache, we must still erase spent entries.
// Otherwise, clear the state of the entry.
if (!m_will_erase) {

8
src/util/overflow.h
View File

@ -31,6 +31,14 @@ template <class T>
return i + j;
}
template <std::unsigned_integral T, std::unsigned_integral U>
[[nodiscard]] constexpr bool TrySub(T& i, const U j) noexcept
{
if (i < T{j}) return false;
i -= T{j};
return true;
}
template <class T>
[[nodiscard]] T SaturatingAdd(const T i, const T j) noexcept
{