Bug 305864 – [CSS] Crash due to release assert in HashTable when MatchedDeclarationsCache::computeHash() returns 0xFFFFFFFF

• Status: RESOLVED
• Resolution: FIXED
• Component: CSS

[David Kilzer (:ddkilzer)]

The computeHash() function in Source/WebCore/style/MatchedDeclarationsCache.cpp returns the result of WTF::computeHash() directly without calling AlreadyHashed::avoidDeletedValue().

The call chain is:
  WTF::computeHash() -> SuperFastHash::hash() -> StringHasher::finalize() -> avoidZero(avalancheBits(hash))

The avalancheBits() function can return the full range of 32-bit values, including 0xFFFFFFFF. While avoidZero() prevents returning 0 (the empty sentinel), it does not prevent returning 0xFFFFFFFF (the deleted sentinel for unsigned hash keys).

When the hash value equals 0xFFFFFFFF and is inserted into HashMap<unsigned, ..., AlreadyHashed>, it triggers RELEASE_ASSERT in checkHashTableKey().

This bug is not testable because the hash includes pointer values that vary at runtime:

```
// MatchedDeclarationsCache.cpp:137
unsigned hash = WTF::computeHash(matchResult, &inheritedCustomProperties);
//                                            ^^^ pointer value

// MatchResult.h:120-121
add(hasher,
    matchedProperties.properties.ptr(),  // <-- pointer value
    matchedProperties.linkMatchType,
    ...
);
```

Crash stack:
```
Thread 0 Crashed:: Dispatch queue: com.apple.main-thread:
0   WebCore  WTFCrashWithInfo + 24 (Assertions.h:969)
1   WebCore  void WTF::checkHashTableKey<...> + 24 (HashTable.h:374)
2   WebCore  WTF::HashTable<...>::inlineLookup<...> + 24 (HashTable.h:670)
3   WebCore  WTF::HashTable<...>::lookup<...> + 24 (HashTable.h:662)
4   WebCore  WTF::HashTable<...>::find<...> + 48 (HashTable.h:1004)
5   WebCore  WTF::HashTable<...>::find<...> + 48 (HashTable.h:515)
6   WebCore  WTF::HashMap<...>::find + 48 (HashMap.h:360)
7   WebCore  WebCore::Style::MatchedDeclarationsCache::find + 48 (MatchedDeclarationsCache.cpp:143)
8   WebCore  WebCore::Style::Resolver::applyMatchedProperties + 1284 (StyleResolver.cpp:706)
9   WebCore  WebCore::Style::Resolver::unadjustedStyleForElement + 3532 (StyleResolver.cpp:341)
10  WebCore  WebCore::SVGElement::resolveCustomStyle + 44 (SVGElement.cpp:647)
[...]
```

<rdar://118164567>

[David Kilzer (:ddkilzer)]

The fix is to wrap the return value with AlreadyHashed::avoidDeletedValue() to transform 0xFFFFFFFF to 0x7FFFFFFF:

```
return AlreadyHashed::avoidDeletedValue(WTF::computeHash(matchResult, &inheritedCustomProperties));
```

This follows the existing pattern in SharedStringHash.cpp and ImmutableStyleProperties.cpp.

[David Kilzer (:ddkilzer)]

Pull request: https://github.com/WebKit/WebKit/pull/56925

[EWS]

Committed 305989@main (45684cc85f9f): <https://commits.webkit.org/305989@main>

Reviewed commits have been landed. Closing PR #56925 and removing active labels.