From 3d274856e792a361336eb4ae1670bc9c1905f0cb Mon Sep 17 00:00:00 2001
From: Steinar H. Gunderson <sesse@chromium.org>
Date: Thu, 12 May 2022 16:42:40 +0200
Subject: [PATCH] Make AhoCorasickNode 4-aligned.
This should fix an issue where std::vector could allocate unaligned
memory for AhoCorasickNode, and we'd then return a pointer to
inline_edges, where a caller would expect the pointer to be aligned
but it wasn't.
Change-Id: Id9dff044c61f8e46062c63b8480b18ebc68c4862
---
diff --git a/components/url_matcher/substring_set_matcher.cc b/components/url_matcher/substring_set_matcher.cc
index e110047..ef0b750 100644
--- a/components/url_matcher/substring_set_matcher.cc
+++ b/components/url_matcher/substring_set_matcher.cc
@@ -424,7 +424,12 @@
edges_.inline_edges[num_edges()] = AhoCorasickEdge{label, node};
if (label == kFailureNodeLabel) {
// Make sure that kFailureNodeLabel is first.
- std::swap(edges_.inline_edges[0], edges_.inline_edges[num_edges()]);
+ // NOTE: We don't use std::swap here, because GCC
+ // doesn't understand that inline_edges[] is 4-aligned
+ // and gives a warning.
+ AhoCorasickEdge temp = edges_.inline_edges[0];
+ edges_.inline_edges[0] = edges_.inline_edges[num_edges()];
+ edges_.inline_edges[num_edges()] = temp;
}
--num_free_edges_;
return;
diff --git a/components/url_matcher/substring_set_matcher.cc b/components/url_matcher/substring_set_matcher.cc
index e110047..ef0b750 100644
--- a/components/url_matcher/substring_set_matcher.h
+++ b/components/url_matcher/substring_set_matcher.h
@@ -154,8 +154,9 @@
static constexpr uint32_t kEmptyLabel = 0x103;
// A node in the trie, packed tightly together so that it occupies 12 bytes
- // (both on 32- and 64-bit platforms).
- class AhoCorasickNode {
+ // (both on 32- and 64-bit platforms), but aligned to at least 4 (see the
+ // comment on edges_).
+ class alignas(AhoCorasickEdge) AhoCorasickNode {
public:
AhoCorasickNode();
~AhoCorasickNode();
@@ -178,6 +179,10 @@
NodeID GetEdgeNoInline(uint32_t label) const;
void SetEdge(uint32_t label, NodeID node);
const AhoCorasickEdge* edges() const {
+ // NOTE: Returning edges_.inline_edges here is fine, because it's
+ // the first thing in the struct (see the comment on edges_).
+ DCHECK_EQ(0u, reinterpret_cast<uintptr_t>(edges_.inline_edges) %
+ alignof(AhoCorasickEdge));
return edges_capacity_ == 0 ? edges_.inline_edges : edges_.edges;
}
@@ -258,6 +263,11 @@
// in the first slot if it exists (ie., is not equal to kRootID), since we
// need to access that label during every single node we look at during
// traversal.
+ //
+ // NOTE: Keep this the first member in the struct, so that inline_edges gets
+ // 4-aligned (since the class is marked as such, despite being packed.
+ // Otherwise, edges() can return an unaligned pointer marked as aligned
+ // (the unalignedness gets lost).
static constexpr int kNumInlineEdges = 2;
union {
// Out-of-line edge storage, having room for edges_capacity_ elements.
