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1//===- ExportTrie.cpp -----------------------------------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This is a partial implementation of the Mach-O export trie format. It's
10// essentially a symbol table encoded as a compressed prefix trie, meaning that
11// the common prefixes of each symbol name are shared for a more compact
12// representation. The prefixes are stored on the edges of the trie, and one
13// edge can represent multiple characters. For example, given two exported
14// symbols _bar and _baz, we will have a trie like this (terminal nodes are
15// marked with an asterisk):
16//
17// +-+-+
18// | | // root node
19// +-+-+
20// |
21// | _ba
22// |
23// +-+-+
24// | |
25// +-+-+
26// r / \ z
27// / \
28// +-+-+ +-+-+
29// | * | | * |
30// +-+-+ +-+-+
31//
32// More documentation of the format can be found in
33// llvm/tools/obj2yaml/macho2yaml.cpp.
34//
35//===----------------------------------------------------------------------===//
36
37#include "ExportTrie.h"
38#include "Symbols.h"
39
40#include "lld/Common/ErrorHandler.h"
41#include "lld/Common/Memory.h"
42#include "llvm/BinaryFormat/MachO.h"
43#include "llvm/Support/LEB128.h"
44#include <optional>
45
46using namespace llvm;
47using namespace lld;
48using namespace lld::macho;
49
50namespace {
51
52struct Edge {
53Edge(StringRef s, TrieNode *node) : substring(s), child(node) {}
54
55StringRef substring;
56struct TrieNode *child;
57};
58
59struct ExportInfo {
60uint64_t address;
61uint64_t ordinal = 0;
62uint8_t flags = 0;
63ExportInfo(const Symbol &sym, uint64_t imageBase)
64: address(sym.getVA() - imageBase) {
65using namespace llvm::MachO;
66if (sym.isWeakDef())
67flags |= EXPORT_SYMBOL_FLAGS_WEAK_DEFINITION;
68if (sym.isTlv())
69flags |= EXPORT_SYMBOL_FLAGS_KIND_THREAD_LOCAL;
70// TODO: Add proper support for stub-and-resolver flags.
71
72if (auto *defined = dyn_cast<Defined>(&sym)) {
73if (defined->isAbsolute())
74flags |= EXPORT_SYMBOL_FLAGS_KIND_ABSOLUTE;
75} else if (auto *dysym = dyn_cast<DylibSymbol>(&sym)) {
76flags |= EXPORT_SYMBOL_FLAGS_REEXPORT;
77if (!dysym->isDynamicLookup())
78ordinal = dysym->getFile()->ordinal;
79}
80}
81};
82
83} // namespace
84
85struct macho::TrieNode {
86std::vector<Edge> edges;
87std::optional<ExportInfo> info;
88// Estimated offset from the start of the serialized trie to the current node.
89// This will converge to the true offset when updateOffset() is run to a
90// fixpoint.
91size_t offset = 0;
92
93uint32_t getTerminalSize() const;
94// Returns whether the new estimated offset differs from the old one.
95bool updateOffset(size_t &nextOffset);
96void writeTo(uint8_t *buf) const;
97};
98
99// For regular symbols, the node layout (excluding the children) is
100//
101// uleb128 terminalSize;
102// uleb128 flags;
103// uleb128 address;
104//
105// For re-exported symbols, the layout is
106//
107// uleb128 terminalSize;
108// uleb128 flags;
109// uleb128 ordinal;
110// char[] originalName;
111//
112// If libfoo.dylib is linked against libbar.dylib, and libfoo exports an alias
113// _foo to a symbol _bar in libbar, then originalName will be "_bar". If libfoo
114// re-exports _bar directly (i.e. not via an alias), then originalName will be
115// the empty string.
116//
117// TODO: Support aliased re-exports. (Since we don't yet support these,
118// originalName will always be the empty string.)
119//
120// For stub-and-resolver nodes, the layout is
121//
122// uleb128 terminalSize;
123// uleb128 flags;
124// uleb128 stubAddress;
125// uleb128 resolverAddress;
126//
127// TODO: Support stub-and-resolver nodes.
128uint32_t TrieNode::getTerminalSize() const {
129uint32_t size = getULEB128Size(info->flags);
130if (info->flags & MachO::EXPORT_SYMBOL_FLAGS_REEXPORT)
131size += getULEB128Size(info->ordinal) + 1; // + 1 for the null-terminator
132else
133size += getULEB128Size(info->address);
134return size;
135}
136
137bool TrieNode::updateOffset(size_t &nextOffset) {
138// Size of the whole node (including the terminalSize and the outgoing edges.)
139// In contrast, terminalSize only records the size of the other data in the
140// node.
141size_t nodeSize;
142if (info) {
143uint32_t terminalSize = getTerminalSize();
144// Overall node size so far is the uleb128 size of the length of the symbol
145// info + the symbol info itself.
146nodeSize = terminalSize + getULEB128Size(terminalSize);
147} else {
148nodeSize = 1; // Size of terminalSize (which has a value of 0)
149}
150// Compute size of all child edges.
151++nodeSize; // Byte for number of children.
152for (const Edge &edge : edges) {
153nodeSize += edge.substring.size() + 1 // String length.
154+ getULEB128Size(edge.child->offset); // Offset len.
155}
156// On input, 'nextOffset' is the new preferred location for this node.
157bool result = (offset != nextOffset);
158// Store new location in node object for use by parents.
159offset = nextOffset;
160nextOffset += nodeSize;
161return result;
162}
163
164void TrieNode::writeTo(uint8_t *buf) const {
165buf += offset;
166if (info) {
167uint32_t terminalSize = getTerminalSize();
168buf += encodeULEB128(terminalSize, buf);
169buf += encodeULEB128(info->flags, buf);
170if (info->flags & MachO::EXPORT_SYMBOL_FLAGS_REEXPORT) {
171buf += encodeULEB128(info->ordinal, buf);
172*buf++ = 0; // empty originalName string
173} else {
174buf += encodeULEB128(info->address, buf);
175}
176} else {
177// TrieNode with no Symbol info.
178*buf++ = 0; // terminalSize
179}
180// Add number of children. TODO: Handle case where we have more than 256.
181assert(edges.size() < 256);
182*buf++ = edges.size();
183// Append each child edge substring and node offset.
184for (const Edge &edge : edges) {
185memcpy(buf, edge.substring.data(), edge.substring.size());
186buf += edge.substring.size();
187*buf++ = '\0';
188buf += encodeULEB128(edge.child->offset, buf);
189}
190}
191
192TrieBuilder::~TrieBuilder() {
193for (TrieNode *node : nodes)
194delete node;
195}
196
197TrieNode *TrieBuilder::makeNode() {
198auto *node = new TrieNode();
199nodes.emplace_back(node);
200return node;
201}
202
203static int charAt(const Symbol *sym, size_t pos) {
204StringRef str = sym->getName();
205if (pos >= str.size())
206return -1;
207return str[pos];
208}
209
210// Build the trie by performing a three-way radix quicksort: We start by sorting
211// the strings by their first characters, then sort the strings with the same
212// first characters by their second characters, and so on recursively. Each
213// time the prefixes diverge, we add a node to the trie.
214//
215// node: The most recently created node along this path in the trie (i.e.
216// the furthest from the root.)
217// lastPos: The prefix length of the most recently created node, i.e. the number
218// of characters along its path from the root.
219// pos: The string index we are currently sorting on. Note that each symbol
220// S contained in vec has the same prefix S[0...pos).
221void TrieBuilder::sortAndBuild(MutableArrayRef<const Symbol *> vec,
222TrieNode *node, size_t lastPos, size_t pos) {
223tailcall:
224if (vec.empty())
225return;
226
227// Partition items so that items in [0, i) are less than the pivot,
228// [i, j) are the same as the pivot, and [j, vec.size()) are greater than
229// the pivot.
230const Symbol *pivotSymbol = vec[vec.size() / 2];
231int pivot = charAt(pivotSymbol, pos);
232size_t i = 0;
233size_t j = vec.size();
234for (size_t k = 0; k < j;) {
235int c = charAt(vec[k], pos);
236if (c < pivot)
237std::swap(vec[i++], vec[k++]);
238else if (c > pivot)
239std::swap(vec[--j], vec[k]);
240else
241k++;
242}
243
244bool isTerminal = pivot == -1;
245bool prefixesDiverge = i != 0 || j != vec.size();
246if (lastPos != pos && (isTerminal || prefixesDiverge)) {
247TrieNode *newNode = makeNode();
248node->edges.emplace_back(pivotSymbol->getName().slice(lastPos, pos),
249newNode);
250node = newNode;
251lastPos = pos;
252}
253
254sortAndBuild(vec.slice(0, i), node, lastPos, pos);
255sortAndBuild(vec.slice(j), node, lastPos, pos);
256
257if (isTerminal) {
258assert(j - i == 1); // no duplicate symbols
259node->info = ExportInfo(*pivotSymbol, imageBase);
260} else {
261// This is the tail-call-optimized version of the following:
262// sortAndBuild(vec.slice(i, j - i), node, lastPos, pos + 1);
263vec = vec.slice(i, j - i);
264++pos;
265goto tailcall;
266}
267}
268
269size_t TrieBuilder::build() {
270if (exported.empty())
271return 0;
272
273TrieNode *root = makeNode();
274sortAndBuild(exported, root, 0, 0);
275
276// Assign each node in the vector an offset in the trie stream, iterating
277// until all uleb128 sizes have stabilized.
278size_t offset;
279bool more;
280do {
281offset = 0;
282more = false;
283for (TrieNode *node : nodes)
284more |= node->updateOffset(offset);
285} while (more);
286
287return offset;
288}
289
290void TrieBuilder::writeTo(uint8_t *buf) const {
291for (TrieNode *node : nodes)
292node->writeTo(buf);
293}
294
295namespace {
296
297// Parse a serialized trie and invoke a callback for each entry.
298class TrieParser {
299public:
300TrieParser(const uint8_t *buf, size_t size, const TrieEntryCallback &callback)
301: start(buf), end(start + size), callback(callback) {}
302
303void parse(const uint8_t *buf, const Twine &cumulativeString);
304
305void parse() { parse(start, ""); }
306
307const uint8_t *start;
308const uint8_t *end;
309const TrieEntryCallback &callback;
310};
311
312} // namespace
313
314void TrieParser::parse(const uint8_t *buf, const Twine &cumulativeString) {
315if (buf >= end)
316fatal("Node offset points outside export section");
317
318unsigned ulebSize;
319uint64_t terminalSize = decodeULEB128(buf, &ulebSize);
320buf += ulebSize;
321uint64_t flags = 0;
322size_t offset;
323if (terminalSize != 0) {
324flags = decodeULEB128(buf, &ulebSize);
325callback(cumulativeString, flags);
326}
327buf += terminalSize;
328uint8_t numEdges = *buf++;
329for (uint8_t i = 0; i < numEdges; ++i) {
330const char *cbuf = reinterpret_cast<const char *>(buf);
331StringRef substring = StringRef(cbuf, strnlen(cbuf, end - buf));
332buf += substring.size() + 1;
333offset = decodeULEB128(buf, &ulebSize);
334buf += ulebSize;
335parse(start + offset, cumulativeString + substring);
336}
337}
338
339void macho::parseTrie(const uint8_t *buf, size_t size,
340const TrieEntryCallback &callback) {
341if (size == 0)
342return;
343
344TrieParser(buf, size, callback).parse();
345}
346