llvm-project

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//===- ExportTrie.cpp -----------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This is a partial implementation of the Mach-O export trie format. It's
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// essentially a symbol table encoded as a compressed prefix trie, meaning that
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// the common prefixes of each symbol name are shared for a more compact
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// representation. The prefixes are stored on the edges of the trie, and one
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// edge can represent multiple characters. For example, given two exported
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// symbols _bar and _baz, we will have a trie like this (terminal nodes are
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// marked with an asterisk):
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//
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//              +-+-+
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//              |   | // root node
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//              +-+-+
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//                |
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//                | _ba
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//                |
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//              +-+-+
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//              |   |
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//              +-+-+
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//           r /     \ z
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//            /       \
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//        +-+-+       +-+-+
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//        | * |       | * |
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//        +-+-+       +-+-+
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//
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// More documentation of the format can be found in
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// llvm/tools/obj2yaml/macho2yaml.cpp.
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//
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//===----------------------------------------------------------------------===//
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#include "ExportTrie.h"
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#include "Symbols.h"
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#include "lld/Common/ErrorHandler.h"
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#include "lld/Common/Memory.h"
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#include "llvm/BinaryFormat/MachO.h"
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#include "llvm/Support/LEB128.h"
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#include <optional>
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using namespace llvm;
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using namespace lld;
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using namespace lld::macho;
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namespace {
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struct Edge {
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  Edge(StringRef s, TrieNode *node) : substring(s), child(node) {}
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  StringRef substring;
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  struct TrieNode *child;
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};
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struct ExportInfo {
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  uint64_t address;
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  uint64_t ordinal = 0;
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  uint8_t flags = 0;
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  ExportInfo(const Symbol &sym, uint64_t imageBase)
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      : address(sym.getVA() - imageBase) {
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    using namespace llvm::MachO;
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    if (sym.isWeakDef())
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      flags |= EXPORT_SYMBOL_FLAGS_WEAK_DEFINITION;
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    if (sym.isTlv())
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      flags |= EXPORT_SYMBOL_FLAGS_KIND_THREAD_LOCAL;
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    // TODO: Add proper support for stub-and-resolver flags.
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    if (auto *defined = dyn_cast<Defined>(&sym)) {
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      if (defined->isAbsolute())
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        flags |= EXPORT_SYMBOL_FLAGS_KIND_ABSOLUTE;
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    } else if (auto *dysym = dyn_cast<DylibSymbol>(&sym)) {
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      flags |= EXPORT_SYMBOL_FLAGS_REEXPORT;
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      if (!dysym->isDynamicLookup())
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        ordinal = dysym->getFile()->ordinal;
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    }
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  }
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};
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} // namespace
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struct macho::TrieNode {
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  std::vector<Edge> edges;
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  std::optional<ExportInfo> info;
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  // Estimated offset from the start of the serialized trie to the current node.
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  // This will converge to the true offset when updateOffset() is run to a
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  // fixpoint.
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  size_t offset = 0;
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  uint32_t getTerminalSize() const;
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  // Returns whether the new estimated offset differs from the old one.
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  bool updateOffset(size_t &nextOffset);
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  void writeTo(uint8_t *buf) const;
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};
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// For regular symbols, the node layout (excluding the children) is
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//
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//   uleb128 terminalSize;
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//   uleb128 flags;
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//   uleb128 address;
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//
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// For re-exported symbols, the layout is
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//
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//   uleb128 terminalSize;
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//   uleb128 flags;
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//   uleb128 ordinal;
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//   char[] originalName;
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//
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// If libfoo.dylib is linked against libbar.dylib, and libfoo exports an alias
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// _foo to a symbol _bar in libbar, then originalName will be "_bar". If libfoo
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// re-exports _bar directly (i.e. not via an alias), then originalName will be
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// the empty string.
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//
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// TODO: Support aliased re-exports. (Since we don't yet support these,
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// originalName will always be the empty string.)
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//
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// For stub-and-resolver nodes, the layout is
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//
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//   uleb128 terminalSize;
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//   uleb128 flags;
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//   uleb128 stubAddress;
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//   uleb128 resolverAddress;
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//
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// TODO: Support stub-and-resolver nodes.
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uint32_t TrieNode::getTerminalSize() const {
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  uint32_t size = getULEB128Size(info->flags);
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  if (info->flags & MachO::EXPORT_SYMBOL_FLAGS_REEXPORT)
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    size += getULEB128Size(info->ordinal) + 1; // + 1 for the null-terminator
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  else
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    size += getULEB128Size(info->address);
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  return size;
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}
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bool TrieNode::updateOffset(size_t &nextOffset) {
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  // Size of the whole node (including the terminalSize and the outgoing edges.)
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  // In contrast, terminalSize only records the size of the other data in the
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  // node.
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  size_t nodeSize;
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  if (info) {
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    uint32_t terminalSize = getTerminalSize();
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    // Overall node size so far is the uleb128 size of the length of the symbol
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    // info + the symbol info itself.
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    nodeSize = terminalSize + getULEB128Size(terminalSize);
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  } else {
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    nodeSize = 1; // Size of terminalSize (which has a value of 0)
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  }
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  // Compute size of all child edges.
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  ++nodeSize; // Byte for number of children.
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  for (const Edge &edge : edges) {
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    nodeSize += edge.substring.size() + 1             // String length.
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                + getULEB128Size(edge.child->offset); // Offset len.
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  }
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  // On input, 'nextOffset' is the new preferred location for this node.
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  bool result = (offset != nextOffset);
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  // Store new location in node object for use by parents.
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  offset = nextOffset;
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  nextOffset += nodeSize;
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  return result;
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}
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void TrieNode::writeTo(uint8_t *buf) const {
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  buf += offset;
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  if (info) {
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    uint32_t terminalSize = getTerminalSize();
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    buf += encodeULEB128(terminalSize, buf);
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    buf += encodeULEB128(info->flags, buf);
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    if (info->flags & MachO::EXPORT_SYMBOL_FLAGS_REEXPORT) {
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      buf += encodeULEB128(info->ordinal, buf);
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      *buf++ = 0; // empty originalName string
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    } else {
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      buf += encodeULEB128(info->address, buf);
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    }
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  } else {
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    // TrieNode with no Symbol info.
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    *buf++ = 0; // terminalSize
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  }
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  // Add number of children. TODO: Handle case where we have more than 256.
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  assert(edges.size() < 256);
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  *buf++ = edges.size();
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  // Append each child edge substring and node offset.
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  for (const Edge &edge : edges) {
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    memcpy(buf, edge.substring.data(), edge.substring.size());
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    buf += edge.substring.size();
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    *buf++ = '\0';
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    buf += encodeULEB128(edge.child->offset, buf);
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  }
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}
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TrieBuilder::~TrieBuilder() {
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  for (TrieNode *node : nodes)
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    delete node;
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}
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TrieNode *TrieBuilder::makeNode() {
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  auto *node = new TrieNode();
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  nodes.emplace_back(node);
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  return node;
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}
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static int charAt(const Symbol *sym, size_t pos) {
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  StringRef str = sym->getName();
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  if (pos >= str.size())
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    return -1;
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  return str[pos];
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}
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// Build the trie by performing a three-way radix quicksort: We start by sorting
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// the strings by their first characters, then sort the strings with the same
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// first characters by their second characters, and so on recursively. Each
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// time the prefixes diverge, we add a node to the trie.
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//
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// node:    The most recently created node along this path in the trie (i.e.
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//          the furthest from the root.)
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// lastPos: The prefix length of the most recently created node, i.e. the number
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//          of characters along its path from the root.
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// pos:     The string index we are currently sorting on. Note that each symbol
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//          S contained in vec has the same prefix S[0...pos).
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void TrieBuilder::sortAndBuild(MutableArrayRef<const Symbol *> vec,
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                               TrieNode *node, size_t lastPos, size_t pos) {
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tailcall:
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  if (vec.empty())
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    return;
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  // Partition items so that items in [0, i) are less than the pivot,
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  // [i, j) are the same as the pivot, and [j, vec.size()) are greater than
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  // the pivot.
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  const Symbol *pivotSymbol = vec[vec.size() / 2];
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  int pivot = charAt(pivotSymbol, pos);
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  size_t i = 0;
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  size_t j = vec.size();
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  for (size_t k = 0; k < j;) {
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    int c = charAt(vec[k], pos);
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    if (c < pivot)
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      std::swap(vec[i++], vec[k++]);
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    else if (c > pivot)
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      std::swap(vec[--j], vec[k]);
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    else
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      k++;
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  }
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  bool isTerminal = pivot == -1;
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  bool prefixesDiverge = i != 0 || j != vec.size();
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  if (lastPos != pos && (isTerminal || prefixesDiverge)) {
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    TrieNode *newNode = makeNode();
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    node->edges.emplace_back(pivotSymbol->getName().slice(lastPos, pos),
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                             newNode);
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    node = newNode;
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    lastPos = pos;
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  }
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  sortAndBuild(vec.slice(0, i), node, lastPos, pos);
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  sortAndBuild(vec.slice(j), node, lastPos, pos);
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  if (isTerminal) {
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    assert(j - i == 1); // no duplicate symbols
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    node->info = ExportInfo(*pivotSymbol, imageBase);
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  } else {
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    // This is the tail-call-optimized version of the following:
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    // sortAndBuild(vec.slice(i, j - i), node, lastPos, pos + 1);
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    vec = vec.slice(i, j - i);
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    ++pos;
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    goto tailcall;
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  }
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}
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size_t TrieBuilder::build() {
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  if (exported.empty())
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    return 0;
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  TrieNode *root = makeNode();
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  sortAndBuild(exported, root, 0, 0);
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  // Assign each node in the vector an offset in the trie stream, iterating
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  // until all uleb128 sizes have stabilized.
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  size_t offset;
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  bool more;
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  do {
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    offset = 0;
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    more = false;
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    for (TrieNode *node : nodes)
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      more |= node->updateOffset(offset);
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  } while (more);
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  return offset;
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}
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void TrieBuilder::writeTo(uint8_t *buf) const {
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  for (TrieNode *node : nodes)
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    node->writeTo(buf);
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}
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namespace {
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// Parse a serialized trie and invoke a callback for each entry.
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class TrieParser {
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public:
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  TrieParser(const uint8_t *buf, size_t size, const TrieEntryCallback &callback)
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      : start(buf), end(start + size), callback(callback) {}
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  void parse(const uint8_t *buf, const Twine &cumulativeString);
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  void parse() { parse(start, ""); }
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  const uint8_t *start;
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  const uint8_t *end;
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  const TrieEntryCallback &callback;
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};
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} // namespace
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void TrieParser::parse(const uint8_t *buf, const Twine &cumulativeString) {
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  if (buf >= end)
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    fatal("Node offset points outside export section");
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  unsigned ulebSize;
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  uint64_t terminalSize = decodeULEB128(buf, &ulebSize);
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  buf += ulebSize;
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  uint64_t flags = 0;
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  size_t offset;
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  if (terminalSize != 0) {
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    flags = decodeULEB128(buf, &ulebSize);
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    callback(cumulativeString, flags);
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  }
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  buf += terminalSize;
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  uint8_t numEdges = *buf++;
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  for (uint8_t i = 0; i < numEdges; ++i) {
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    const char *cbuf = reinterpret_cast<const char *>(buf);
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    StringRef substring = StringRef(cbuf, strnlen(cbuf, end - buf));
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    buf += substring.size() + 1;
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    offset = decodeULEB128(buf, &ulebSize);
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    buf += ulebSize;
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    parse(start + offset, cumulativeString + substring);
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  }
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}
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void macho::parseTrie(const uint8_t *buf, size_t size,
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                      const TrieEntryCallback &callback) {
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  if (size == 0)
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    return;
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  TrieParser(buf, size, callback).parse();
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}
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